NXB i hc quc gia H Ni 2007, 190 Tr.

T kho: Cu trc tinh th, cu trc tinh th ca cc oxit, cc kiu khuyt tt mng tinh

th, stki, frenken, tm mu, l trng, nguyn t xm nhp, o cu trc, mt trt,

lch mng, dung dch rn, gin pha, h mt cu t, h hai cu t, h ba cu t, tnh

cht vt l ca vt liu v c, dn in, dn nhit, dn electron, pht quang, laze .

Ti liu trong Th vin in t H Khoa hc T nhin c th c s dng cho mc

ch hc tp v nghin cu c nhn. Nghim cm mi hnh thc sao chp, in n phc

v cc mc ch khc nu khng c s chp thun ca nh xut bn v tc gi.

Mc lc

Chng 1 CU TRC TINH TH ...................................................................................... 7 1.1 Cc phng php m t cu trc tinh th................................................................... 7

1.1.1 M t theo kiu t bo mng li....................................................................... 7 1.1.2 M t cu trc theo kiu xp kht cc khi cu................................................ 14 1.1.3 M t cu trc bng cch ni cc khi a din trong khng gian .................... 19

1.2 Cu trc tinh th ca cc oxit v mt s hp cht quan trng ................................. 20 1.2.1 Cu trc tinh th ca mt s oxit...................................................................... 21 1.2.2 Hp cht gia cc oxit...................................................................................... 29

1.3 Nhng nt c bit ca tinh th cng ho tr v tinh th kim loi............................ 53 1.4 Cc yu t nh hng n kiu cu trc tinh th ..................................................... 57

1.4.1 Tnh hp thc SPT ca cc nguyn t .......................................................... 57 1.4.2 nh hng ca kiu lin kt ............................................................................ 58 1.4.3 nh hng ca bn knh nguyn t, ion .......................................................... 59

Chng 2 TINH TH THC - CC KIU KHUYT TT - DUNG DCH RN ...... 67 2.1 Cc kiu khuyt tt ................................................................................................... 67

2.1.1 Khuyt tt Stki................................................................................................ 67 2.1.2 Khuyt tt Frenken ........................................................................................... 68 2.1.3 Nhit ng hc ca s hnh thnh khuyt tt ................................................... 69

Vt liu v c (Phn l thuyt c s)

GS. Phan Vn Tng

2.1.4 Tm mu........................................................................................................... 73 2.1.5 L trng v nguyn t xm nhp trong tinh th bt hp thc.......................... 74 2.1.6 Khuyt tt o cu trc..................................................................................... 77 2.1.7 Cc khuyt tt ko di - Mt trt ................................................................... 78 2.1.8 Lch mng l loi khuyt tt ph bin trong tinh th....................................... 80

2.2 Dung dch rn ........................................................................................................... 81 2.2.1 Dung dch rn thay th ..................................................................................... 82 2.2.2 Dung dch rn xm nhp .................................................................................. 83 2.2.3 Nhng c ch phc tp trong s hnh thnh dung dch rn thay th................ 84 2.2.4 Nhng nhn xt tng qut v cc iu kin hnh thnh dung dch rn ............ 87 2.2.5 Cc phng php nghin cu dung dch rn.................................................... 88

Chng 3 GII THCH GIN PHA........................................................................... 94 3.1 M u ..................................................................................................................... 94 3.2 H mt cu t (h bc 1) .......................................................................................... 97 3.3 bc hai (K = 2).......................................................................................................... 99

3.3.1 Trng hp to thnh tecti n gin.............................................................. 99 3.3.2 Trng hp c to thnh hp cht mi .......................................................... 101 3.3.3 H bc hai trng hp c to thnh dung dch rn ........................................ 104

3.4 H bc ba (K = 3) ................................................................................................... 107 3.4.1 H bc ba to thnh tecti n gin ............................................................... 108 3.4.2 H bc ba trng hp c to thnh hp cht ho hc .................................... 110 3.4.3 H bc ba trng hp to thnh dung dch rn .............................................. 113

3.5 H tng tc bc ba................................................................................................ 116 Chng 4 MT S TNH CHT VT L QUAN TRNG CA VT LIU V C ............................................................................................................... 123

4.1 Nhm tnh cht in ............................................................................................... 123 4.1.1 Cht dn in ion, cht in li rn.................................................................. 123 4.1.2 Cht dn electron............................................................................................ 143 4.1.3 Cc tnh cht in khc .................................................................................. 151

4.2 Nhm tnh cht t................................................................................................... 163 4.2.1 Phn l thuyt.................................................................................................. 164 4.2.2 V d mt s vt liu t, cu trc v tnh cht................................................ 169

4.3 Cc tnh cht quang, vt liu pht quang v laze ................................................... 179 4.3.1 S pht quang v cht pht quang.................................................................. 179 4.3.2 Laze ................................................................................................................ 185

3

Li m u

Vt liu V c c mt ni dung kh rng, kh lng trnh by ht trong mt cun gio trnh. Khc vi cun gio trnh c nh my v pht nhn bn cho sinh vin nm 1998, ln ny chng ti chia thnh 3 phn l:

Phn l thuyt c s Phn k thut tng hp Phn gii thiu tng loi vt liu

Ba phn tuy ni dung khc nhau nhng c mi lin h mt thit vi nhau v b sung cho nhau.

Cun gio trnh ny ch trnh by phn l thuyt c s nhm vo i tng chnh l sinh vin v hc vin cao hc i v lnh vc vt liu v c ni ring v ho hc cht rn ni chung. hc vin c th t kim tra kin thc ca mnh, chng ti c a ra mt s cu hi v bi tp sau mi chng.

Xin chn thnh cm n i hc Quc gia H Ni, trng i hc Khoa hc T nhin, khoa Ho hc v b mn V c gip chng ti c iu kin xut bn cun gio trnh ny. Cm n Thc s V Hng Sinh nhit tnh gip trong vic hon thin ch bn bn tho.

TC GI

4

M u

1. Vai tr ca vt liu i vi s pht trin khoa hc k thut

Lch s ca loi ngi gn lin vi lch s pht minh v s dng ca tng loi vt liu chnh. Ni v cc thi i trc, ngi ta thng phn chia ra thnh: thi i , thi i ng v thi i st thp.

T gia th k XX n nay, s xut hin ca nhiu loi vt liu khc nhau c cc c tnh vt c st thp v ang thay th dn st thp trong nhiu lnh vc lm cho ngi ta a ra nhiu tn gi v thi i: thi i nhm v hp kim nhm, thi i gm thu tinh, thi i ca cht do v thi i compozit,

Gia th k XX vt liu bng hp kim nhm ng vai tr quan trng trong nhiu lnh vc khc nhau. Nh c tnh nh, cng v bn i vi mi trng nn hp kim nhm ang gi mt v tr quan trng trong cng ngh sn xut cc phng tin giao thng vn ti. Hp kim nhm gp phn quan trng trong vic gii quyt cc vn v nng lng ca k nguyn ny. Mt chic xe t du lch c ch to bng hp kim nhm nh hn khi c lm bng st thp 600 kg, do tnh ht thi gian s dng tit kim c khong 10.000 lt xng. Hp kim nhm cho php ch to c cc loi my bay phn lc siu thanh t c tc khong 3000 km/gi. Cc loi tn la, tu v tr u ch to bng hp kim nhm. Nh c bn cao vi mi trng khc nghit nn hp kim nhm c s dng xy dng nh my in thu triu khai thc nng lng ca i dng. Trong i sng hng ngy chng ta tip xc vi nhiu dng bng nhm, thi gian gn y hp kim nhm c s dng ph bin trong xy dng

Gm l loi vt liu c v tr c bit i vi lch s loi ngi c i, cn i, ng i v chc chn l trong th k XXI, vt liu gm cn a li nhiu iu k diu na i vi khoa hc k thut. S ra i ca gm mi c nh hng dn ti cuc cch mng trong ngnh in t ni ring v trong khoa hc k thut ni chung. Thnh tu trong lnh vc tng hp gm mi a ngnh in t hc t th h th nht (s dng bng n in t chn khng) sang th h th hai (s dng bng bn dn - tranzito) v sang th h th ba (s dng cc vi mch hay cn gi l mch t hp). Nh m c th thu nh cc thit b, my mc cng knh thnh nhng my mc gn nh, b nh hn, rt thun li trong s dng v c bit l tit kim c trit nng lng trong i sng hng ngy, trong sn xut cng nh trong nghin cu khoa hc. Cng ngh gm mi cn to ra c cc vt liu siu cng, chu c nhit rt cao, vt liu siu dn, iu c bit l nguyn liu sn xut cc loi gm mi hu ht u i t cc nguyn t ph bin nht trong t nhin nh oxi, nit, cacbon, silic.

Cng ngh hin i i hi nhng loi vt liu c cc tnh cht c bit nh: rn cao, chu mi mn va p, nh, bn nhit, bn i vi mi mi trng n mn khc nghit, Compozit l loi vt liu t hp gia kim loi gm polime p ng c nhng i hi . Trong t nhin ngi ta bit c nhiu loi compozit nh vy. V d g gm cc si xenlulo do v dai c bao bc bng loi vt liu cng l lignhin. Xng ng vt l compozit ca colagen protein dai nhng mm v apatite cng nhng gin. Hoc mt s loi

5

vt liu compozit m t xa ngi ta ch to c nh thp peclit c rn cao nhng do l do s t hp gia pha xementit (Fe3C) rt rn nhng gin vi pha ferrite (dung dch rn xm nhp ca cacbon trong -Fe) mm v do. Phi hp gia ct thp, rm, ct v pha nn l xi mng pooclng hirat ho cho ta vt liu b tng ct thp c cc tnh cht c bit p ng yu cu ca cng ngh xy dng Nhng loi vt liu compozit do kt qu ca cc cng trnh nghin cu l thuyt c a ra sn xut quy m cng ngh u tin l compozit bnh kp gm polyeste - si thu tinh - kim loi nh ch to my bay tng hnh Mosquito do ngi M cng b t 1940. T n nay vt liu compozit xm nhp vo rt nhiu ngnh cng nghip khc nhau:

- Cng nghip quc phng - ch to cc loi my bay qun s, tn la, cc loi qun trang, qun dng.

- Giao thng vn ti - sn xut cc phng tin giao thng nh cc loi my bay dn dng, v v khung xe hi, tu thu, tu ha

- Cng ngh xy dng - sn xut cc loi tm lp, vt liu cch m cch nhit, cc loi vt liu xy dng c bit

- Y hc - sn xut xng gi, rng gi, da

- Cng ngh sn xut cc dng c sinh hot, dng c th thao nh xe p ua, giy th thao, vt tennis, thuyn, cc loi dng trong gia nh.

Trong lnh vc ch to vt liu mi i hi s hiu bit su sc v cu to, tnh nng v c bit l cng ngh sn xut ca tng loi vt liu.

Trong hai thnh phn ca gi thnh sn phm l gi nguyn liu v gi tr tu, th gi tr tu ngy cng tng ln, cn gi nguyn liu ngy cng gim i mt cch nhanh chng. Hin nay c nhiu sn phm gi tr tu chim trn 80% gi thnh. C th ni th k XXI l th k ca tr tu, nc no bit khai thc tt tr tu th s tr nn giu mnh. iu ny ng cho mi lnh vc, c bit l trong lnh vc pht trin khoa hc cng ngh vt liu.

2. Phn loi vt liu

Theo quan im ho hc c th phn thnh cc nhm vt liu v c l:

a) Vt liu kim loi v hp kim

b) Vt liu gm (gm sinh hot, gm xy dng, gm m ngh, gm k thut nhit, c, in, quang) ch yu di dng tinh th.

c) Vt liu thy tinh ch yu di dng v nh hnh

d) Vt liu kt dnh: Xi mng v cc cht kt dnh khc.

e) Vt liu t hp (compozit)

Theo c tnh k thut li phn thnh 5 nhm vt liu l:

a) Vt liu kim loi: c trng ca loi vt liu ny l c cc electron chuyn ng t do trong mng tinh th. iu ny lm cho loi vt liu ny c cc tnh cht nh dn nhit v

6

dn in tt, khng trong sut, khi c mi nhn th c b mt rt bng, rt bn nhng li d bin dng, d dt mng, d ko si,

b) Vt liu gm: l hp cht gia kim loi v phi kim, thng thng l cc oxit, cacbua, borua, nitrua, silixua. y l nhm vt liu a dng nht gm cc loi: gm, xi mng, thu tinh. c tnh chung ca nhm vt liu ny l cch in, cch nhit, bn nhit cao v bn vi mi mi trng khc nghit hn so vi nhm vt liu kim loi cng nh vt liu polime. Gm c rn cao nhng rt dn.

c) Polime: Gm cc cht do v cao su, ch yu l hp cht hu c c cu trc phn t ln. c tnh chung ca loi vt liu ny l nh, do v d to hnh.

d) Compozit: Bao gm nhiu kiu vt liu t hp vi nhau. V d si thu tinh kt hp vi vt liu polime va c c tnh bn ca thu tinh va c tnh do ca polime. Ni chung phn ln cc vt liu mi thuc nhm ny.

e) Vt liu bn dn: L nhm vt liu c tnh cht trung gian gia cht dn in v cht cch in v c bit nht l tnh cht in ca loi vt liu ny rt nhy cm vi s c mt ca tp cht nng cc nh. Tnh cht bn dn ca nhm vt liu ny a ti cuc cch mng v in t hc v cng ngh my tnh.

Gio trnh ny gii thiu nhng kin thc c s ca vt liu v c nh: cu trc tinh th, cc loi khuyt tt, dung dch rn, gin cn bng pha, mt s tnh cht vt l quan trng. L thuyt v phn ng gia cc pha rn c trnh by trong cun cc phng php tng hp vt liu gm(*) .

(*) Phan Vn Tng: Cc phng php tng hp vt liu gm, NXB i hc Quc gia H Ni 2006.

7

Chng 1

CU TRC TINH TH

1.1 Cc phng php m t cu trc tinh th

Cu trc tinh th lin quan n mi tnh cht ca vt liu. Do tng hp c loi vt liu c cc tnh cht mong mun phi hiu r cu trc bn trong ca n v t la chn phng php ch to hp l.

C nhiu cch m t cu trc tinh th: Da vo kiu t bo mng, vo cch sp xp kht khi cu, da vo cch ni cc a din trong khng gian. Trong cc gio trnh tinh th hc u c trnh by cc phng php . y ch trnh by tm tt nhng vn lin quan n mn vt liu hc.

1.1.1 M t theo kiu t bo mng li

Trong cht rn dng tinh th, cc tiu phn (nguyn t, ion, phn t,) c sp xp mt cch u n, tun hon to thnh mt mng li khng gian. Gi s ta chn mt tiu phn A bt k lm gc to , ri dng h trc to AX, AY, AZ theo 3 hng trong khng gian. Gi gc lp bi 3 trc l , , v gi khong cch u n gia cc tiu phn theo trc AX l a (thng s n v theo trc AX), theo trc AY l b, theo trc AZ l c. Th tch b nht trong khng gian ABCDABCD c cha mi yu t i xng c trng cho khng gian gi l t bo mng li.

