B O L E T Í N DE LA S O C I E D A D E S P A Ñ O L A DE

Cerámica y Vidrio A R T I C U L O

• • •

The superconducting oxycarbonates (TI, M)/Sr, Ba)4Cu2(C03)07 and (Hg, M)̂ (Sr, Ba)4Cu2(C03)07:

three classes of closely related structures B. RAVEAU, C. MICHEL, M. HERVIEU AND A. MAIGNAN

Laboratoire CRISMAT, CNRS, URA1318 , I SM RA/U Diversité de Caen Bd du Maréchal Juin, 14050- Caen cedex- France

Starting from the «1201» structure and the Sr2Cu02C03 (S2CC) structures more than sixteen oxycarbonates with an original structure can be generated. They can be classified according to their structure in three series. The single intergrowth of the «1201» and S2CC structures formulated [1201]-j[S2CC]̂ represent the first class. The two other classes, called collapsed oxycarbonates, derive from the first series by she-aring mechanisms. The second class corresponds to the appearance of a shearing phenomenon in the (100) plane, i.e. a shifting of c/2 in the [1201] JS2CC]j intergrowth may appear every m octahedra (m = 3,4 or m = 3 and 4 alternately). The third class is characterized by a shea-ring phenomenon in the (110) plane of c/2 every m octahedra (m = 5, or m = 5 and 6 alternately, or m = 4 and 5 alternately). Incommensurate distributions of the (100) and (110) shear planes are also observed. One remarkable feature deals with the fact that all these oxycarbonates are superconductors with T̂ ranging from 17 K to 80 K.

Keywords : Superconductivity, Oxycarbonates, Inter growths, Shearing mechanism

Los oxicarbonatos superconductores (Tl,M)j (Sr̂ Ba)̂ CU2 (C03)0^ y (Hg,M)̂ (Sr,Ba)4 CU2 (C03)0^: tres clases de estructuras estrechamente relacionadas

Partiendo de la estructura «1201» y las estructuras Sr2 CUO2 CO3 (S2CC), más de 16 oxicarbonatos, con una original estructura, pueden ser formados. Estos pueden ser clasificados de acuerdo a su estructura, en tres series. La formada a partir de la interpenetración de las estruc-turas «1201» y S2CC y formulada como [1201], [S2CC], representa la primera clase. Las otras dos clases, denominadas oxicarbonatos colap-sados, derivan de la primera serie a través de mecanismos de distorsión. La segunda clase corresponde a la aparición de un fenómeno de distorsión en el plano (100), producido, por ejemplo, por un desplazamiento de c/2 en la Estructura [1201],[S2CC], en cada m octaedros (m= 3,4 ó m= 3 y 4 alternativamente). La tercera clase está caracterizada por un fenómeno de distorsión en el plano (110) de c/2 cada m octae-dros (m= 5 ó m= 5 y 6 alternativamente, ó m= 4 y 5 alternativamente). Inadecuadas distribuciones de las distorsiones en los planos (100) y (110) son también observadas. Otra característica reseñable es que todos estos oxicarbonatos son superconductores en un rango de tem-peraturas críticas de 17K a 80K.

Palabras clave: Superaconductividad, Interpenetración, mecanismos de distorsión.

1. INTRODUCTION

A great deal of work has been devoted to superconducting oxy-carbonates these last four years. Most of these compounds derive from the cuprates by introducing triangular carbonate groups bet-ween the layers of CuO^ pyramids or of CuO^ octahedra. Among them, the oxycarbonates with the generic formulation (Tl,M)^(Sr,Ba)4Cu2(C03)07 and (Hg,M)^(Sr^Ba)4Cu2(C03)07, that have been recently discovered, are of great interest, owing to their close structural relationships with the «1201» cuprates such as Tlo5Pbo5Sr2Cu05 (1) or HgBa2Cu04^g (2). The great flexibihty of these structures makes that intergrowth and shearing phenomena can be combined in the same matrix, allowing the generation of new materials without destroying superconductivity.

