determination of crystal structure (chapt. 10)
DESCRIPTION
Determination of Crystal Structure (Chapt. 10). 1.Use the angular positions of the diffraction lines to determine the shape and size of the unit cell. Assume either cubic, tetragonal, orthorhombic, rhombohedral, hexagonal, monoclinic, or triclinic. - PowerPoint PPT PresentationTRANSCRIPT
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Determination of Crystal Structure (Chapt. 10)
Crystal Structure Diffraction PatternUnit Cell Line PositionsAtom Positions Line Intensities
Guess
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1. Use the angular positions of the diffraction lines to determine the shape and size of the unit cell. Assume either cubic, tetragonal, orthorhombic,
rhombohedral, hexagonal, monoclinic, or triclinic. Assign Miller indices to each reflection (“index the
pattern”). If a match is not obtained, the assumption of should be
changed and the pattern indexed again. Calculate the size of the unit cell based on the positions
and Miller indices of the diffraction lines.2. With the measured density of the material, the chemical
composition, and the size of the unit cell, calculate the number of atoms per unit cell.
3. Find the positions of the atoms in the unit cell by using the relative intensities of the diffraction lines.
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Preliminary Treatment of Data
We want the values of sin2 for each diffraction line (in order to find the cell size and shape), however, there can be errors to what we measure, including extraneous lines in the diffraction pattern, and systematic errors (misalignment, film shrinkage, absorption, etc.)
Extraneous Lines1. X-ray beam with multiple wavelengths:
2. Contaminants or impurities in the sample, or the specimen mount!
Systematic Errors1. Film shrinkage (see Fig. 6-5 Cullity)2. Specimen is off-centered in Debye-Scherrer camera (see Fig. 11-3 Cullity)3. Absorption in the sample
Answer mix in a reference material and calibrate. (see Fig. 10-1 Cullity)
K
K
d
d
sin2
sin2
2
2
2
22
sinsin4
KKd
222
2
sinsinK
K
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Cubic Crystals (Indexing the Patterns)
2222
22
4sin lkh
a
:222 lkh Simple cubic: 1, 2, 3, 4, 5, 6, 8, 9, 10, 11, 12, 13, 14, etc.Body-centered cubic: 2, 4, 6, 8, 10, 12, 14, 16, . . .Face-centered cubic: 3, 4, 8, 11, 12, 16, 19, 20, . . .Diamond cubic: 3, 8, 11, 16, 19, 24, 27, 32, . . .
Simple cubic bcc fccLinesin2(h2 + k2 + l2)
2
2
4a
(h2 + k2 + l2)2
2
4a
(h2 + k2 + l2)2
2
4a
a(Å)hkl
10.140 1 0.140 2 0.070 3 0.04673.5711120.185 2 0.093 4 0.046 4 0.04633.5920030.369 3 0.123 6 0.062 8 0.04613.5922040.503 4 0.126 8 0.063 11 0.04573.6131150.548 5 0.110 10 0.055 12 0.04573.6122260.726 6 0.121 12 0.061 16 0.04543.6240070.861 8 0.108 14 0.062 19 0.04533.6233180.905 9 0.101 16 0.057 20 0.04533.62420
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Corrections for Systematic Errors
RS4
RR
SS
RS
ln4lnlnln
RR
SS
RS ,
2sin22 xONS
cossin4
2sin2Rx
Rx
SS
C
Absorption error can be lumped into this error.
cossin,,, Rx
RR
SS
ACRS
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Correction for Systematic Error
For a cubic crystal:
Differentiation of Bragg’s Law: sin2d
sin2d
2
2
sin4
cossin22
sin4
cos2sin2
d
dddd
cotdd
222 lkhda dd
aa
Fractional error in a
(goes to zero as 90)
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Correction for Systematic Error
sincos ,cossin , ,90
cossincossin
cossin
sincos
Rx
RR
SS
dd
At small (large ), this could be approximated as:
22 cossin KKdd
2cosKaaa
aa
dd
o
o
2cosKaaa oo
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Correction for Systematic Error
0
0.5
1
1.5
2
2.5
3
3.5
4
-20 0 20 40 60 80 100
Mag
nitu
de
0.01*cos2
cos2
(p/2 - )*cos()/sin()
0.1*cos2
10*cos2
100*cos2(p/2 - )*cos()/sin()
+ cos2()
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Nelson-Riley
0
0.5
1
1.5
2
2.5
3
3.5
4
-20 0 20 40 60 80 100
Mag
nitu
de
Nelson Rileyfunction
2*cos2
cos2
= (cos2/sin2) + (cos2/)(p/2 - )*cos()/sin()
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Indexing Patterns for Non-Cubic Crystals
Tetragonal
2
222
22
2
2
22
2
11
ac
lkh
ac
l
a
kh
d
2
222loglog2log2
ac
lkhad
2
222
22
22
212
12
121 logloglog2log2
ac
lkh
ac
lkhdd
Depends on (c/a), but not on a.