A

B C

D

A'

B' C'

D'

X

Y

Z

ab

Hnh 1 Mng li khng gian

Ty theo cc gi tr a, b, c, , , , ngi ta phn ra thnh 7 h tinh th vi cc kiu mng c s khc nhau, mi mng c s li phn thnh cc kiu mng li khc nhau v c k hiu nh sau: mng c s n gin k hiu l P, nu tm ca cc mt mng c s c cha mt tiu phn na th gi l mng li tm mt v k hiu l F, nu ch tm ca hai y c cha thm tiu phn th gi l mng li tm y v k hiu l C, nu ti tm im ca

8

mng c s c cha mt tiu phn th gi l mng li tm khi v k hiu l I. Bng 1 di y gii thiu 7 h tinh th v 14 kiu t bo mng li.

Bng 1.7 h tinh th v 14 kiu t bo mng

H Cc thng s t bomng Yu t i xng c

trng nht Cc kiu mng

Lp phng (cubic) a = b = c = = = 90o

4 trc bc ba P. F. I

Bn phng (tetragonal) a = b c = = = 90o

1 trc bc bn P. I

Trc thoi (orthorhombic) a b c = = =90o

3 trc bc hai P. F. I. C

Lc phng (hexagonal, trigonal) a = b c = = 90o,

= 120o

1 trc bc su P

Mt thoi (Rhombohedral) a = b = c = = 90o

1 trc bc ba P

n t (monoclinic) a b c = = 90o,

90o

1 trc bc hai P. C

Tam t (triclinic) a b c 90o

khng P

H lc phng (Hexagonal) v h tam phng (Trigonal) u c thng s t bo mng nh nhau.

Ct th 3 trong bng 1 ch a ra yu t i xng c trng nht ca mi h. Cn s yu t i xng ca cc h th c rt nhiu. V d c nhiu yu t i xng nht l h lp phng. H lp phng c 3 trc i xng bc 4 (3A4) l cc ng thng ni tm im ca hai mt i din nhau, 3 ng ny trc giao vi nhau ti tm t bo, 4 trc i xng bc ba (4A3) l cc ng thng ni hai nh i din nhau, su trc i xng bc hai (6A2) l cc ng thng ni im gia 2 cnh i din nhau, ba mt i xng M (3M) l mt phng i qua tm im ca 4 cnh song song vi nhau, su mt i xng M (6M) l cc mt ct khi lp phng theo tng cp ng cho mt, mt tm i xng (C). Nh vy khi lp phng c cc yu t i xng l:

3A4, 4A3,6A2, 3M, 6M, C Cng vy cc yu t i xng ca h t phng l 1A4, 2A2, 2A2, M,

2M, 2M, C. Cc yu t i xng ca h trc thoi l A2, A2, A2, M, M, M, C. Cc yu t i xng ca h lc phng l A6, 3A2, 3A2, M, 3M, 3M, C. Cc yu t i xng ca h mt thoi l A2, 3A2, 3M, C. H n t c cc yu t i xng: A2, M, C.

9

P

ab

c

C

c

ab

I

ab

c

F

c

ab

Hnh 2 Bn t bo mng li ca h trc thoi

P: l mng li n gin F: l mng li tm mt

C: l mng li tm y I: l mng li tm khi

Trong mng li tinh th c rt nhiu h mt phng song song v cch u nhau. Mi mt h mt phng song song vi nhau c c trng bng 3 ch s h k l (gi l ch s Mile (Miller)). xc nh ch s h, k, l ca mt mt phng bt k trong mng li tinh th, trc ht cn chn gc to O v ba trc xut pht t O l Ox, Oy, Oz. Thng s n v theo trc Ox l a, theo Oy l b v theo Oz l c. V d mt 1 trn hnh 3 ct Ox im ng vi 1/2 thng s n v (a/2), ct Oy im ng vi mt thng s n v (b/1) ct Oz im ng vi 1/3 thng s n v (c/3). Ly gi tr nghch o ca cc s ta c ch s h k l ca mt 1 l 2 1 3. C mt h cc mt phng song song v cch u mt 1 , trn hnh v c ghi mt 2. H mt phng gi l h mt 2 1 3 c mt 1 gn vi gc to nht. Hnh 4 gii thiu ch s Mile ca mt s mt phng khc nhau.

yz

x1 2

ab

c

0o a/2

c/3 b

Hnh 3 Xc nh ch s Mile hkl ca mt phng trong mng li tinh th

(a) (b) (c)

z

x

y

b

a

c Oy

z

x

a

b

c o

b

ac

ab

x

z

y

b

a

c

O1 2

a

10

Hnh 4 Ch s Mile ca mt s mt phng khc nhau: a(111); b(101); c(010)

Mt phng gch gch hnh 4a ct Ox, Oy, Oz im ng vi mt thng s n v a, b, c nn gi l mt 1 1 1. Hnh 4b v mt phng ct trc Ox, Oz im ng vi mt thng s n v v song song vi trc Oy (ct Oy ) nn gi l mt 1 0 1. Hnh 4c c cc mt c v d song song vi nhau, ta chn mt d xc nh ch s Mile ca h mt phng ny, v mt c i qua im gc O khng th xc nh c cc gi tr h k l. Mt d song song vi trc Ox v Oz ct Oy mt thng s n v b nn gi l mt 0 1 0.

Thng tin quan trng nht khi kho st mng li khng gian l gi tr khong cch gia cc mt mng dhkl. T kt qu ghi ph nhiu x tia X cho ta bit cc gi tr ca mu nghin cu, do bit c s c mt ca cc pha rn trong mu. Mi h tinh th c mt mi lin h gia cc gi tr dhkl vi cc thng s ca t bo mng.

Vi h lp phng ta c: 2 2 2

2 2hkl

1 h k ld a

+ += (1)

v th tch t bo V = a3

Vi h t phng ta c: 2 2 2

2 2 2hkl

1 h k ld a c

+= + (2)

v th tch t bo

V = a2.c

Vi h trc thoi ta c: 2 2 2

2 2 2 2hkl

1 h k ld a b c

= + + (3)

v th tch t bo V = a.b.c

Vi h lc phng ta c: 2 2 2

2 2 2hkl

1 4 h kh k l3d a c

+ += + (4)

th tch t bo

V= 23.a .c

2

= 0,866a2.c

Vi h n t: 2 2 2 2

2 2 2 2 2hkl

1 1 h k .sin l 2hlcosd sin a b c ac

= + + (5)

th tch t bo

11

V= abc.sin Vi h tam t ta c:

( 2 2 2 2 2 2 2 2 2 2 2 22 2hkl

1 1 h b c sin k a c sin l a b sin d V

= + + (6) 2 2

2

2hkabc (cos.cos.cos) 2kla bc(cos.cos.cos )2hlab c.(cos.cos.cos))

+ + +

Th tch t bo:

V=abc(1- cos2- cos2 - cos2 + 2cos.cos.cos )1/2 Di y kho st mt vi gi tr c trng ca t bo mng li kim loi. Trc ht quy

c rng mng li kim loi gm cc nguyn t xp kht nhau, nhng d hnh dung, trong cc hnh v chng ta biu din cc nguyn t bng nhng vng trn nh.

Mng li lp phng tm khi: thng s t bo mng l a, mi t bo cha hai nguyn t,

quan h gia bn knh nguyn t v hng s mng l: a 3r4

= , t xc nh c chc c C (compact).

= th tch ca 2 nguyn tCth tch t bo

= 3 3

3 3

4 4 a 32( .r ) 2( .( ) )3 3 4 0, 68a a

= =

iu c ngha l trong t bo lp phng tm khi c 32% khong trng. Mi nguyn

t c bao quanh bng 8 nguyn t khc vi khong cch u l a 32

, ngha l s phi tr

(SPT) bng 8, khi lng ring 32MdNa

= (M l nguyn t lng, N l s Avgar).

T

TT

T

Hnh 5. T bo mng lp phng tm khi

Hnh 6 V tr cc hc trng bt din (hc O), k hiu

Hnh 7 V tr cc hc trng t din (hc T), k hiu y

C hai loi hc trng l hc bt din (hc O) v hc t din (hc T).

+ Hc O:

Tm ca 6 mt u l hc O chung cho 2 t bo cnh nhau.

12

im gia 12 cnh u l hc O chung cho 4 t bo cnh nhau.

Do mi t bo c: (6 1/2) + (12 1/4) = 6 hc O (hnh 6). + Hc T: Mi mt c 4 hc T chung cho 2 t bo cnh nhau. Do mi t bo c (4 6

1/2) = 12 hc T (hnh 7).

Mng li lp phng mt tm: Thng s t bo mng l a. Mi t bo cha 4 nguyn t.

Quan h gia bn knh nguyn t v cnh l: a 2r4

= , c chc

= th tch ca 4 nguyn tCth tch t bo

= 3 3

3 3

4 4 a 24( .r ) 2( .( ) )3 3 4 0,74a a

= =

Mi nguyn t c bao quanh bng 12 nguyn t khc vi khong cch bng a 22

,

SPT =12, khi lng ring 34MdNa

= .

Hnh 8 Mng li lp phng tm mt

Hnh 9 V tr hc O, k hiu

Hnh 10. V tr cc hc O, k hiu x

Trong t bo lp phng mt tm c 26% khong trng l cc hc O v hc T. + Hc O:

Tm t bo c 1 hc O (hnh 9).

13

Gia cc cnh u c hc O chung cho 4 t bo (hnh 10).

Vy mi t bo c 1 + 12 1/4 = 4 hc O. + Hc T: Mi t bo c 8 hc T nm trong t bo cc to : (1/4, 1/4, 1/4); (3/4, 1/4,

1/4); (3/4, 3/4, 1/4); (1/4, 3/4, 1/4); (1/4, 1/4, 3/4); (3/4, 1/4, 3/4); (3/4, 3/4, 3/4); (1/4, 3/4, 3/4) (hnh 11).

Mng li lc phng: Thng s t bo mng l a, c (hnh 12). Mi t bo c 2 nguyn

t. Quan h gia bn knh nguyn t v thng s t bo l ar2

= .

chc c = Th tch 2 nguyn tCTh tch t bo

Th tch 2 nguyn t 3

14 aV 23 2

= ; Th tch t bo V2 = abc

a

c

aA1

B1 C1

D1

A

BCD

(a)

A B

CD

A1B1

D1 C1

60o120o

(b)

Hnh 11 V tr cc hc T

Hnh 12 Khi gm 3 t bo (a), t bo mng li lc phng (b)

T

5C8

83C

Hnh 13 V tr cc hc T (k hiu y)

T T

T

T

T T

T

T

14

Hnh 14 V tr cc hc O (k hiu )

a 3b ;2

= 2

2a 3V c;

2= mt khc a 3 ;

c 2 2=

Do C 0,743 2

= =

SPT =12; Khi lng ring 2

4MdNa 3c

=

Mi t bo c 2 hc O v 4 hc T (hnh 13 v 14).

1.1.2 M t cu trc theo kiu xp kht cc khi cu

Theo nguyn l xp kht, th khi khng c s nh hng ca lin kt, cc tiu phn to thnh tinh th c khuynh hng sp xp sao cho khong khng gian t do c th tch b nht, ngha l c chc c ln nht. Nu cc tiu phn to thnh tinh th u c dng qu cu vi ng knh bng nhau th c 2 kiu xp kht gi l xp kht lc phng (kiu ABABA) v xp kht lp phng (kiu ABCABCAB). Hnh 15 trnh by cch xp kht ca mt lp qu cu . Trong lp xp kht ny (gi l lp A) mi mt qu cu (v d qua cu K) c bao quanh bng 6 qu cu khc. Trong hnh b, mi qu cu ch tip xc vi 4 qu cu khc, y khng phi l mt xp kht. Trong lp xp kht c 3 hng xp kht (XX, YY, ZZ) (hnh 15a), hnh 15b ch c 2 hng xp kht. Trong mt xp kht ny c cc dy l trng R v dy l trng P.

15

RP

1

2

34

5

6

P

y y'

x

x'z

z'

(a)

pppR R

K

(b) Hnh 15 Mt phng gm cc qu cu xp kht nht (a), cch xp khng kht (b)

By gi chng ta t lp xp kht th 2 (gi l lp B) ln trn lp A. Mun cho khng gian t do c th tch b nht th phi t sao cho cc qu cu ca lp B nm ng v tr lm gia 3 qu cu ca lp A v ngc li, cc qu cu ca lp A phi nm ng cc v tr lm ca lp B. Mun vy th cc qu cu ca lp B hoc l phi nm vo tt c cc v tr P, hoc l phi nm vo tt c cc v tr R ca lp A (xem hnh 15). Ta c 2 lp xp kht (hnh 16).

t lp th 3 ln lp th 2 ta c 2 cch. Nu t sao cho cc qu cu ca lp th 3 nm vo v tr S ca lp th 2 (hnh 16) th tt c cc qu cu ca lp 3 u trng vo v tr tng ng ca lp th nht. Ngha l chu k lp li ca cc lp l 2, cc lp xp theo th t ABABA. Kiu xp kht nh vy gi l xp kht lc phng.

A

B

TTT

SS S

S S

T T TT

Hnh 16 Hai lp xp kht A v B

A

B

C

16

Hnh 17 Ba lp xp kht ABC to thnh kiu xp kht lp phng

Nu t lp th 3 sao cho cc qu cu nm lt vo v tr T (xem hnh 16) th s hnh thnh mt lp mi (lp C), n lp th 4 mi lp li chu k tc l lp A. Cch xp nh vy gi l xp kht lp phng v th t lin tc ca cc lp l ABCABCAB (hnh 17). Xp kht lp phng v xp kht lc phng l hai kiu cu trc n gin v quan trng nht. Ngoi ra cng cn c nhiu kiu xp kht vi th t cc lp phc tp hn, v d ABCACB... hoc ABAC to thnh nhng chu k lp li ln hn (xem cu trc tinh th ca -Al2O3 - hnh 29).

Trong 2 kiu xp kht lc phng v xp kht lp phng, mi qu cu u tip xc vi 12 qu cu khc (SPT = 12).

chc c ca c 2 kiu xp kht ny u bng nhau v bng 0,74. iu ny c ngha l trong c hai kiu xp kht nht vn cn 26% th tch l cc khong trng. C hai loi hc trng gi l hc trng t din (hc T) v hc trng bt din (hc O). Hc T l khong khng gian gia 4 khi cu xp kht vo nhau. Tu thuc vo khi cu nh ca t din nm trn hoc di m phn thnh hc T+ hoc T (hnh 18). Hc O l khong khng gian nm gia 6 qu cu xp kht ca 2 lp st nhau, su qu cu gm 4 qu cng nm trn mt mt phng v 2 qu nm v hai pha ca mt phng .