We report herein on the structure and superconductivity of these materials that can be classified in three series : one class of single intergrowths of the «1201» and «S2CC» (Sr2Cu02C03) structures for-

mulated (1201)^(S2CC)p and two classes of collapsed oxycarbonates derived from the (1201)̂ (S2CC)̂ structure by shearing phenomena along the (100) and (110) planes respectively.

2. THE SINGLE INTERGROWTHS (1201)̂ . (Sf,Q\

The comparison of the «1201» structure (Fig. la) observed for TIQ sPbQ 5Sr2Cu05 (1) with the structures of the oxycarbonate Sr2Cu02C03 (3) (Fig. lb) suggests the possibility to reahse an inter-growth, due to their bidimensional accord. This is the case of the oxycarbonate TIQ^Pbg 5Sr4Cu2(C03)07 (4) whose structure (Fig. Ic) consists of a regular stacking of these two kinds of layers. The impor-tant point deals with the fact that the oxycarbonate ^^o.5^^o.5^^4^^2(^^3^^7 ^^hibits a sharp superconducting transition with a high diamagnetic volume fraction, with T̂ up to 70 K after optimisation (Fig. 2), whereas the two components of the structure

Bol. Soc. Esp. Cerám. Vidrio, 34 [5-6] 421-426 (1995) 421

B. RAVEAU, C. MICHEL, M. HERVIEU, A. MAIGNAN

TIO

O * o o 1201

SaCC

c * o * o ^ o * o - o * o * o

Fig. 1 : Schematic representation of the structure of: (a) «1201» (b) Sr2Cu02CO¡(S2CC) and (c) single intergrowth [1201]^[S2CC]j. Triangles represent the carbonate groups.

- 0 . 8 Fig. 2 : x'(T) curves for optimized TlQ^PbQ^Sr^Cu2(C0^)0j

0 10 20 30 40 50 60 70 80 0.0

0 10 20 30 40 50 60 70 80

T(K) Fig. 3 : x'(T) curves for Hg^_^ßrfu2iC0^}0-j oxijcarbonates with M = Pb,x = 0.7 (a) ; M = Bi, X = 0.5 (b) ;M^V,x = 0.4 (c) ;M = Mo,x = 0.5 (d) ; M = Cr; x = 0.6 (e).

have never been formed superconductors whatever this annealing treatment they were submitted. The replacement of lead by bismuth allows an isotypic phase to be synthesized, with the formulation Tl .̂̂ Bi^Sr4Cu2(C03)07 (5) ; the critical temperature of the latter is however lower than for the lead phase, ranging from 48 K for x = 0.3 to 54 K for X = 0.5. It is remarkable that no thallium oxycarbonate that would correspond to the composition TlSr4Cu2(C03)07 could be synthesized up to date. Curiously the partial substitution of molybdenum for thallium, allowed a similar intergrowth to be iso-lated for the composition TIQ gMog 2Sr4Cu2(C03)07 (6) ; the critical temperature of this phase can be increased up to 80 K by annealing in an hydrogen argon flow (10% H2) at low temperature (280 °C).

Hg (II), owing to its electronic d̂ ^ configuration similar to Tl(III) and to its similar size, exhibits also isotypic oxides, in spite of the fact that mercury adopts the twofold linear coordination. The oxides