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Dull-Harvey Chart
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Indexing Patterns for Non-Cubic Crystals
Tetragonal
2
2
2
222
22
2
2
22
2
sin411
ac
lkh
ac
l
a
kh
d
2
222
2
22 log
4logsinlog
ac
lkh
a
2
222
22
22
212
12
122
12 loglogsinlogsinlog
ac
lkh
ac
lkh
Depends on (c/a), but not on a.
2
222
2
22
4sin
ac
lkh
a
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Scales for Left Side of Above Equations
1 102 3 4 65 7 8 9
0.010.11 0.5 0.05
d scale
sin2 scale
1 102 3 4 65 7 8 9
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Hexagonal Hull-Davey Chart
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Zinc Example (Cu K)
From table 10-2 (sin2)
0.010.11 0.5 0.05
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0.010.11 0.5 0.05
10-7
0.010.11 0.5 0.05
10-5
0.010.11 0.5 0.05
10-8
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Chapter 10 Example
sc fcc bcc diamond
line sin2 (h2 + k2 + l2) 2/4a2 (h2 + k2 + l2) 2/4a2 (h2 + k2 + l2) 2/4a2 (h2 + k2 + l2) 2/4a2
1 0.0462 1 0.0462 3 0.0154 2 0.0231 3 0.01542 0.1198 2 0.0599 4 0.02995 4 0.02995 8 0.0149753 0.1615 3 0.053833 8 0.020188 6 0.026917 11 0.0146824 0.179 4 0.04475 11 0.016273 8 0.022375 16 0.0111885 0.234 5 0.0468 12 0.0195 10 0.0234 19 0.0123166 0.275 6 0.045833 16 0.017188 12 0.022917 24 0.0114587 0.346 8 0.04325 19 0.018211 14 0.024714 27 0.0128158 0.391 9 0.043444 20 0.01955 16 0.024438 32 0.0122199 0.461 10 0.0461 24 0.019208 18 0.025611 35 0.01317110 0.504 11 0.045818 27 0.018667 20 0.0252 40 0.012611 0.575 12 0.047917 32 0.017969 22 0.026136 43 0.01337212 0.616 13 0.047385 35 0.0176 24 0.025667 48 0.01283313 0.688 14 0.049143 36 0.019111 26 0.026462 51 0.0134914 0.729 15 0.0486 40 0.018225 30 0.0243 56 0.01301815 0.799 16 0.049938 43 0.018581 32 0.024969 59 0.01354216 0.84 17 0.049412 44 0.019091 34 0.024706
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Extended Hull-Davey Chart
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Hull-Davey for Cubic
110100
Hull Davey cubic
(c/a
) =
1
dia.
bcc
dia.
fcc
dia.
sc
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Sin2 Scale of Table 10-5
0.010.11 0.5 0.05
0.010.11 0.5 0.05
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Chapter 10 Example
fcc
line sin2 (h2 + k2 + l2) 2/4a2
1 0.0462 3 0.01542 0.1198 8 0.0149753 0.1615 11 0.0146824 0.179 12 0.0149175 0.234 16 0.0146256 0.275 19 0.0144747 0.346 24 0.0144178 0.391 27 0.0144819 0.461 32 0.01440610 0.504 35 0.014411 0.575 40 0.01437512 0.616 43 0.01432613 0.688 48 0.01433314 0.729 51 0.01429415 0.799 56 0.01426816 0.84 59 0.014237