1

2 3

45

6

(a) (b) (c) Hnh 18 Cc hc trng trong mng li xp kht nht Hc T+ (a), Hc T- (b), Hc O (c)

Vic chn mt phng cha 4 qu cu c th ly t do theo nhiu cch. V d, cc qu 1264 hoc 2345 hoc 1356. Hnh 19 trnh by cc hc trng gia 2 lp xp kht. Mng tinh th ca cc oxit gm cc ion O2 xp kht, cn cc cation c phn b vo cc hc T v O. V rng trng tm ca t din gn y hn nh nn cation hc T khng ng vo v tr chnh gia 2 lp, cn cation hc O th nm ng chnh gia 2 lp. Cc cation khi chui vo hc T v O s lm gin n phn mng oxi.

Hc t din T-

Hc t din T+

Hc bt din O

Hnh 19 S phn b cc hc trng gia hai lp xp kht (lp cu pha trn v vng

17

m, lp cu pha di v ng chm chm) T bo mng ca kiu gi ghm chc c lp phng (hay cn gi l lp phng tm

mt) c trnh by trn hnh 20a, cn hnh 21 trnh by t bo mng ca kiu gi ghm chc c lc phng.

18

123

4

5

6

7

23

4

56

7

(a) (b) Hnh 20 T bo mng lp phng tm mt c to thnh t kiu gi ghm cc qu cu theo kiu chc c lp phng

Kiu gi ghm chc c lp phng trng vi kiu t bo lp phng mt tm (hnh 20a). Ni chung iu ny khng d dng thy c, v rng cc mt ca t bo lp phng khng trng vi lp xp kht, trong mi qu cu ch tip xc vi 4 qu cu khc ch khng phi 6 qu. Trong t bo lp phng mt tm th lp xp kht l cc mt song song vi mt phng 111. thy c r hn, ta b i qu cu v tr 1 ca hnh 20a th thy ngay lp di gm cc qu cu 2, 3, 4, 5, 6, 7 (hnh 20b) nm trn cng mt mt phng (song song vi mt 111). T ta c th kt lun rng cu trc gi ghm chc c lp phng c 4 mt xp kht trc giao vi ng cho ca khi lp phng.

T bo mng kiu lc phng trng vi kiu gi ghm chc c lc phng, iu ny d thy c trn hnh 21, y cc mt c s ca mng u trng vi mt xp kht.

60o 1 20 o

a

c

a

a

Hnh 21 T bo mng lc phng c to thnh t s gi ghm chc c lc phng cc khi cu

a s cc kim loi u kt tinh theo mt trong ba kiu lp phng tm mt (ABCABC), lc phng (ABAB) v lp phng tm khi. Bng 2 cho bit kiu t bo, thng s t bo ca mt s kim loi.

19

Bng 2 Cu trc v thng s t bo mng li ca mt s kim loi Lp phng tm mt

(kiu ABCABC) Lc phng (kiu ABAB) Lp phng tm khi

Kim loi a () Kim loi a () c () Kim loi a () Cu 3,6150 Be 2,2859 3,5843 Fe 2,8664

Ag 4,0862 Mg 3,2095 5,2104 Cr 2,8839

Au 4,0786 Zn 2,6650 4,9470 Mo 3,1472

Al 4,0494 Cd 2,9793 5,6181 W 3,1648

Ni 3,5238 Ti 2,9500 4,6860 Ta 3,3058

Pd 3,8898 Zr 3,2320 5,1470 Ba 5,0250

Pt 3,9231 Ru 2,7058 4,2819

Pb 4,9506 Os 2,7341 4,3197

Re 2,7600 4,4580

Vic mt kim loi no kt tinh theo kiu mng li ny hay mng li khc l mt vn cho n nay vn cha r. Trong hai kiu cu trc lp phng tm mt v lc phng u c chc c ln (C = 0,74) cn chc c ca kiu lp phng tm khi nh hn (C = 0,68). Kt qu tnh ton cho thy nng lng mng li ca cc kim loi c cu trc lc phng v lp phng tm mt gn tng t nhau, do s khc nhau v cu trc c th do s khc nhau v cu trc vng ca chng.

C mt s kim loi c bin ho th hnh, ngha l c th c cc kiu cu trc khc nhau. V d st, tu thuc vo nhit c th c cu trc lp phng tm khi (Fe-) hoc lp phng tm mt (Fe-); coban ngoi kiu cu trc lp phng tm mt v lc phng li cn c th to ra cc dng khc vi chu k lp li ca cc lp xp kht phc tp hn. y l trng hp ng vi dng a hnh c bit (polytypism) khi m s khc nhau v cu trc ca nhng dng th hnh ch xy ra theo mt hng. Trong cc kim loi c cu trc gi ghm chc c ca tt c cc lp nguyn t kch thc ging nhau, cn s khc nhau v cu trc ch l cch sp xp ln nhau ca cc lp . Tuy rng ch c hai cch chnh sp xp cc lp l ABC (lp phng tm mt) v AB (lc phng) nhng cng c th hnh thnh rt nhiu kiu sp xp lun phin phc tp hn. l trng hp ca coban kim loi. C nhng vt liu trong c dng th hnh vi chu k lp li gia cc lp xp kht ti vi trm lp nguyn t. S hnh thnh nhng cu trc nh vy ang l vn cha gii thch c. V d nh c kiu cu trc vi chu k lp li ti vi trm lp, dy ti 500. Cng c quan im cho rng chu k lp li ln nh vy lin quan vi c ch xon c khi tinh th ln dn sinh ra bin v xon.

1.1.3 M t cu trc bng cch ni cc khi a din trong khng gian

V d tinh th mui n (NaCl) c th m t bng cch ni cc khi bt din [NaCl6]5 qua mt cnh chung (hnh 22), mng tinh th nh vy c trnh by hnh 23.

Tuy nhin khng phi ton khi khng gian c lp y bng cc bt din, m vn cn li cc khong trng, i vi tinh th NaCl th l cc khong trng t din nh trn hnh 23.

Kiu m t nh vy rt thun li cho cch trnh by mng tinh th ca silicat. V d mng tinh th SiO2 c trnh by bng cch ni cc t din [SiO4]4 qua nh, mng tinh th

20

khong vt st c trnh by theo cch ni cc t din [SiO4]4 vi cc bt din [Al(OH)6]3

qua nh, mng tinh th zeolit c m t theo cch ni cc bt din ct [Si24xAlxO48]x+ qua cc mt bn (xem phn silicat). V cc a din u c cation nm tm, anion cc nh, nn ni cc a din qua nh, cnh hoc cc mt chung th phi lu n lc y gia cc cation, c bit l cc cation c kch thc b v in tch ln, v d nh Si4+. Tt nhin mng li bn nu cch ni sao cho lc y ny b nht, ngha l khong cch gia cc cation xa nht. Hnh 24 v bng 3 cho thy khong cch gia cc cation gim dn khi ni cc a din qua nh, cnh, mt.

Hnh 22 T bo mng NaCl c hnh thnh theo cch ni cc bt din theo cnh chung

Hnh 23 Mng tinh th NaCl c xy dng theo cch ni cc bt din [NaCl6]5 qua cnh chung

o

(a)

o

o

o

o

oo

oo o

MM

X

(b)

MMX

90o

X

MM

X

71o

(c)

X M M = 2MX (M - O, X - o) M - M= 2 MX = 1,411MX M - M =

o2(1 cos71 )MX = 1,16M

Hnh 24 Khong cch cation-cation khi ni cc bt din qua nh (a); qua cnh (b) v khi ni cc t din qua cnh (c)

Khong cch M-M khi ni 2 a din a din

Bng nh Bng cnh Bng mt Hai t din 2MX 1,16MX 0,67MX Hai bt din 2MX 1,41MX 1,16MX Bng 3 Khong cch gia cc tm a din cnh nhau MX4 v MX6

1.2 Cu trc tinh th ca cc oxit v mt s hp cht quan trng

st vi i tng nghin cu vt liu v c, trong phn m t cu trc tinh th di y s trnh by loi hp cht in hnh nht l oxit v hp cht gia cc oxit nh M2O, MO, M2O3, MO2, AB2O4, ABO3, AB2O7,... v silicat. Cc hp cht v c khc (halogenua, sunfua, nitrua,) s c ni n khi c cu trc tng t vi oxit.

21

1.2.1 Cu trc tinh th ca mt s oxit

1.2.1.1 Oxit c cng thc chung MO

Na+ Cl

Hnh 25 T bo mng li kiu NaCl

Oxit kim loi ho tr 2 nu c t l rM2+/rO2 nm trong khong 0,414 n 0,732 v c lin kt ch yu l lin kt ion th c mng li tinh th thuc kiu NaCl. Ta c th biu din mng li tinh th ny theo c 3 phng php trn. y biu din t bo mng. C th xem t bo mng ca loi ny gm hai phn mng lp phng mt tm ca cation M2+ v ca anion O2 lng vo nhau mt khong bng 1/2 cnh ca lp phng. Mi t bo gm 4 phn t MO, SPT ca cation v anion u bng nhau v bng 6.

Bng 4 Mt s hp cht cu trc theo kiu NaCl Hp cht a () Hp cht a () Hp cht a ()

MgO 4,2130 MgS 5,2000 MgSe 5,462 CaO 4,8105 CaS 5,6948 CaSe 5,924 SrO 5,1600 SrS 6,0200 SrSe 6,246 BaO 5,5390 BaS 6,3860 BaSe 6,600 TiO 4,1770 MnS 5,2240 CaTe 6,660 MnO 4,4450 SnAs 5,7248 BaTe 7,000 FeO 4,3070 TiC 4,3285 UC 4,955 CoO 4,2600 LaN 5,3000 ScN 4,440 NiO 4,1769 TiN 4,2400 UN 4,890 CdO 4,6653

Hp cht c t l rcation/ranion nm gia 0,225 v 0,414 v lin kt ch yu l ion th c cu trc kiu ZnS.

ZnS c hai dng th hnh l blend (hoc cn c tn l sphalerit) v vuazit.

Blend c t bo tinh th lp phng tm mt ca S2, ion Zn2+ nm 4 trong 8 hc t din ca phn mng lp phng tm mt S2 (hnh 26). To ca Zn2+ l: 1/4, 1/4, 1/4; 3/4, 3/4, 1/4; 3/4, 1/4, 3/4 v 1/4, 3/4, 3/4. Hng s mng ca blend a = 5,43; d = 4,092. Bng 5 gii thiu cc hp cht c cu trc kiu blend.

22

Zn

S

Hnh 26 T bo mng ca blend (ZnS)

Bng 5

Mt s hp cht cu trc kiu blend

Hpcht a () Hp cht

a () Hp cht

a () Hp cht

a () Hp cht

a ()

CuF 4,255 BeS 4,8624 -CdS 5,818 BN 3,616 GaP 5,448 CuCl 5,416 BeSe 5,070 CdSe 6,077 BP 4,538 GaAs 5,653

-CuBr 5,690 BeTe 5,54 CdTe 6,481 BAs 4,777 GaSb 6,095 -CuI 6,051 -ZnS 5,406 HgS 5,852 AlP 5,451 InP 5,869 -AgI 6,495 ZnSe 5,667 HgSe 6,085 AlAs 5,662 InAs 6,058 -MnS 5,600 ZnTe 6,102 HgTe 6,453 AlSb 6,135 InSb 6,478 -MnSe 5,880 SiC 4,358

Khi un nng ti 1020oC th blend chuyn thnh vuazit. T bo mng ca vuazit c phn mng S2 gi ghm chc c lc phng (hnh 27). Cc ion Zn2+ chim ht tt c hc t din T+, cn cc hc t din T v hc bt din O u trng.

Hnh 27 trnh by mt t bo mng ca vuazit. C th hnh dung t bo mng ny gm hai phn mng xp kht lc phng cation v anion lng vo nhau mt khong bng 3/8 chiu cao. Trong thc t hai phn mng ny lng vo nhau khng ng vi gi tr 3/8 (0,375) chiu cao, m tu theo tng loi cation gi tr dch chuyn khc nhau (gi tr u trong bng 6).

Hng s mng ca vuazit a = 3,811; c = 6,234; d = 3,98 4,08. Mt trong nhng tnh cht c trng ca phn mng xp kht lc phng l gi tr c/a.

Trng hp l tng th t s bng 1,633. Cc gi tr trn bng 6 u khc vi 1,633, iu ny chng t rng cc mt khng hon ton xp kht.

23

60 o120 o

a

c

38 c

60 o

38 c

a

c

120 o

Hnh 27 T bo mng kiu vuazit

Bng 6

Gii thiu mt s hp cht c cu trc vuazi

Cht a () c () u c/a Cht a () c () u c/a

ZnO 3,2495 5,2069 0,345 1,602 MnS 3,976 6,432 1,618

ZnS 3,8110 6,2340 1,636 MnSe 4,120 6,720 1,631

ZnSe 3,9800 6,5300 1,641 AlN 3,111 4,978 0,385 1,600

ZnTe 4,2700 6,9900 1,637 GaN 3,180 5,166 1,625

BeO 2,6980 4,3800 0,378 1,623 InN 3,533 5,693 1,611

CdS 4,1348 6,7490 1,632 TaN 3,050 4,940 1,620

1.2.1.2 Oxit c cng thc M2O3 l cc oxit Al2O3 (corun), Fe2O3 (hmatit), Cr2O3, Ga2O3, Ti2B3, Al2O3 c nhiu

dng th hnh. iu ny cng d hiu v t l rAl3+/rO2 = 0,42 tc l nm gia hai s phi tr l 4 v 6. Trong cc dng th hnh ch c 3 dng quan trng l , , .

Al2O3 l dng th hnh bn vng nht, loi ny cn c tn l corun, tinh th corun gm phn mng xp kht lc phng ca ion oxi. Cation Al3+ chim 2/3 hc bt din, cn hc T+ v T u b trng.

O2-

Al3+

O2-

Al3+

Hnh 28 Cu trc tinh th corun Al2O3

Khong cch gia hai lp xp kht oxi bng 2,16. T bo nguyn t ca corun thuc h mt thoi, chiu di cnh bng 5,12, gc nhn gia cc cnh bng 5o17. T bo nguyn t c 4 ion nhm v 6 ion oxi. Cc oxit c cu to ging corun gm hmatit Fe2O3, Cr2O3, Ga2O3, Ti2O3.

24

-Al2O3. Tn gi Al2O3 ch mt nhm hp cht c cng thc M2O.nX2O3. Gi tr ca n thay i t 8 n 11. M l cation ho tr +1 nh Cu+, Ga+, Tl+, In+, NH4+, H3O+, X l cc cation Al3+, Ga3+, Fe3+. Quan trng nht trong cc loi hp cht ny l natri -Al2O3 (M l Na+, X l Al3+) c bit t lu nh l mt sn phm ph khi sn xut thu tinh. Natri -Al2O3 c hnh thnh trong ni thu tinh khi tng tc gia Na2O c trong khi nng chy vi Al2O3 trong thnh phn gch chu la. Tn gi -Al2O3 rt d lm ngi ta nhm ln vi mt dng th hnh ca tinh th Al2O3, nhng do thi quen dng t lu nn by gi khng thay i na. Thc ra vai tr Na2O nh l mt tc nhn n nh cu trc tinh th ca loi hp cht ny. Hnh 29 trnh by mng anion trong -Al2O3. Trn hnh ny cho thy c 4 lp oxi gi ghm chc c li c mt lp ch c 1/4 ion O2 cn 3/4 v tr O2 trng. Cc ion Na+ c phn b trong lp thiu oxi . Do bn knh ion O2 rt ln hn bn knh ca Na+, mt khc lp ny rt rng nn Na+ chuyn ng kh d dng theo mt phng ca lp ny.