HgQ 3PbQ 7Sr2Cu04^g (7) and Hg^ 5BÍQ 5Sr2Cu04^5 (8), are characteri-zed by the «1201» structure like the thallium cuprates, and differ only from the latter by an oxygen deficiency. Like the thallium pha-ses, these mercury cuprates do not superconduct. In a very similar way, the assemblage of the «1201» mercury cuprate structure with the S2CC structure allows new superconductors to be generated. This is the case of the oxycarbonates Pbp ĤgQ 3Sr4Cu2(C03)07_g (9) and HgQ ^BÍQ 3Sr4Cu2C(03)07_g (10) that both exhibit the same struc-ture as TlQ5PbQ5Sr4Cu2(C03)07 (Fig. Ic), mercury replacing tha-llium, and anionic sites at the mercury level being partly vacant. Contrary to the «1201» parent oxides and to Sr2Cu02C03, these two oxycarbonates are superconductors; the lead phase Pbo 7Hgo 3Sr4Cu2(C03)07 has a T̂ of 70 K (Fig. 3 curve a), after opti-misation in a reducing atmosphere, i.e. very similar to Pbg 5TI0 5Sr4Cu2(C03)07. Curiously, a much lower T̂ of 17 K (Fig. 3

422 Boletín de la Sociedad Española de Cerámica y Vidrio. Vol. 34 Num. 5-6 Septiembre-Diciembre 1995

THE SUPERCONDUCTING OXYCARBONATES (Tl, M),(Sr, Ba j^Cu j iCOj jO ; A N D (Hg, /vD^iSr, Ba)4Cu2(C03)07: THREE CLASSES OF CLOSELY RELATED STRUCTURES

^ m m mmm m % ii m m Y V * .

Fig. 4 : [200] UREM image of TIQ ^Pbg ̂ Sr^Cu2(C0^)0y showing the regular stacking of the dif-ferent layers. The heavy black dots are correlated to Tl and Pb atoms and the brightest dots to the carbonate groups.

Fig.5 : Idealized drawing of the structure of the collapsed oxycarbonate TlBa2Sr2Cu2(CO^)Oj. Two blocks of the single intergrowth [1201]JS2CC]j, four octahedra wide, are evidenced.

curve b) is observed for HgQ 3BÍQ ̂ Sxflvi2{CO^Orj. In fact, this class of mercury oxycarbonate can be easily stabiHsed by introducing besi-des mercury a transition element. Three single intergrowths have indeed been synthesized for M - Mo, V, Cr. The critical temperatu-re of these phases depends on the nature of the transition element M, the highest T̂ has been obtained for the vanadium phase, Hgo^Vo4Sr4Cu2(C03)07_5 (Fig. 3 curve c) for which T̂ = 76 K (11), whereas the molybdenum oxycarbonate HgQ 5M0Q 3Sr4Cu2(C03)07 _ g exhibits a T̂ of 74 K (Fig. 3 curve d) (12), the lowest T̂ of 37 K being obtained for Hgg ̂ CrQ ̂ Sr4Cu2(C03)07_5 (12) (Fig. 3 curve e).

All these oxycarbonates are tetragonal with a ranging from 3.824Â to 3.875Â and c ranging from 16.156Â to 16.528Â. Their structure is easily identified by their (010) high resolution electron microscopy (HREM) images that show a regular stacking along "ĉ of their car-bonate layers with Tl (or Hg), Sr, and Cu layers as shown for instan-ce for T\Q^\^Sxfl\i^O^Orj (Fig. 4) where the rows of britghest dots correspond to the carbonate layers.

3. THE COLLAPSED (100)-(1201)i(S2CC)i INTERGROWTH

Starting from the single intergrowth, (1201)|(S2CC)^ (Fig. Ic), a shearing mechanism can be applied along the (100) plane of the structure by introducing barium on the strontium sites, leading to a new structure, that consists of shces of the (1201)^(S2CC)| inter-growths, shifted of c/2 with respect to each other.

The most current member of this series is represented by the oxy-carbonates TlBa2Sr2Cu2(C03)07 (13), Tl^.^Hg^Ba2Sr2Cu2(C03)07 with X = 0.2 and 0.5 (14). In this structure two successive (1201)|(S2CC)^ slices that are m = 4 CuO^ octahedra thick are shifted of •c /̂2 in the (100) plane as schematised on figure 5. From this she-aring mechanism, it appears that the (Cu02)^ layers and the (SrO)^ layers of each block remain unchanged and form infinite layers para-llel to (001). This is not the case for the (TIO)^ ribbons which are limi-ted to four Tl atoms along b and are connected to ribbons of four CO3 groups. Thus the (TIO)^ and (CO)^ layers of the structure ^10.5^^0.5^^4^^(^03)07 are replaced by mixed layers ((T10)4(CO)4)^ characterized by a sequence of four thallium atoms and four carbo-nate groups along b.