C

B A

Sp

Sp

Sp

c

A C

C B A C A B C A B

B A

Hnh 29 Cc lp oxi xp kht trong -Al2O3

Mng tinh th -Al2O3 cha cc bloc tng t spinen (Sp). Bloc Sp gm 4 lp oxi gi ghm chc c lun phin theo th t ABCA c trng ca cu trc lp phng. Ion Al3+ chim mt s v tr t din v bt din gia hai lp oxi. Cc bloc Sp tch bit nhau bng mt lp thiu oxi, ion Na+ nh v trong lp thiu oxi . T bo mng li lc phng c cc thng s a = 5,60, c = 22,50. Cc lp oxi trc giao vi trc c. T bo ca -Al2O3 c cha 2 bloc Sp. Cu trc spinen y c xem nh cu trc MgAl2O4 cha khuyt tt (xem hnh 35). Trong spinen MgAl2O4 c t l ion Mg2+ v Al3+ l 1 : 2 trong khi spinen ca -Al2O3 ch c Al3+ (khng k n cc ion hp kim ho nh Li+, Na+), do cn bng in tch trong cc bloc spinen phi c cha cc l trng Al3+.

Tinh th ca cc oxit t him c mc oxi ho +3 thng dng trong vt liu gm c cu trc khc vi cu trc corun v bn knh Ln3+ ln hn bn knh ca Al3+. Theo Holsmit, c th phn thnh 3 loi oxit t him: loi A c cu trc mt thoi hoc lc phng, loi B thuc h n t, loi C thuc h lp phng. Oxit t him cng c nhiu dng th hnh.

25

Bng 7 Tnh cht ca cc dng th hnh ca Al2O3

Ch s khc x

Dng th hnh

H tinh th

Dng D

(g/cm3) ng np

Bin ho khi nung

-Al2O3 (corun)

lc phng Bn, mt

thoi 3,99 1,786 1,76

2050oC nng

chy

-Al2O3 lc phng 3,33,4 1,680 1,65 1600oC thnh

-Al2O3 lp phng

Bt din 3,6 1200oC thnh

Loi A (kiu La2O3) c La2O3, Ce2O3, Pr2O3.

Loi B c Ge2O3, Pr2O3, Nd2O3, Sm2O3, Eu2O3, Cd2O3, Lu2O3, Tb2O3, Dy2O3 (trn 2000oC).

Loi C (kiu Tl2O3 hoc Mn2O3) c: Sm2O3, Eu2O3, Cd2O3, Lu2O3, Tb2O3, Dy2O3, Ho2O3, Er2O3, Tu2O3, Yb2O3.

Cc oxit Sc2O3, Y2O3, In2O3 v Tl2O3 cng c cu trc loi C nh oxit t him (thuc h lp phng, mi t bo c 16 phn t).

1.2.1.3 Oxit c cng thc MO2 Tu thuc vo t l rcation/ranion hp cht loi ny c 3 kiu cu trc l kiu florit (CaF2),

kiu rutin (TiO2) v kiu oxit silic (SiO2).

Kiu florit: Cc anion chim hc t din trong phn mng lp phng mt tm ca cation. Hay c th hnh dung khi phn chia mang c s lp phng mt tm ca cation thnh 8 khi lp phng b bng nhau th cc anion nm tm ca 8 khi lp phng b . SPT ca cation bng 8 cn ca anion bng 4. Mi t bo c 4 phn t AB2.

Oxit, sunfua, selenua kim loi ho tr mt c cu trc kiu antiflorit, ngha l vn ging kiu florit CaF2 nhng y v tr ca ion canxi li do ion oxi chim, cn v tr ca ion flo th cation kim loi kim chim.

(a)Ca F (b)

TiO

(c)

SiO

26

Hnh 30 T bo ca cc hp cht c cng thc AB2a) kiu florit, b) kiu rutin, c) kiu silic oxit (cristobalit)

Bng 8

Cc hp cht c cu trc florit v antiflorit

florit antiflorit

Hp cht

a () Hp cht

a () Hp cht

a () Hp cht

a ()

CaF2 5,4626 PbO2 5,3490 Li2O 4,6114 K2O 6,449

SrF2 5,8000 CeO2 5,4110 Li2S 5,7100 K2S 7,406

SrCl2 6,9767 PrO2 5,3920 Li2Se 6,0020 K2Se 7,962

BaF2 6,2001 ThO2 5,6000 Li2Te 6,5170 K2Te 8,168

BaCl2 7,3110 UO2 5,3720 Na2O 5,5550 Rb2O 6,740

CdF2 5,3895 NpO2 5,4334 Na2S 6,5390 Rb2S 7,650

HgF2 5,5373 PuO2 5,3860 Na2Se 6,823

Kiu rutin TiO2: T bo mng li thuc h t phng khi tm ca cation Ti4+ cn anion to thnh bt din u bao quanh titan (2 ion oxi nm mt trn dc theo mt ng cho, 2 ion oxi nm mt di cng theo hng ng cho , 2 ion oxi na nm ngay trong t bo nhng theo hng ng cho khc). Mi t bo mng c 2 phn t TiO2. S phi tr ca titan bng 6, cn ca oxi bng 3. To ca Ti l (0 0 0), (1/2 1/2 1/2) ca oxi l (x x 0), ( x x 0), (1/2+x, 1/2-x, 1/2), (1/2-x, 1/2+x, 1/2). Kt qu thc nghim xc nh c x = 0,3 (xem hnh 30b v 31).

Bng 9 Mt s hp cht c cu trc tinh th kiu rutin

Cht a () c () x Cht a () c () x

TiO2 4,5937 2,9581 0,305 SnO2 4,7373 3,1864 0,307

CrO2 4,4100 2,9100 TaO2 4,7090 3,0650

GeO2 4,3950 2,8590 0,307 WO2 4,8600 2,7700

IrO2 4,4900 3,1400 CoF2 4,6951 3,1796 0,306

-MnO2

4,3960 2,8710 0,302 FeF2 4,6966 3,3091 0,300

MoO2 4,8600 2,7900 MgF2 4,6230 3,0520 0,303

NbO2 4,7700 2,9600 MnF2 4,8734 3,3099 0,305

OsO2 4,5100 3,1900 NiF2 4,6506 3,0836 0,302

PbO2 4,9460 3,3790 PbF2 4,9310 3,3670

RuO2 4,5100 3,1100 ZnF2 4,7034 3,1335 0,303

C th m t cu trc mng tinh th rutin theo kiu xp kht nh hnh 31. Phn mng anion oxi xp kht theo kiu lc phng (ABAB). Cation titan chim 1/2 s hc bt din theo trt t: c mt dy hc bt din b trng n mt dy hc bt din b chim ht. Trt t ny thay i lun phin theo 1/2 n v ca c.

Kiu silic oxit

27

Silic oxit (SiO2) c nhiu dng th hnh, y ni v mt trong cc dng gi l cristobalit (hnh 30c). Cristobalit thuc h lp phng. Nguyn t Si chim cc nh, tm cc mt v 4 trong 8 hc t din theo hai hng ng cho khc nhau (hoc c th hnh dung chia khi hnh lp phng thnh 8 khi lp phng nh bng nhau th Si l tm ca 4 khi lp phng nh ). Oxi l cu ni gia cc nguyn t silic. Mi t bo c 8 phn t SiO2. S phi tr ca Si bng 4, ca oxi bng 2.

Ngoi cristobalit ra SiO2 cn rt nhiu dng hnh th khc na l: thch anh, tridimit, cristobalit.

Mi dng chnh nh vy cn tn ti di nhiu dng ph.

Cc dng n tinh: cancedoan, m no, jat (ngc).

Dng v nh hnh: thu tinh thch anh, opal.

b

a

(b)

To cc nguyn t

Ti4+ 0, 0, 0 - 0.5, 0.5, 0.5

O2- 0, 0.3, 0.3 - 0, 0.7, 0.70.8, 0.2, 0.5 - 0.2, 0.8, 0.5

a

c

(a)

Hnh31 a) Cu trc rutin biu din theo mt phng xp kht ca ion oxi b) Hnh chiu ca t bo mng rutin trn mt y

m t cu trc ca cc dng SiO2 th tt nht l dng phng php ghp cc t din vi nhau qua nh oxi chung (hnh 32). im khc nhau ca 3 dng tinh th chnh (thch anh, tridimit, cristobalit) l v tr tng i ca 2 t din SiO44 (hnh 33).

thch anh ta c gc lin kt Si-O-Si bng 150o, tridimit, v cristobalit th gc lin kt Si-O-Si bng 180o. T thch anh bin thnh cristobalit ch cn nn thng gc Si-O-Si t 150o thnh 180o, trong khi chuyn thnh tridimit th ngoi vic nn thng gc ny cn phi xoay t din SiO44 quanh trc i xng mt gc bng 180o

28

T+

T- T+

T+

T+

T-

T-

T-

Hnh 32 Cch ghp cc t din SiO4 ( Si4+, { O2-)

(a)

A

BC

A'

B'C'

D ) 150o

A

BC

A'

B'C'

D

(b) (c)

A

BC

A'

B'

C'

D

Hnh 33 S t hp hai t din SiO4 a) Thch anh; b) Cristobalit ; c) Tridimit

S khc nhau ca cc dng th hnh th cp , , ch do mc khc nhau ca gc lin kt Si-O-Si .

Qu trnh chuyn ho ca cc dng th hnh -thch anh -tridimit -cristobalit u l qu trnh c bin, cn qu trnh chuyn ho cc dng th hnh th cp u l qu trnh h bin.

29

tri dimit

t

t ridimit

r idimit

T hch a n h

T h c h a n h

cristobalit

cristobalit N n g ch y163

oC

117o

C

5 7 3 o C 253o C

870o

C 1728o C1470o C

1050o C

Hnh 34 S biu din s chuyn ho gia cc dng th hnh ca SiO2

Trong thc t nhit chuyn ho cc dng th hnh ca SiO2 cn ph thuc vo nhiu yu t nh s c mt ca nhiu cht khong ho, ch nng nhit v.v Trn y ch nu ln cc dng th hnh ca SiO2 p sut kh quyn. Di p sut cao cn c cc dng th hnh khc na (xem gin trng thi ca SiO2 hnh 92 v bng 30).

1.2.2 Hp cht gia cc oxit

1.2.2.1 Spinen Spinen l tn gi khong vt c cng thc MgAl2O4. C th xem spinen nh l hp cht

ca hai oxit: oxit baz ca kim loi ho tr 2 v oxit lng tnh ca kim loi ha tr 3.

MgO + Al2O3 = MgAl2O4

Spinen l i din cho mt lot cc hp cht c cng thc tng qut AB2O4. Trong A l cation ho tr 2 v B l cation ho tr 3. Mng li spinen gm cc ion oxi gi ghm chc c lp phng mt tm, cc cation A2+ v B3+ c sp xp vo cc hc t din v bt din (T+, T , O). Mi t bo mng gm 8 phn t AB2O4, ngha l c 8 khi lp phng b (hnh 35) trong c 32 ion oxi, 8 cation A2+ v 16 cation B3+. Ta c th tnh ton s cation, s anion v s hc t din T, s hc bt din O khi tng tng ghp 8 khi lp phng tm mt li vi nhau.

Hnh 35 T bo mng ca spinen

S ion oxi gm:

30

8 nh ca lp phng ln: 8 1/8 = 1 6 mt lp phng ln : 6 1/2 = 3 12 mt nh trong lp phng: 12 1 = 12 Tng s c 32 oxi 24 mt nh pha ngoi : 24 1/2 = 12 12 cnh ca lp phng ln: 12 1/4 = 3 Tm ca lp phng ln = 1

S hc T (cn gi l phn mng A). Mi lp phng nh c 8 hc t din nm trong lp phng . T bo mng spinen c 8 lp phng nh. Nh vy mi t bo spinen c 8 8 = 64 hc T.

S hc O (cn gi l phn mng B) gm:

8 tm ca 8 lp phng b: 8 1 = 8 24 cnh bin ca lp phng b: 24 1/4 = 6 24 cnh gia ca 6 mt bin: 24 1/2 = 12 6 cnh nm trong lp phng: 6 1 = 6 Nh vy, mi tinh th spinen c 64 + 32 = 96 hc T v O. M s cation ch c 8 + 16 =

24 cation. Ngha l ch 1/4 s hc trng cha cation, cn 3/4 hc trng khng.

Nu 8 cation A nm trong 8 hc trng T, cn 16 cation B nm vo hc O th gi l mng li spinen thun, k hiu A[BB]O4.

Nu 8 cation A nm trong 8 hc trng O, cn 16 cation B phn lm hai: 8 cation nm vo hc T, 8 cation nm vo hc O th gi l spinen nghch o, k hiu B[A.B]O4.

Nu 24 cation A v B c phn b mt cch thng k vo cc hc T v hc O th gi l spinen trung gian.

- -3 2 2 31 x x 41 x 1 x

B A OA B+ + + +

+ vi 0 < x

31

a l hng s mng li spinen (), rA v rB l bn knh khi cu nm trong khng gian t din v khng gian bt din, rO l bn knh oxi.

S tinh th kt tinh theo mng li spinen kh ph bin trong hp cht v c. Trong cng thc tng qut AB2O4 th A2+ c th l Cu, Be, Mg, Ca, Sr, Ba, Zn, Cd, Mn, Pb, Fe, Co, Ni. Cation B3+ c th l Al, Cr, Fe, Mn, t khi gp Ga, In, La, V, Sb, Rh. T hp cc cation li cho thy c rt nhiu hp cht spinen. Tuy nhin cng cn phi ni thm rng khng phi tt c cc hp cht c cng thc AB2O4 u kt tinh theo h lp phng nh spinen. V d nh BeAl2O4, CaCr2O4 thuc h hnh thoi, cn SrAl2O4 thuc h t phng. Trong khi mt s hp cht oxit ng vi cng thc A2BO4 (ng vi A2+, B4+), v d Mg2TiO4 li kt tinh theo h lp phng v c sp xp vo nhm spinen. l cc hp cht nh titanat, stanat ca coban, st(II), magi, km, Ngoi cc oxit phc tp ra, cn c cc spinen c anion l chalcogen (S2, Se2, Te2) hoc halogen. V d Li2NiF4.

Do kh nng thay th ng hnh, ng ho tr hoc khng ng ho tr cc cation trong spinen oxit lm cho s lng hp cht spinen tng ln rt ln.

V d t hp cc cation trong spinen oxit cho ta cc loi sau: Spinen MgAl2O4 Mg2TiO4 LiAlTiO4 Li0,5Al2,5O4 LiNiVO4 Na2WO4

T hp cation 2;3 2;4 1;3;4 1;3 1;2;5 1;6

Cc hp cht spinen c gi tr rt ln trong k thut. Chng c s dng lm bt mu, vt liu chu la, vt liu k thut in t, qu, Do vn nghin cu tng hp spinen v cc tnh cht, c, in ca chng l i tng nghin cu ca nhiu nh khoa hc. Theo dn in, c th nh gi c cu to bn trong ca spinen. V d Fe3O4 v Mn3O4 u c cu trc spinen, nhng trong khi Mn3O4 l cht in mi (khng dn in) cn Fe3O4 li c dn in cao nh kim loi. l do Fe3O4 c cu trc spinen o: Fe3+T[Fe2+Fe3+]OO4, cn Mn3O4 c cu trc spinen thun: Mn2+T[Mn23+]OO4. Trong mng tinh th spinen Fe3O4, cc ion Fe2+ v Fe3+ c phn b trong cc bt din. Cc bt din ny tip xc vi nhau theo mt cnh chung, do cation st c mc oxi ho khc nhau m gn nhau nn trao i in tch d dng, l trng dng c th chuyn t Fe2+ sang Fe3+. Cn mng tinh th spinen Mn3O4 th Mn3+ nm trong bt din, Mn2+ nm trong t din. Bt din v t din ch tip xc vi nhau qua nh, ngha l cc cation Mn3+ v Mn2+ nm cch xa nhau hn so vi khong cch Fe2+ v Fe3+ trong Fe3O4. iu ny lm cho s trao i in tch trong Mn3O4 kh khn hn nhiu.

Bng 10

Tnh cht ca mt s spinen

Cng thc Tinh th a () cng d (g/cm3) tonc Gin n

(-105) R(900oC)

MgAl2O4 Lp

phng

(8,09) 8 3,57 2135 0,593 75,3.104

ZnAl2O4 -nt- (8,09) 7,5-8 4,58 1930 0,596 18,6.104

CoAl2O4 -nt- (8,1) >7 4,37 1960 - 11,7.104

NiAl2O4 -nt- (8,04) - 4,45 2020 - 36,4.104

CuAl2O4 -nt- - 4,58 Khng tng hp -

MnAl2O4 -nt- - 4,12 - - -

FeCr2O4 -nt- >7 4,392 1780 0,90 -

32

MgCr2O4 -nt- (8,31) 6 4,429 2330 0,96 10,3.102

ZnCr2O4 -nt- (8,31) >7 5,393 - + 97,2.102

FeCr2O4 -nt- (8,34) - 4,998 2180 0,85 54,7.10

CoCr2O4 -nt- (8,31) - 5,580 - - 40,1.10

NiCr2O4 -nt- - 5,160 - - -

MnCr2O4 -nt- >7 4,870 - - -

CuFe2O4 -nt- - 5,700 - - -

MgFe2O4 -nt- (8,36) 6,65 4,560 1750 1,15 25,1

ZnFe2O4 -nt- (8,42) 6,5 5,330 - - 78,0

CdFe2O4 -nt- - 5,800 - - -

MnFe2O4 -nt- 6 4,900 - - -

FeFe2O4 -nt- (8,40) 6 5,210 1598 1,53 5,4(300o)

NiFe2O4 -nt- (8,35) 5 5,340 51,3

CoCo2O4 -nt- - 6,073 P.hu - -

MgCo2O4 -nt- - 4,960 - - -

MgV2O4 -nt- (8,42) - 4,240 - - -

CaV2O4 -nt- - 4,570 - - -

MnV2O4 -nt- (8,52) - - - - -

BeAl2O4 Hnh thoi - 8,5 3,720 1870

ph

0,573

BaAl2O4 Lp

phng

8 - 1820

Mg2TiO4 Lp

phng

>6 3,560 1,12

Zn2TiO4 Lp

phng

5,360

Mn2SnO4 Lp

phng

4,640 1850 - -

Vai tr ca s xen ph obitan d cng nh hng mnh n tnh dn in. V d spinen LiMn2O4 v LiV2O4 u c cu trc ging nhau l Li[Mn3+Mn4+]O4, Li[V3+V4+]O4.

Trong c hai spinen, v tr bt din u cha ng thi mt loi cation c mc oxi ho khc nhau l +3 v +4. Tuy nhin, s xen ph d trong vanai ln hn trong mangan, iu ny c phn nh tnh cht in, LiMn2O4 l cht bn dn theo c ch nhy, cn LiV2O4 l cht dn in kiu kim loi.

Vn nghin cu tng hp spinen v nghin cu tnh cht quang, c nhit, in, t ca chng l i tng ca nhiu nh ho hc v c. Theo dn in c th nh gi gin tip v cu to bn trong ca spinen cng nh ca dung dch rn spinen. V dn in th c th xp cc spinen thuc v loi hp cht bn dn v c th phn thnh 3 nhm:

Aluminat c dn in rt b (in tr ring 990oC R = 105106 m.cm), cromit c dn in trung bnh (R 990oC t 103104 m.cm), ferit c dn in cao (R 990oC t 10102 m.cm) ring FeFe2O4 c dn in gn bng dn in ca kim loi.

33

Trong aluminat, tnh dn in c quyt nh bi cation ho tr 2, cn cromit v ferit (tr hp cht FeO) li c quyt nh bi cation ho tr 3.

Mt trong cc c tnh quan trng ca spinen l d dng to thnh dung dch rn thay th vi nhau do thng s mng ca chng gn bng nhau.

V d, cc h spinen MgAl2O4-MgCr2O4, FeCr2O4-FeFe2O4 c gin trng thi thuc kiu tnh tan khng hn ch. Cromit cng d trn ln vi ferit. Mt s spinen c th to dung dch rn vi nhm oxit, c bit vi -Al2O3 c mng li ging vi mng li tinh th ca spinen. Ni chung, tnh cht ca spinen c quyt nh bi tnh cht v hm lng ca cc oxit hp phn. Khi tng hp spinen hoc khi hnh thnh dung dch kiu spinen u c s tng th tch ca pha tinh th. Mt nt c trng cn quan tm l phn ng thay th trong spinen, v d cc aluminat vi oxit c 3 kiu tng tc:

1) MgO + BeAl2O4 = MgAl2O4 + BeO

2) MgO + NiAl2O4 = (Ni,Mg)O + (Mg,Ni)Al2O4

3) Cc aluminat trn ln hon ton cn cc oxit th trn ln khng hon ton. V d trong cc h oxit km.

c tnh rt quan trng i vi k thut ca spinen l chu la cao, bn vi cc tc nhn oxi ho cng nh tc nhn kh (xem bng 10).

Trong cc spinen trn y th FeFe2O4 c ngha quan trng nht, trong st hai mc oxi ho l II v III. Da vo kt qu nghin cu dn in v t tnh c th kt lun c rng FeFe2O4 thuc kiu spinen nghch o, ngha l FeII nm trong hc O cn FeIII mt na nm trong hc O, mt na nm trong hc T.

Vi cc ferit c th biu din bng cng thc tng qut sau:

_ _2 3 2 3x 1 x 41 x 1 x

M Fe [M Fe ]O+ + + ++

Trong cation nm trong hc T c t ngoi du mc, cn nm trong hc O th trong du mc.

Khi x = 1 ta c spinen thun, x = 0 ta c spinen nghch o. Spinen trung gian th 0 < x < 1. i lng x gi l mc nghch o ca spinen cho bit lng M2+ nm trong hc T. S ph thuc gia mc nghch o v nhit c th biu din bng h thc:

_ E / kT_ 2

x(1 x) e(1 x)

+ = (9)

k l hng s Bonzman, T l nhit tuyt i, E l bin thin nng lng t do ca phn ng:

+3B

+2A

+3A

+2B Fe+M=Fe+M E

trong : A, B ch phn mng A v phn mng B trong AB2O4.

Nh vy, mc nghch o lin quan n iu kin ch ho nhit ca ferit. Kt qu nghin cu bng phng php nhiu x ntron cho thy cu trc ca spinen thun c ferit km v ferit caimi Zn[Fe2O4], Cd[Fe2O4]. Spinen nghch o c Fe3+[Fe2+Fe3+O4], Fe3+[CoFe3+O4], Fe3+[NiFe3+O4], Fe3+[Li0,5Fe1,5O4]. Spinen trung gian gn thun (x = 0,8) Mn0,8 Fe0,2[Mn0,2Fe1,8O4]. Spinen trung gian gn nghch (x = 0,1) Mn0,1Fe0,9[Mn0,9Fe1,1O4].

34

C ba yu t nh hng n s phn b cc cation A v B vo v tr t din, bt din.

a) Bn knh ion: Hc T c th tch b hn hc O do ch yu cc cation c kch thc b hn c phn b vo hc T. Thng thng rA2+ ln hn rB3+ ngha l xu th ch yu l to thnh spinen nghch o.

b) Cu hnh electron: Tu thuc vo cu hnh electron ca cation m chng thch hp vi mt kiu phi tr nht nh. V d Zn2+, Cd2+ (c cu hnh 3d10) ch yu chim cc hc T v to nn spinen thun, cn Fe2+ v Ni2+ (c cu hnh 3d6 v 3d8) li chim hc O v to thnh spinen nghch o.

c) Nng lng tnh in: Nng lng tnh in ca mng spinen (nng lng Madelung) to nn bi s gn nhau ca cc ion khi to thnh cu trc spinen. S phn b sao cho cc cation A2+ nm vo hc T, B3+ nm vo hc O l thun li v nng lng nht.

1.2.2.2 Perpkit Perpkit l tn gi ca khong vt c cng thc CaTiO3. Mng li ca perpkit thuc h

lp phng, cation titan nm tm lp phng, 6 ion oxi nm tm cc mt, 8 ion canxi nm cc nh (hnh 36). Mi t bo c cha mt phn t CaTiO3.

Hnh 36 T bo mng CaTiO3

Cng c th m t t bo perpkit theo mt cch khc. V d t bo mng ca SrTiO3 nh trn hnh 37a cho ta thy:

Ion titan nm cc nh, c to (0, 0, 0) c s phi tr bng 6; di lin kt Ti O bng a/2 = 3,905/2 = 1,956.

Ion sronti tm t bo, c to (1/2, 1/2, 1/2). S phi tr 12, khong cch Sr O bng

76,2=2

2a , ion oxi nm tm cc cnh c to (0, 0, 1/2). Mi ion oxi c bao quanh

bng 2 ion titan (vi khong cch 1,953) v 4 ion Sr (vi khong cch 2,76). Trong cu trc perpkit cc ion oxi khng to thnh mt phng xp kht nht, nhng cng vi ion Sr to thnh mt phng xp kht nht. thy r iu ny nn so snh vi mng li NaCl. y ion Cl nm gia cc cnh v tm ca t bo. Trn hnh 37b c v mt phng xp kht theo kiu lp phng. Mt phng cha ion Sr (nh s 1) v 6 ion oxi (nh s 2, 3, 4, 5, 6, 7). Mt phng ny song song vi mt phng 111 cha ion Ti A, B, C.

35

Trong mt xp kht ion Sr chim 1/4 nguyn t, cn 3/4 l ion oxi. Kiu cu trc perpkit bao gm mt s ln hp cht v c cng thc tng qut ABX3, trong X c th l ion oxi hoc halogen.

Trng hp chung A l cation c kch thc ln, B l cation c kch thc b. Cation A cng vi X to thnh kiu gi ghm chc c lp phng mt tm, cn ion vi kch thc b B nm tm khi lp phng . Tng in tch dng ca A v B phi bng tng in tch m ca X. Do cc t hp c th c ca hp cht ABO3 l +1 v +5, +2 v +4, +3 v +3 vi hp cht ABX3 (X l halogen) ch c mt trng hp duy nht l +1 v +2.

Bng 11 gii thiu mt s hp cht kt tinh theo kiu perpkit. Hp cht c cu trc perpkit ng vai tr quan trng trong vt liu p in, vt liu bn dn. Hu ht hp cht perpkit c cng thc ABO3 u c tnh cht xecnhet in. Theo Holsmit th gia tnh cht xecnhet in v cu trc perpkit ABO3 c mi lin h c nh gi bng yu t tlrng (t).

A O

B O

r rt2(r r )

+= + (10)

Sr

Ti

x

y

z

(a)

2

3

4

5

6

7

A

B

C

1

2 3

4

56

7

(b) Ti4+, Sr2+, O2-

Hnh 37 a) T bo mng ca SrTiO3; b) mt xp kht

Bng 11 Cc hp cht kt tinh theo kiu perpkit

in tch cation Hp cht perpkit

+1 v +5 NaNbO3, KNbO3, AgNbO3, AgTaO3,

+2 v +4 SrTiO3, SrZrO3, SrHfO3, SrSnO3, BaTiO3, BaZrO3, BaSnO3, BaCeO3, BaPrO3, BaThO3

+3 v +3 LaAlO3, TiTiO3, LaCrO3, LaMnO3, LaFeO3

+1 v +2 KMgF3, KNiF3, KZnF3

Yu t tlrng t ny nm trong khong 0,95 n 1 th ta c cu trc lp phng v c tnh xecnhet in, t nh hn 0,95 th thuc loi lp phng bin dng v khng c tnh xecnhet in.

36

y, cn lu mt iu l trong tnh ton chng ta xut pht t quan nim cho rng lin kt trong perpkit thun tu l lin kt ion. Nh trn chng ta u bit iu ny khng thc s ng, do khng th p dng quy tc ca Golsmit mt cch my mc c.

Trong mng li tinh th perpkit thng c l trng cation A, cn cation B th hu nh cha gp l trng. C th xem cu trc ca WO3 thuc dng perpkit m khng c cation A. Hp cht NaxWO4 (x = 0,3 1) c mng li n gin cu trc perpkit bin dng trong mi gi tr ca x.

Ngi ta cng gp perpkit c l trng anion. V d titanatsronti c cng thc SrTiO2,5 ng hnh vi cu trc lp phng ca SrTiO3, trong c l trng O2.

Mt trong cc hp cht quan trng ca nhm perpkit l BaTiO3. y l cht p in u tin thu c di dng gm v c s dng rng ri trong k thut hin i.

Bari titanat c 2 dng th hnh chnh. Trn 1460oC barititanat tn ti di dng lc phng, khng c tnh p in; di 1460oC chuyn thnh dng lp phng v c tnh p in. Nhng khi lm ngui xung di 1460oC dng lc phng c th cha chuyn thnh lp phng v tn ti dng khng bn n nhit phng. Mt tnh cht l th ca dng lp phng l bin ho mng li khi thay i nhit .

Khi lm lnh n 120oC, t bo perpkit b ko di theo mt cnh lm cho mng li t lp phng bin thnh t phng, c/a ln hn 1. S chuyn pha lc ny km theo bin i bt thng v in mi chng t c pht sinh s phn cc ca t bo. Lm lnh tip tc n 0oC li xy ra bin ho th hnh th hai. S thay i cu trc lc ny c th hnh dung nh ko di khi lp phng dc theo ng cho ca mt mt bn. nhit thp hn na (-90oC) c s bin ho th hnh th 3, ln ny t bo b ko di theo ng cho ca khi lp phng to thnh khi mt thoi (xem hnh 38).

tt c ba nhit hng ca momen lng cc xecnhet in u song song vi hng ko di t bo mng. l do s chuyn dch v tr ca ion titan trong bt din oxi. Ban u khi h nhit s hng v pha mt ion oxi, sau hng v pha gia hai ion oxi v cui cng hng v gia ba ion oxi. Cch gii thch ny c Kay H. F. v Vonsden a ra t nm 1949.