É; W:>-. TI,C •Cu

\ß\fk

Fig.6: [100] HREM image ofTlBa2Sr2Cii2(CO^)Oy.

This shearing phenomenon with respect to TIQ 3PbQ 5Sr4Cu2(C03)07 is evidenced by the (100) HREM images. The contrast can be described from rectangular areas (Fig. 6) which correspond to half a cell «b/2 x c». Considered separately, these rectangles exhibit exactly the same contrast as the oxycarbonate TIQ 5PbQ 3Sr4Cu2(C03)07; along b, each rectangle is four CuO^ octahedra wide. Then the three groups of layers with different contrast stacked along c can be identified by their similarity with TIQ ̂ PbQ 5Sr4Cu2(C03)07. The first group consists of three rows of alternated black dots which are correlated to the sequen-ce of rows «(Ba,Sr)0 - TlO - (Ba,Sr)0», also observed in some thallium cuprates characterized by thallium monolayers. The second one corresponds to a single layer of grey dots which is correlated to a (Cu02)^ layer. The last group consists of three alternated rows of white dots, characteristic of the rows of CO3 groups. The fundamen-tal difference with the lead based oxycarbonate (Fig. 4) deals with the fact that two adjacent rectangles are deduced one from the other by a translation of b/2+c/2 in agreement with the A-type space group.

Such a shearing phenomenon, which implies the coexistence of tha-llium and carbon within a same layer rises the issue of the adaptation of these elements owing to their large size differences which imply Tl-O apical distances of about 2.00 Â, much larger than the C-O distan-

Boletín de la Sociedad Española de Cerámica y Vidrio. Vol. 34 Num. 5-6 Septiembre-Diciembre 1995 423

. RAVEAU, C. MICHEL, M. HERVlEU, A. MAICNAN

£"%

^Hî 'i\r ä%Vt^^ 1|*lt*

1 :' ̂^ ••^*f^fcii*l|fÄWtetl«fiM*^^^^^

, ,. , .. ^ , , ,: ^^^^^^^^^^^W^^^^^

~TI,M

'C

WiWPIIi«*i*i i

Fig. 8 : [010] HREM image of the (llO)-collapsed Tl^ifry^SrfupfO^ oxycarhonate. The simulated mage (t ~ 31A and focus value ~ 50Â) in insert has been calculated for the unmodu-lated subcell. The carbon segments appear as five successive dots.

ces along c (about 1.30 Ä). The answer to this question is given by changing the focus of the (100) HREM image. For some focus values (Fig. 7), the atoms of the (Cu02)^ layers are highlighted : it can be seen that these layers undulate with a rather large amplitude. Thus the structure of TlBa2Sr2Cu2(C03)07 (Fig. 5) consists of single perovskite undulating layers involving the (Cu02)^ and the ((Sr,Ba)0)^ sheets connected through {(110)¿CO)^^ layers. As a result the cell is^orthor-hombic, and characterized by a superstructure along "^ with respect to the single intergrowth : a~ap ~ 3.8Â, b - 8 x a , c - 17Â.

In a more general way, the (100) plane is a crystallographic shear plane, along which a shifting may appear every «m» octahedra, so that the (100) - collapsed oxycarbonates will be characterized by a super lattice «a - a ,̂ b - 2m â c - 17» ; the above phases corres-pond to m = 4.