Cng nh spinen, cc hp cht c cu trc perpkit c th to thnh dung dch rn thay th vi nhau trong mt gii hn rt ln. V d PbTiO3, SrTiO3, BaZrO3, BaSnO3, BaHfO3, KNbO3, c th to thnh dy dung dch rn khng hn ch vi BaTiO3.

37

oAa' = 5,668

b' = 5,683c' = 3,876

oAoA

-90 oC:

Lp phng

T phng

n t

Mt thoiHnh thoi

a3 a2a1

a1 = a2 = a3 = 4,009 Ao

a1

a1 = a2 = 4,003 Ao

130 oC: c = 4,022 oA

0 oC:

-90 oC:

a1 = a2 = 3,992 Ao

c = 4,035 oA

0 oC: a1 = a2 = 4,012 Ao

c = 3,989 oA

a1 = a2 = a3 = 3,998 Ao

= 89 51'o

= 89 52'o

0 oC

120 oC

-90 oC

0 oC: oAa' = 5,667

b' = 5,681c' = 3,898

oAoA

a1

a3 a2

c a2

a2a1

c

Hnh 38 Bin ho ca t bo mng BaTiO3

S thay th Ba2+, Ti4+ trong barititanat bng cc cation ho tr 2 v bng Zr4+ c th ci thin nhiu tnh cht vt l ca gm p in bari titanat. Gm PZT l mt loi dung dch rn nh vy. y Pb2+ thay th Ba2+ cn Zr4+ thay th Ti4+, loi gm PZT hin ti vn cn gi vai tr trong k thut.

1.2.2.3 Pyrclo A2B2O7. Pyrclo l tn gi ca nhm khong vt cha t him c cng thc khng n nh

(Na,Ca,)2(Nb,Ti)2O6.F.OH. Ch pyros t gc Hylp c ngha l la, cloros c ngha l mu lc. Loi khong vt ny khi th bng phn ng ng thi th c mu lc. Thnh phn khong vt ny rt khng n nh (bng 12).

Bng 12 Thnh phn ca mt s mu khong vt pyrclo

Cht Hm lng (%)

Cht Hm lng (%)

Cht Hm lng (%)

Nb2O5 063 Na2O 16 (Ce,La)2O3 215,3 Ta2O5 077 K2O 01,4 (Y,Er)2O3 05,1 TiO2 213 CaO 418,1 SnO2 04,1 ThO2 05 MnO 07,7 ZrO2 05,7 UO2 011,4 FeO 010 WO3 00,3 UO3 015,5 Fe2O3 09,8 H2O 06

Ngoi ra cn cha mt t Sb2O5, MgO, PbO, HfO2, SiO2, Al2O3, SrO, BeO, CuO, GeO2.

38

Hnh 39 Cu trc pyrclo Er2Ti2O7

Khong vt pyrclo thuc h lp phng. Mng tinh th tng t florit CaF2. Khi chuyn t florit thnh pyrclo th v tr ca F b chim bi O2. Nhng mi t bo mng c mt ion O2 b khuyt, 7 ion O2 cn li b chuyn dch lm cho v tr l tng ban u bin i. Trong cng thc tng qut ca pyrclo A2B2O7 th A l cation c kch thc ln hn c bao quanh bng 8 ion O2 to thnh mt khi lp phng bin dng. Cation nh B4+ nm trong bt din dng BO68. Cc bt din BO6 ni vi nhau qua nh to thnh mt dy dy xch dc gn song song vi nhau. Cc dy xch ny li ni vi nhau qua bt din BO6 lp nn dy dy xch ngang trc giao vi dy dy xch dc (hnh 39).

V tr trung gian gia cc bt din BO6 c xp bng cc ion A. Nhm pyrclo c gi tr nht trong vt liu mi l titanat t him c cng thc tng qut l Ln2Ti2O7. Theo tnh ton ca nhiu tc gi th ch c nhng nguyn t t him no c bn knh tho mn h thc:

3

4

Ln

Ti

r1,22 1,5

r+

+< <

th hp cht titanat mi c cu trc pyrclo.

Bng 13 cho bit bn knh ca cc ion t him v t s 3

4

Ln

Ti

rr

+

+ (rTi4+=0,64).

Bng 13

Bn knh ca mt s cation t him

Ion La3+ Ce3+ Pr3+ Nd3+ Pm3+ Sm3+ Eu3+

r () 1,061 1,034 1,013 0,995 0,979 0,964 0,050

rLn3+/rTi4+ 1,660 1,610 1,580 1,550 1,530 1,500 1,480

Ion Gd3+ Tb3+ Dy3+ Ho3+ Er3+ Tm3+ Yb3+ Lu3+

r () 0,938 0,923 0,908 0,894 0,889 0,899 0,858 0,84

rLn3+/rTi4+ 1,460 1,440 1,420 1,400 1,390 1,390 1,34 1,32

39

Nh vy l t La n Pm th cc titanat Ln2Ti2O7 kt tinh theo h n t, cn tt c cc t him cn li (t Sm n Lu) th titanat Ln2Ti2O7 kt tinh theo h lp phng kiu pyrclo. Trong dy ny c s ph thuc tuyn tnh gia thng s mng a v bn knh ion Ln3+.

S chuyn t cu trc florit n pyrclo lm xut hin cc vch siu cu trc trn gin tia X, c bit trn gin ph hng ngoi chng ta thy mt cch kh r rt s chuyn pha pyrclo khi xt h titanat v zircnat t him (RZr4+) = 0,82 (xem hnh 40).

Hnh 40. Ph hng ngoi ca titanat v zirconat t him

i vi h titanat, cc pic c trng cho cu trc pyrclo ch c t Sm n Lu. Cn vi h zirconat th pic c trng cho pha pyrclo li ch thy i vi La, Pr, Nd, Sm, Eu, Gd, vi cation t him c bn knh b hn Gd th khng to thnh pha c trt t.

c tnh ca pyrclo l tnh bt hp thc. Tnh bt hp thc y l do hai nguyn nhn.

a) C s thay th mt phn Ti4+ bng cc ion t him Ln3+, lc ny s xut hin l trng b sung trong phn mng anion.

b) C s thay th mt phn Ln3+ bng Ti4+ .

Do cng thc tng qut ca pyrclo c th vit

2 x 2 x 7 x / 2 1 x / 2Ln Ti O , Nu thay th 1 ion Ln3+ vo tr Ti4+ th thiu mt in tch dng, m bo cn bng

v in phi loi ra mt in tch m, hay ni cch khc loi ra 1/2 v tr ion oxi, do lc u ch c mt l trng ca ion oxi by gi thm 1/2 l trng oxi v cng thc tng qut s l:

Ln2+x Ti2-x O7-x/21+x/2 Trong dy titanat t him c nhiu cng trnh nghin cu kh k cu trc ca

Er2Ti2O7 v Y2Ti2O7 bng cc phng php nhiu x tia X v gin ntron. T bo mng li ca Er2Ti2O7 c cha 16 Er3+, mi ion Er3+ c bao quanh bng 8 ion oxi trong 2 ion nm v tr i din nhau v cch nhau mt khong 2,185,cn 6 ion oxi cn li nm trn cng mt mt phng cch nhau 2,471 (hnh 39).

40

1.2.2.4 Pha siu dn 1:2:3 Gm siu dn nhit cao (gn 100 K) l loi vt liu mi c ra i vo cui nm

1986, hin nay nhp nghin cu quanh lnh vc ny ang c pht trin vi quy m cha tng thy. Trong vng 4, 5 nm thp k 80 ca th k XX, mi nm c ti hng nghn cng trnh nghin cu tng hp, nghin cu cu trc v cc tnh cht ca loi vt liu ny. Ti liu s 9 trnh by tng quan v lnh vc ny cho n nm 1988.

Cho n nay bit c nhiu loi gm c tnh siu dn nhit cao, trong loi quan trng nht c cng thc ng vi thnh phn RBa2Cu3O7-x. y R l Y hoc Ln. Ln lt theo th t s nguyn t ytri, bari, ng trong cng thc ny, ngi ta gi tt l pha gm 1:2:3. Gm RBa2Cu3O7-x c cu trc tinh th thuc h hnh thoi. C th xem cu trc ca pha ny nh l dn xut ca peropkit c nhn ba gi tr trc C theo trt t Ba-Y-Ba. T bo mng ca pha 1:2:3 gm cc lp phng peropkit c nh l nguyn t Cu, hai lp phng c tm l bari kp gia mt lp phng c tm l ytri.

c tnh quan trng cu cu trc pha 1:2:3 l v tr ca ng khng ging nhau do s b tr cc l khuyt oxi. Nguyn t Cu v tr 1 l tm ca mt hnh vung trong mt phng bc. Nh vy s phi tr ca nguyn t Cu ny l 4. Nguyn t Cu v tr th 2 nm trong khi chp c y l hnh vung. Nh vy s phi tr ca nguyn t Cu ny l 5 (xem hnh 41).

Cc hnh chp CuO5 v mt CuO4 to thnh tng lp song song vi mt ab. Hai hnh chp CuO5 kp gia mt lp CuO4. Hai hnh chp quay nh v pha hnh vung CuO4 v ni vi nhau qua cu oxi.

Tu theo iu kin tng hp (c bit l p sut ring phn ca oxi trong h phn ng) s l trng oxi thay i. Do h s ca oxi trong cng thc khng xc nh (bng 7-x) v trng thi oxi ho ca ng cng thay i lm nh hng n tnh cht in ca gm.

Cuprat lantan La2CuO4 c cu trc tinh th ca K2NiF4 l cht bn dn nhng khi ch ho nhit p sut ring phn PO2 cao, hoc to ra cc khuyt tt cation trong cc phn mng th c th bin thnh trng thi siu dn 40K. C th gii thch iu ny nh sau: Hp cht cuprat khi mi iu ch c cc khuyt tt stki (xem chng sau) va c l trng cation va c l trng anion do sn phm thu c c cng thc La2-2xCu1-xO4-4x. p sut ring phn cao ca oxi, th oxi c th lp y cc l trng anion, do mt s cation ng (II) s chuyn thnh cc mc oxi ho cao La2-2xCu1-xO4.

Hoc c th iu ch pha siu dn cuprat lantan bng phn ng tng hp vi t l d CuO di p sut bnh thng ca oxi. Lc ny giai on ban u to thnh cc l trng anion v hnh thnh hp cht trung gian La1,9Cu2+O3,85. Hp cht ny c kh nng chim ly oxi to nn La1,9Cu2+3+O4.

Phng php th ba to thnh cuprat siu dn l hp kim ho La2CuO4 bng cc oxit kim th. V d hp kim ho bng sronti th thu c pha tinh th La2-xSrxCuO4 vi gi tr x t 0,15 n 0,2. C th thay th Cu bng kim loi chuyn tip 3d khc.

41

(a) (b)

(a) (b) Hnh 41 Cu trc ca pha siu dn 1:2:3 (YBa2Cu3O7x ) (a) v (a) thuc h hnh thoi; (b) v (b) thuc h t phng

1.2.2.5 Silicat Silicat l loi hp cht ph bin nht trong v Qu t cng nh trong vt liu v c.

Cu trc ca silicat c to thnh t s gi ghm chc c cc ion oxi. Trong cc hc trng ca b khung anion c sp xp cc cation kim loi v c bit ion Si4+ chim t l ln

42

nht. Tu thuc vo kch thc ca cation m chng c xp vo cc hc trng t din, bt din, lp phng, lc phng. Cc silicat t nhin thng thng c cc cation Si4+, Al3+, Fe3+, Cr3+, Mg2+, Fe2+, Ca2+, Na+, K+ Khi t l rc/ra nm gia 0,41 v 0,732 th cation c SPT 6, khi t l nm gia 0,732 v 1 th cation c SPT 8. Bng 14 gii thiu SPT ca mt s cation ch yu trong silicat.

Bng 14

Bn knh ion v s phi tr ca vi ion quan trng trong silicat

Nguyn t

Ion Bn knh ion ()

(h thng Paolinh) rc/ra SPT n v cu trc

Oxi O2+ 1,36 - - -

Silic Si4+ 0,39 0,29 4 t din

Nhm Al3+ 0,57 0,42 4

6

t din

bt din

St

Fe3+

Fe2+

0,59

0,83

0,59

0,63

6

6

bt din

bt din

Magi Mg2+ 0,78 0,57 6 bt din

Natri Na+ 0,98 0,725 6

8

bt din

lp phng

Canxi Ca2+ 1,06 0,8 8 lp phng

Kali K+ 1,33 1,0 12 lc phng

Trong silicat c nhiu ion c th nm trong cc n v cu trc khc nhau. V d Al va c th nm trong t din va c th nm trong bt din. Na va c th nm trong bt din va c th nm trong khi lp phng

C th biu din mng li silicat bng cch sp xp cc khi t din SiO4 v bt din M(OH)6 (M c th l Al3+, Mg2+, Fe3+ ) theo mng li 3 chiu trong khng gian. S hnh thnh cc khong vt thng xy ra s thay th cation trong mng li. Bi vy, cng thc khong vt thng c vit di dng tng qut. V d khong vt almanin thuc nhm grnat c cng thc l thuyt l 2 3

3 2 4 3Fe Al [SiO ]+ + . Nhng v tr nt mng ca Al3+ c th thay

th bng Fe3+, v tr ca Fe2+ thng b thay th bng Mg2+, Mn2+, Ca2+. Kh nng thay th ph thuc vo s c mt ca cation M2+, M3+ trong qu trnh hnh thnh mng li almanin, nhit , pH Do cng thc khong vt almanin thng c vit di dng tng qut: (Fe2+,Mg,Mn,Ca)3(Al,Fe)2[SiO4]3, trong cng thc ny cn hiu mt phn t c cha tng s 3 cation M2+ v hai cation M3+. Nu kt qu phn tch pha (phng php nhiu x tia X, phng php DTA, phng php hin vi phn cc) cho bit mu ca ta ch c mt khong vt almanin duy nht, th t kt qu phn tch ho hc ca mu ta c th vit c cng thc chnh xc ca phn t almanin. Sau y l phng php xc nh cng thc cu to. V d t kt qu thnh phn ho hc bng 15 ta c:

C 2,472 nguyn t oxi th c 0,615 nguyn t silic, trong phn t almanin c 12 nguyn t oxi, vy mt phn t almanin c 2,985 hoc quy trn l 3 nguyn t silic. Cng tnh nh vy ta c s nguyn t kim loi trong mt phn t almanin ghi ct 6 ca bng 15 v cng thc cu to ca mu nghin cu l (Fe2+2,315Mg0,366Mn0,169Ca0,155)(Al1,961Fe3+0,038)[SiO4]3.

43

T phn trm khi lng ca cc nguyn t trong mu nghin cu v cc thng s t bo mng, t trng (g/cm3), ta c th tnh ton s nguyn t ca mi t bo mng. V d mu ca chng ta c t trng l d, th tch ca mt t bo l V(3) hoc V.1024(cm3). Khi lng ca mt t bo l V.d.1024g. Thnh phn phn trm khi lng ca nguyn t kho st l Q, tng s phn trm ca cc nguyn t l T, ta tnh c khi lng ca nguyn t trong mt t bo l Q.V.d.1024/T.

Bng 15 Thnh phn ho hc ca mu almanin

Oxit (1)

%khi lng (2)

S pt. oxit (3)

S cation (4)

S ion O2 (5)

(6)

SiO2 36,96 0,615 0,615 1,230 2,985 3 Al2O3 20,65 0,205 0,406 0,609

Fe2O3 0,70 0,004 0,008 0,012

1961

1,999 20.038

FeO 34,30 0,477 0,477 0,477

MnO 2,45 0,035 0,035 0,035

MgO 3,17 0,077 0,077 0,077

CaO 1,81 0,032 0,032 0,032

2,315

0.1693,005 3

0,366

0,155

Tng 100,04 2,472

Nguyn t lng ca nguyn t l A. Nu gi s nguyn t c trong T gam mu l R (gi tr ghi trong ct 4) th s nguyn t trong mt t bo l:

Z = _

_

24

24

Q.V.d.10 R.V.D1,6602.TA.1.6602.10

= (16) v Q/A = R

V d almanin c t bo mng thuc h lp phng, cnh a = 11,54. T trng d = 4,22 g/cm3. Trn bng 15 c tng l 100,04. Trong 100,04 gam mu c cha 2,472 nguyn t oxi (R = 2,472). Do s nguyn t oxi c trong mt t bo l:

51,96=04,100.6602,1

472,2.22,4.)54,11( 3

Kt qu thu c l s l, iu ny l do sai s khi phn tch thnh phn ho hc, khi xc nh gi tr a, d. Nhng kt qu kho st nhm khng gian ca h tinh th grnat cho thy s nguyn t oxi, kim loi M2+, M3+, silic trong mt t bo nguyn t ch c th l 16, 24, 32, 48, 96. Vy y ta ly gi tr l 96.

Silicat c phn thnh cc lp sau y:

1) Octosilicat: (cn gi l nezosilicat), gm cc t din ring l _4

4SiO . Bn ho tr d c bo ho bng mt cation ho tr 4+ (v d zircon ZrSiO4), hoc bng hai cation ho tr 2+ (v d fosterit Mg2SiO4, faialit Fe2SiO4). Cc cation Fe2+ v Mg2+ c kch thc gn bng nhau do c th thay th ln nhau trong fosterit v trong faialit to thnh mt dy dung dch rn lin tc c tn gi l olivin (hoc cn gi l periot) vi cng thc (Mg,Fe)2SiO4.

44

1

23

4

-

-

-

-

-

1

2 3

4

--

-

Hnh 42 T din 4-4SiO trong mng octosilicat (y Si4+, {O2-)

Hnh 43 Sorosilicat

Octosilicat thng kt tinh trong h tinh th i xng cao, c rn cao, ch s khc x cao. i khi c cha ion O2 khng lin kt trc tip vi silic. Lp silicat ny c cc nhm sau:

Nhm olivin (Mg,Fe)2 [SiO4]

Nhm grnat (Ca,Fe,Mn,Mg)3(Al,Fe,Cr)2 [SiO4]3

Nhm silicat nhm Al2[OSiO4], topaz Al2[F2SiO4], v sphen CaTi[OSiO4]

Silimanit c ba dng th hnh u c cng thc chung Al2[OSiO4] gi l silimanit, andaluzit, disten.

2) Sorosilicat: n v cu trc gm hai t din c mt oxi chung. Nh vy trong n v cu trc c 6 ho tr cha bo ho v cng thc ca b khung anion l [Si2O7]6. V d khong vt llilit Ca2Mg[Si2O7]. Anion OH c th kt hp vi nhm anion ny. V d khi kt hp thm 2 anion OH th c ion phc [Si2O7(OH)2]8 nh khong vt laosonit CaAl2[Si2O7 (OH)2].2H2O. Mng li ca lp ny c khi c mt t din c lp.

V d epiot Ca2(Al,Fe)3[O.OH.SiO4.Si2O7 ]

v ircaz Ca10(Mg,Fe)2[(OH)4(SiO4)5(Si2O7)] Bng 16.

Mt vi khong vt thuc lp octosilicat

Khong vt Cng thc H tinh th d H Ch s khc x

Olivin (Mg,Fe)2[SiO4] hnh thoi 3,3 6,57,0 1,671,87 1,631,83

Zircon ZrSiO4 t phng 4,7 7,5 1,921,96 1,962,02

Phenacit Be2[SiO4] mt thoi 34 7,58,0 1,641,67 Willemit Zn2[SiO4] mt thoi 3,94,2 5,5 1,691,72 Pyrop Mg3Al2[SiO4]3 lp phng 3,51 7,07,5 1,70

Almanin Fe3Al2[SiO4]3 lp phng 4,25 7,0 1,83

Spossartit Mn3Al2[SiO4]3 lp phng 4,18 7,07,5 1,80

Grossulair Ca3Al2[SiO4]3 lp phng 3,53 6,5 1,73

Andradit Ca3Fe2[SiO4]3 lp phng 3,75 7,0 1,89

Uvarovit Ca3Cr2[SiO4]3 lp phng 3,77 7,0 1,86

45

Silimanit+ Al2[OSiO4] hnh thoi 3,30 67 1,671,65 clas AlBe[OHSiO4] n t 3,10 7,5 1,671,65

Topaz Al2(F,OH)2SiO4 hnh thoi 3,43,6 8,0 1,621,61

Sphen CaTi[(OSiO4)] n t 3,50 5,5 2,091,90

Datolit CaB[(OH)SiO4] n t 2,83 55,5 1,671,63

3) Xiclosilicat (silicat mch vng) Bng 17

Mt vi khong vt thuc lp silicat vng

Tn Cng thc H tinh th d H Ch s

Benitit BaTi[Si3O9] Lc phng 3,7 6,5 Khc x

Catapleit Na2Zn[Si3O9] Lc phng 2,8 6,0

Endialit (Na,Ca,Fe)6Zr[OH.Si3O9] Mt thoi 3,0 5,5

Axinit Ca2(Fe,Mn)Al2[OH.BO3 Si3O12] Tam t 3,3 6,5-7 1,7-1,68

Beryl Al2Be3[Si6O18] Lc phng 2,7 7,5-8 1,6-1,58

Cordierit Mg2Al3[AlSi5O18] Hnh thoi 2,6 7-7,5 1,54-1,63

(Fe v Mn c th thay th Mg ti 10%

Tucmalin R+,R2+3Al6[(OH) BO3) Si6O18] Mt thoi 3,5 7,5

Da vo thnh phn ha hc ngi ta phn bit

Tucmalin kim (Enbaite) Na(Al,Li)3Al6[(OH)4BO3)3 Si6O18]

Tucmalin magi (Dravit) Na Mg3Al6[(OH)4BO3)3 Si6O18]

Tucmalin st (Schor) NaFe3Al6[(OH)4BO3)3 Si6O18]

Tucmalin canxi magi (uvit) CaMg3Al5 [(OH)4BO3)3 Si6O18]

1,65-1,7

Dipta Cu6[Si6O18].6H2O Mt thoi 3,3 5 1,65-1,7

n v cu trc gm 3, 4, 6 t din ni nhau qua nh oxi to thnh vng (hnh 44). Nhng vng c th c lp hoc chng ln nhau to thnh khng gian trng cha cc cation c kch thc ln hn (Ca, K...) chim. Bng 17 gii thiu vi khong vt thuc lp ny.

4) Inosilicat (Silicat mch) c hai nhm: + Nhm pyrxen: Cc t din ni nhau bng 2 oxi. Cng thc khung anion c th vit

[SiO3]2 hoc [Si2O6]4 (hnh 45, bng 18).

-- -

-

-

-

-

-

-

-

-

-

- -

-

-

-

-

-

--

-

---

-

46

[Si3O9]6- [Si4O12]8- [Si6O18]12- Hnh 44 Xiclosilicat

-- -

--- -

- -

[Si2O6]4-

Hnh 45 Nhm pyrxen

Bng 18

Mt vi khong vt nhm pyrxen

Tn Cng thc Tinh th d H Ch s khc x

ng np

ipzit CaMg[Si2O6] n t 3 5 1,694-1,664

Jadeit NaAl[Si2O6] n t 3,4 7 1,667-1,665

git Ca(MgFeAl)[Si2O6] n t 3,5 5,5 1,711-1,686

Spodulen LiAl[Si2O6] n t 3,2 6,5 1,676-1,661

Enstatit (MgFe)2[Si2O6] hnh thoi 3,5 5,5 1,77-1,73

Mt phn MgO c th thay th bng FeO. Trong enstatit cha di 5% FeO. Nu FeO c gi tr gia 5 n 14% th gi l bronzit, nu FeO c trn 14% th gi l hypecten.

+ Nhm amphibn: Cng thc ca b khung anion l [Si4O11]6. Tm ca mi lc gic c th c nhm OH lc cng thc ca b khung anion l [(OH)Si4O11]7.

-

-

-

- -

- ---

-- - - -

[Si4O11]6-

Hnh 46 Nhm amphibn

47

Bng 19. Mt vi khong vt nhm amphibn

Tn Cng thc Tinh th d H Ch s khc x ng np

Trmolit Ca2Mg5[(OH)Si4O11]2 n t 2,3 5-6 1,633 -1,60 Actinot Ca2(MgFe)5[(OH)Si4O11]2 n t 1,634 1,61

Glaocophan Na2Mg3Al2[(OH.Si4O11]2 n t 3,0 6-6,5 1,63 1,61 Wollastonit Ca3[Si3O9] tam t 2,9 5 1,634 1,62

Rodonit CaMn4[Si5O15] tam t 3,5 6 1,76 1,72

5) Phylosilicat (silicat lp): Cc mch pyroxen hoc amphibn kt hp li vi nhau to thnh mt phng ln v hn. Nu xt mt n v nh nht c trng cho c lp th trong cu trc ny mi t din ch cn mt oxi cha bo ho ho tr. Do cng thc ca b khung anion c th vit di dng [Si4O10]4. Nhng lp phng ny chng ln nhau v cng quay nh t din

_44SiO vo nhau. Gia hai lp c cha ion OH

v cc cation kim loi. Do cng thc cu trc ca b khung anion c c nhm OH v d [Si4O10(OH2)]6.

Silic trong t din c th b thay th bng nhm n 50% nguyn t. Su nh ca 6 t din

_44SiO to thnh cc mt lc gic, ion OH

nm tm ca lc gic . Pha y cc t din cng c nhng mt lc gic. Khi hai mt y chng ln nhau s to nn cc khong trng dng khi lc lng. Cc cation kim loi c kch thc ln (s phi tr 12) nm trong khi lc lng ny.

Tal (Mg3[(OH)2Si4O10], mica KAl2[(OH)2AlSi3O10], bitit K(MgFe)3[(OH)2AlSi3O10] v cc khong vt st thuc lp phylosilicat.

-

-

-

-

-

-

--

-

- -

-

-

-

-

-

-

--

[ ] 44 10Si O

Hnh 47 Nhm phylosilicat

Bng 20 Mt vi khong vt thuc lp phylosilicat

Tn Cng thc Tinh th d H Ch s khc x

Tal Mg3[(OH)2Si4O10] n t 2,7 1 1,58-1,54 Pyrofilit Al2[(OH)2 Si4O10] n t Apofilit KCa4[F.Si8O20] t phng 2,3 4,5 1,53

Muscovit KAl2[(OH.F)2 AlSi3O10] n t 1,565-1,535

48

Phlogopit KMg3[(OH.F)2 AlSi3O10] n t 2,9 3 1,60-1,56 Biotit K(MgFe)3[(OH.F)2AlSi3O10] n t 3 3 1,66-1,60

Lepidolit KLi2Al[(OH.F)2 Si4O10] n t 3 3 1,56-1,54

Trong phylosilicat c mt nhm c bit l khong vt st. V khong vt st cn c nhiu ng dng trong ha vt liu nn di y chng ta xt k hn.

Khong vt st

Khong vt st c cu trc lp, mi lp gm mt hoc hai l t din SiO24 v mt l bt din Al(OH)63, hai l t din cng quay nh v l bt din. Ti cc nt chung ca t v bt din ion OH ca bt din c thay th bng O2 ca t din. Ty theo cch xp sp ca l t din (Te) v l bt din (Oc) m phn thnh cc nhm st sau y:

a) Nhm caolinit (Te-Oc). Caolinit l khong vt i din cho nhm ny. Mi lp caolinit gm c mt l t din (Te) v mt l bt din (Oc) (xem hnh 48). V rng bn knh O2 v OH u ln hn rt nhiu bn knh Si4+, Al3+, nn c th hnh dung mng tinh th caolinit gm cc anion O2- v OH- gi ghm chc c, cn cc cation Si4+ v Al3+ c sp xp vo cc hc trng ca phn mng anion .

Hnh 48 Cu trc tinh th caolinit

xc nh t bo mng ca caolinit, chng ta xem mt lc gic gm cc ion oxi l y. T dng cc mt phng thng ng (theo trc c) ct ln lt qua mt oxi, mt oxi v hiroxil v mt hiroxil (hnh 49).

Cnh a ca t bo khong 5 n 5,44, cnh b khong 9 n 9,422. Kch thc mt y ny gn nh khng thay i vi tt c cc loi khong st. Cn chiu cao (cnh c) i vi mng caolinit c gi tr khong 7,1 n 7,15.

Trn hnh v cho thy vi mt t bo caolinit c: - Mt lc gic oxi gm: 4 + 4/2 = 6O2

2 + 4/2 = 4Si4+ - Mt hn hp gm: 2 + 4/2 = 4O2

1 + 4/4 = 2OH 3 + 2/2 = 4Al3+

- Mt hiroxil: 5 + 2/2 = 6OH

Nh vy cng thc ca mt t bo caolinit l Al4[Si4O10](OH)8. Vit di dng oxit l 2(Al2O3.2SiO2.2H2O). Ta c th tnh c khi lng ring ca caolinit nh sau. Mi t bo c 2 phn t Al2O3.2SiO2.2H2O, ngha l khi lng mt t bo bng (2102) + (460) + (418) = 516 516/6,02.1023g.

49

Th tch mt t bo l 597,1 = 319,53 = 319,5.1024cm3 Vy d = 516/(6,02.1023 319,5.1024) = 2,68g/cm3 Nt c trng ca khong vt caolinit l lin kt hir gia mt gm cc ion O2 v mt

gm cc ion OH lm cho cc lp gn vi nhau. Cp ht ca caolinit thng ln hn cc khong vt st khc.