Different shearings may appear that lead to various superstructu-res : (100) - shearing phenomena were indeed observed in the system TlSr4.^Ba^Cu2(C03)07 (15) leading to the superlattice «a x 6a x 17A» that corresponds to m = 3, and to the super lattice «a x 7â x 17» that corresponds to a (100) shearing phenomenon every three (m =3) and four (m = 4) octahedra alternately. This shearing mecha-nism is sometimes more complex as shown for instance in the oxy-carhonate Tlo 3Hgo 7Ba2Sr2Cu2(C03)07 (14) that exhibits an incom-mensurate structure «a^ x 7.5ap x 17», which implies that the dis-tribution of the (100) shear planes is modulated along the b direction of the actual cell.

It is remarkable that all these (lOO)-collapsed (1201)̂ (S2CC)̂ oxy-carbonates are superconductors with critical temperatures ranging between 60 K and 70 K. This is in agreement with the fact that the (Cu02)^ planes are not interrupted by the shearing phenomenon. Moreover this shows that the waving of the copper oxygen layers does not destroy the superconducting properties of these materials. At the present time, the role of barium in the (lOO)-shearing phe-nomenon is not explained. In particular, it is not known whether barium is absolutely necessary for the appearance of (100) shear planes.

4. THE COLLAPSED (110)-(1201),(S2CC), INTERGROWTH

The introduction of transition elements on the thallium sites of the S2CC-I2OI oxycarhonate allows also a shearing phenomenon to be induced. This is the case of the oxycarbonates TL/3CrT/,Sr,Cu, (003)07 and TlosVo2Sr4Cu2C(03)07 (11).

Nevertheless, in these oxycarbonates the shearing plane is no more the (100) plane but the (110)s2cc-i20i Plane. In order to eviden-ce the atomic arrangements of these oxides, the crystals were viewed along (100). This direction corresponds to (110)s2cc 1201 or (110) . Two images of the Tl2/3Cr^/3Sr4Cu2(C03)07 compound are given in figures 8 and 9 illustrating the typical contrasts which have been registered; their interpretation, especially the identification of the layers, was made with the help of the (110)s2cc-i20i simulated ima-ges, calculated for an ideal unmodulated structure. In figure 8 the heavy Tl(Cr or V) and Sr atoms appear as dark spots, for a focus value close to -50Â. The contrast of this image consists mainly of three rows of bright and grey spots : in the intermediate row, five bright spots correlated to the carbonate positions alternate with five dark spots correlated to the heavy ion positions (Tl,Cr), leading to a 5x ap

THE SUPERCONDUCTING OXYCARBONATES (Tl, M),(Sr, Ba)4Cu2(C03)07 A N D (Hg, M),(Sr, Ba)4Cu2(C03)07: THREE CLASSES OF CLOSELY RELATED STRUCTURES

ir^mi^^i^t^n^t^

rf'iîiHSiiintf á á i S ^ •Éiiiiiifíii JMiiriLji

"̂ "NSíiHi-wif n . ^ r

3jiÉÉ»ÉJM(iiÉiMaijyiiwlÉMai4¿4^ I iiiiiiiT : - i r i M Í T ' * ^ I

,tilitli««itt!I^AÎtiî' tliilliiltlllÄIIIÄti liiiáilSillllll^lAt^S»

iliÄlliiÄIlit^iit

•Î1,M

tr" "•:*

a»ii;i»l»injii|i/,.

Fig. 9 : fOlO] HREM image for a focus value close to -550Â where the cation positions are corre-

lated to the bright spots. The calculated image is in insert.

IIÜP

I I %{ï

mm Fig. 10 : Idealized drawing of the (llO)-collapsed structures : a) (Tl, M)Sr¿¡^Cu2(C0^)0j and b)

Hg03P>>05B

. RAVEAU, C. MICHEL, M. HERVIEU, A. MAIGNAN

ACKNOWLEDGEMENTS

The authors are grateful to the European Community for its finan-cial support. •

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426 Boletín de la Sociedad Española de Cerámica y Vidrio. Vol. 34 Num. 5-6 Septiembre-Diciembre 1995