O

O

O

O O

OOO

O

O

O

O

1/2

1/2

1/2

1/2

(a)

9Ao

5Ao

OH

OH

OH

OH

OH1/4 1/4

1/41/4

1/21/2

1/21/2 O

O

O

O

OO

O

O

(b)

1/2

(c)

OH OHOH

OH

OH

OHOH

OH

OH

OHOH

OH

OH

OHOH

1/2

Hnh 49 Cc mt ct ca t bo caolinit a) Qua cc mt lc gic oxi; b) Qua cc mt hn hp oxi v hiroxil c) Qua cc mt hiroxil

b) Nhm monmorillonit (Te-Oc-Te)

S cu trc ca nhm ny gm mt l bt din nm gia hai l t din. Theo cch xc nh trn y th mt t bo mng monmorillonit c:

- Mt lc gic oxi gm: 4 + 4/2 = 6O2 2 + 4/2 = 4Si4+

Te

- Mt hn hp gm : 2 + 4/2 = 4O2 1 + 4/4 = 2OH 3 + 2/2 = 4Al3 2 + 4/2 = 4O2 1 + 4/4 =2OH

Oc

- Mt lc gic oxi gm: 2 + 4/2 = 4Si4+ 4 + 4/2 = 6O2

Te

Tng cng 2(Si4Al2O10(OH)2)

Ngha l mi t bo ng vi 2 phn t Al2O3.4SiO2..H2O. Nh vy t l SiO2/Al2O3 trong monmorillonit ln gp 2 ln t l trong caolinit. Gia cc lp monmorillonit khng c lin kt hiro, do cp ht thng b hn nhiu so vi caolinit.

50

O2-

Si4+

OH-

Al3+, Mg2+

Fe3+, Fe2+ Hnh 50 Mng li tinh th monmorillonit

51

O2-

Si4+

OH-

Al3+,Fe3+..

Mg2+

Hnh 51 Mng li tinh th clorit

Nt c trng monmorillonit l thng xuyn c s thay th Al3+ trong v tr bt din bng cc ion Mg2+, Fe2+, Fe3+, Cr3+ v thay th v tr Si4+ trong t din bng Al3+. in tch dng thiu do s thay th khng cng in tch c b tr bng cc cation trao i. Dung tch trao i cation ca nhm khong st ny c th t t 90 150 mlg/100 gam mu. Cc cation trao i ny thng xuyn c km theo cc phn t nc hirat.

c) Nhm hiromica (Te-Oc-Te)

Cu trc tinh th cng ging nh ca nhm monmorillonit, nhng hiromica cation b tr ch l K+. V rng kch thc ca K+ ln hn nhiu ca Na+, Mg2+ cho nn v tr K+ c gi kh chc khng tham gia phn ng trao i.

d) Nhm clorit (Te-Oc-Te-Oc)

Gia cc lp ca monmorillonit cn c thm mt lp bt din bruxit [Mg(OH)6]4- (hnh 51).

7) Tectosilicat (silicat vi cu trc 3 chiu)

C th hnh dung mng tinh th tectosilicat gm cc t din SiO44 ni vi nhau qua nh oxi nh trong tinh th SiO2. Nhng y v tr ca Si4+ b thay th bng Al3+ c khi ti 50%. S in tch dng thiu do s thay th c b tr bng cc ion dng c s phi tr ln nh Ca2+, K+, Na+. Do lp silicat ny gi l alumosilicat. C 3 nhm tectosilicat l phenpat, phenpatoit, zeolit.

a) Nhm phenpat: Phenpat l khong vt ph bin nht ca v qu t (chim 50% khi lng). Cng thc ca phenpat thay i t X[AlSi3O8] n Y[Al2Si2O8] tu theo cation X v Y ngi ta phn thnh 2 nhm:

- Phenpat kim (octoz v microlin) u c cng thc chung K[AlSi3O8] nhng octoz thuc h n t, t trng 2,6, rn 6 c th cha mt t natri (di dng anbit), st, bari, rubii, cn microlin thuc h tam t.

52

Anbit (0 10%An, 90 100%Ab)

Olgoclaz (10 30%An, 70 90% Ab)

Aesin (30 50%An, 50 70%Ab)

Labrade (5070%An, 30 50% Ab)

Bitonit (7090%An, 10 30% Ab)

Anoctit (90 100%An, 0 10%Ab)

- Phenpat kim th (plagiocla). Nhm ny gm mt dy dung dch rn lin tc ca cc khong vt t anbit Na[AlSi3O8] n anoctit Ca[Al2Si2O8]. Cc khong vt trung gian c phn loi theo thnh phn phn trm ca anbit (Ab) v anoctit (An).

b) Nhm phenpatoit: L loi alumosilicat natri v kali xut hin cc khi macma ngho silic, giu kim. Trong c lxit K[AlSi2O6] thuc h t phng, c th cha mt lng rt t natri v rubii, anaxim Na[AlSi2O6].H2O, nephelin KNa3[AlSiO4]4.

c) Nhm zeolit: Zeolit l alumosilicat natri, canxi ngm nc. Cng thc tng qut ca zeolit l MxAyO2y.nH2O. Trong M l Na+,Ca2+; A l Si4+, Al3+. Tng s ion Si4+ v Al3+ bng y, ngha l aSi + aAl = y vi iu kin aAl < aSi.

C s cu trc ca mng tinh th zeolit l t din SiO44-. Cc t din ni vi nhau qua c 4 nh to thnh b khung 3 chiu trong khng gian. Do s thay th Si4+ bng Al3+ nn b khung xut hin in tch m. Cation b tr cho in tch m nm cc hc, cc khong trng ca b khung zeolit. Do s thay th nh vy nn b khung zeolit c mch lin kt -Si-O-Al-O-Si-. Do tng tc tnh in nn khng th c hai t din AlO45- st nhau, iu c ngha l trong zeolit khng c mch lin kt -O-Al-O-Al-O-. Cc t din ni vi nhau qua nh to thnh nhng bt din ct. Hnh 52 l mt bt din ct c nh l Si4+ v Al3+.

Mi bt din ct c 24 nh do cng thc ca mt bt din ct l: Mx+Si24xAlxO48.nH2O. Mi bt din ct c 6 mt hnh vung v 8 mt lc gic u. Khi cc bt din ct ni vi nhau qua mt hnh vung s to nn mng li khng gian thuc h lp phng n gin, gi l zeolit A. T bo mng ca zeolit A nh vy c cha mt bt din ct. Do l cng thc zeolit A ng bng cng thc bt din ct. Khi lng nhm t gi tr ti a th x = 12, gi s cation trao i ch c Na+ th lng nc hp th t 27 phn t v cng thc natri zeolit A l Na12Si12Al12O48.27H2O. Hng s mng li ca zeolit ny l 12,3. Khong trng bn trong ca t bo mng zeolit A c ng knh 11,4(thng c gi l hc ). Khong trng bn trong ca hc bt din ct c kch thc bng 6,6 (gi l hc ).

53

Hnh 52 Bt din ct, n v c s ca mng li zeolit

Hnh 53 Zeolit A

Khi cc bt din ct ni vi nhau qua mt lc gic th to thnh mng lp phng phc tp kiu kim cng. Zeolit thuc kiu ny c 3 loi gi l zeolit X, zeolit Y, faujazit. T bo lp phng ny c cnh a = 25. Mi t bo c 192 t din (SiAl)O4. V d faujazit c thnh phn: Na13Ca12Mg11(AlO2)59(SiO2)133.235H2O. Tu thuc vo phng php tng hp m thu c sn phm c t l Si/Al khc nhau. i vi zeolit X v Y, s t din AlO4 trong mt t bo mng lin h vi t l Si/Al theo h thc sau:

NAl = Al/Si+1

192 (11)

Vi zeolit X s cation Al3+ trong mt t bo nguyn t thay i t 77 n 96, trong zeolit Y con s thay i t 48 n 76. Khi thay th Al3+ vo v tr ca Si4+, hng s mng ca zeolit X v Y b thay i t 25 n 24,6. Breck v Flanigen kho st trn 37 mu zeolit X v Y vi cc phng php phn tch chnh xc thnh phn Al trong tinh th v xc nh gi tr a ca mng lp phng xc lp c h thc:

a = 0,00868 NAl + 24,191 (12)

y NAl l s ion nhm c trong mt t bo mng zeolit X hoc Y.

Zeolit c rt nhiu ng dng trong nn cng nghip hin i lm cht trao i ion, hp th chn lc, ry phn t, cht mang xc tc v c lm cht xc tc trong k ngh ho hc, c bit l trong cng nghip ch bin du m.

1.3 Nhng nt c bit ca tinh th cng ho tr v tinh th kim loi

Cu to tinh th v tnh cht ca cc hp cht cacbua, nitrua, borua, silixua kim loi c nhiu nt ring bit do kiu lin kt ho hc. Khc vi cc hp cht trnh by phn trn c lin kt ion l ch yu, trong tinh th ca cacbua, nitrua, silixua kim loi th lin kt cng ho tr l chnh hoc lin kt hn tp gia cng ho tr ion kim loi.

Lin kt cng ho tr rt bn vng nn nhng hp cht loi ny c rn v nhit nng chy cao, dn. Trong khi lin kt kim loi truyn cho cacbua, nitrua, borua, silixua mt s c tnh ca kim loi nh do, c dn in cao, siu dn nhit thp.

bn v hng ca lin kt cng ho tr ph thuc vo s lng t ca in t lin kt. V d lin kt ca cc obitan p bn hn lin kt ca obitan s. Vi kim loi chuyn tip th li

54

do lin kt ca obitan d. Mt khc hp cht gm c lin kt cng ho tr th vai tr quan trng li l lin kt ca cc obitan lai ho. Vng xen ph ca cc obitan lai ho ln hn do loi hp cht ny c lin kt rt bn v tnh nh hng th hin rt r rt. S phi tr ca cc nguyn t trong tinh th c lin kt cng ho tr khng ln nh trong tinh th c lin kt ion.

Hnh 54 Cch ni cc bt ct qua mt lc gic cc nh l Si4+ hoc Al3+ ti tm ca t din Si(Al)O4

(a )

(b)

Hnh 55 Cu trc graphit

i vi tinh th c lin kt kim loi th c mt s c tnh khc hn. l khng tun theo quy lut ho tr, cc nt mng gn nhau v c th l cc nguyn t ca cng mt nguyn t. Mt s loi cht dng trong vt liu gm m c lin kt kim loi (v d berilit, aluminit) c th xem nh hp cht gia cc kim loi, hoc l hp kim c trt t.

kho st cc hp cht c lin kt cng ho tr dng trong vt liu v c chng ta nhc li cu trc ca graphit.

Trong graphit 1 in t s v 2 in t p ca cacbon to thnh lai ho sp2. Cc obitan lai ho ny to thnh 3 lin kt vi 3 nguyn t cacbon khc trong cng mt mt phng. in t ho tr th t lin kt yu vi nguyn t cacbon mt khc. Do mng li graphit gm cc lp lin kt vi nhau bng mt lc kh yu. Cc nh ca hnh lc gic trong cng mt lp nm v pha trn hoc pha di ng tm ca hnh lc gic ca lp gia (hnh 55a v b).

55

Hnh 56 Kiu cu trc blend (ZnS), kim cng (z Zn, { S)

Hnh 57 Cu trc ca nitrua bo (z B, { N)

Do cu trc nh vy nn graphit c tnh ng hng mnh. Graphit thuc mng li lc phng.

Kim cng gm cc nguyn t cacbon trng thi lai ho sp3. S xen ph ca cc obitan lai ho sp3 to thnh mng li khng gian 3 chiu. S phi tr ca cacbon y bng 4. Kim cng thuc h lp phng. Mng li ca kim cng ging mng li ca blend (hnh 56), cng vi loi cu trc ny c cacbua silic (SiC).

Cu to tinh th ca nitrua bo cng tng t cu to graphit. im khc nhau ch ch cch b tr gia cc lp lc gic: nitrua bo th cc nh ca lc gic nm thng hng theo trc c, nh vy gia cc lp c mch lin kt lun phin NBNBN. Lin kt gia cc nguyn t trong cng mt lp cng bn hn lin kt gia cc lp.

C hai kiu cu trc ca graphit v nitrua bo p sut v nhit cao u c th tr thnh cu trc lp phng kiu blend. p sut cao hn na hp cht NB c th tr thnh cu trc zinkit ZnO.

Cacbua, nitrua, borua, silisua ca cc nguyn t chuyn tip c cc kiu lin kt ho hc hn tp. Nhiu hp cht trong nhm ny c cu to mng li kh n gin. Theo c tnh cu to tinh th c th phn hp cht nhm ny thnh hai loi l pha xm nhp v pha thay th.

56

Nu t l gia bn knh nguyn t phi kim (X) v ca kim loi (M) trong hp cht khng ln hn 0,59 th cc nguyn t kim loi to thnh mng gi ghm chc c thng thng nh trong mng kim loi (lp phng mt tm v lc phng), cn phn t phi kim xm nhp vo cc khong trng. iu ny tho mn vi cacbua, nitrua v mt phn borua ca kim loi chuyn tip. Bn knh ca nguyn t silic qu ln do silisua cu to theo kiu thay th. Ngoi yu t hnh hc ra cu trc tinh th ca hp cht nhm ny cn ph thuc vo thnh phn ho hc, iu kin bn ngoi (p sut, nhit ) v mt s yu t khc na.

ng v quan im vt liu gm th cacbua, nitrua c thnh phn AX c cu to kiu NaCl, trong cc nguyn t kim loi c kiu gi ghm chc c lp phng, cn phi kim chui vo cc hc bt din. V d tinh th TiC, ZrC, HfC, NbC, TaC, TiN, ZrN, CrN, HfN, UN...

Cacbua ca wolfram v ca molipen c mng li tinh th ging kiu cu trc ca NiAs (hnh 58). Trong cc cacbua ny nguyn t kim loi chim v tr ca niken, cn cacbon chim v tr ca As. l mng li lc phng. Mi t bo mng c 6 nguyn t cacbon, 6 nguyn t wolfram hoc molipen. S phi tr ca chng u bng 6 nhng dng a din phi tr th khc nhau: nguyn t cacbon nm tm ca lng tr tam gic, cn cc nguyn t cacbon phi tr quanh nguyn t kim loi th phc tp hn. Cc tinh th vi lin kt ion th cha gp kiu cu trc ny.

Gm cacbua c cu trc chuyn tip t pha xm nhp n pha thay th. Borua l hp cht ho hc c thnh phn rt khc nhau M1B, M2B, MB, M3B4, MB2, M2B5, MB4, MB6 v MB12. Mng tinh th rt phc tp, trong va c lin kt cng ho tr va c lin kt kim loi.

NiAs

a

Cr (Si)

Al (Mo)c

Hnh 58 Cu trc tinh th nikenlin NiAs

Hnh 59 Cu trc Cr2Al

Trong gm th ph bin nht c iborua MB2, y M l Cr, Ti, Zr, Hf, Nb, Ta, W... Tt c cc borua ny u c mng tinh th ging nhau v ging mng tinh th ca AlB2. y l mt kiu cu trc phc tp gm cc lp nguyn t bo nh trong graphit phn b chnh xc lp ny trn lp kia.