Physica C 235-240 (1994) 885-886 North-Holland PHY$1CA

Synthesis and structural characterisation of YBCO related phases containing cobalt

A. J. Wright and C. Greaves

School of Chemistry, University of Birmingham, Edgbaston, Birmingham, B15 2TT, U. K.

The synthesis of (NdL~Ceo.5)Sr2CoCu209, a fluorite-type block material containing Co ~', was recently reported in an impure form with its structure related to YBCO with y3. replaced by a fluorite-type block of composition (Nd,Ce)~O~, Ba ~÷ by Sr ~" and the copper on the Cul sites wholly by Co ~*. This material was reported to be tetragonal, in contrast to its non fluorite block analogue YSr~CoCu,O7, which exhibits long range directional ordering of chains of corner linked CoO~ tetrahedra, producing orthorhombic symmetry. In this study the presence of a high level of impurities was confirmed and attributed to the for~tiar~ af ¢_he perovskite impurity NdCoQ and subsequently a single phase product was synthesised when neodymium was replaced by yttrium, forming (Y, ~Ceos)Sr~CoCu~O,. Neutron diffraction indicates the presence of long range directional order of CoO 4 tetrahedra chains. No superconductivity was observed.

1. INTRODUCTION

The incorporation of tetrahedral layers into superconductor-related materials has been found to have important structural consequences. Structural studies on the first of these materials, YSr2Cu2GaO7 ~, have revealed ordered chains of corner linked GaO, te t rahedra leading to an enlarged orthorhombic supercell (a=2%, b=~/2b~, c=~/2ap) when compared to YBa2Cu307.~ (a~, b~, %). These chains can link up in one of two senses (L or R) but in this material only one chain tyr e is present.

Extensive studies in this area hav~ yielded other superconductor-related materials containing tetrahedral layers. One such example is YSr2Cu2CoO 7 which has been shown to contain corner linked chains of CoO, tetrahedra ~. However, the structure of YSr2Cu2CoO 7 has been shown to subtly differ from its gallium analogue by the loss of ordering of chain type and hence the presence of equal quantities of the two chain types (L and R) but still retaining chain directional order and t:lerefore orthorhombic symmetry.

Recently, the phase (Nd,Ce)2Sr2Cu~CoO,~ has been reported 3 with no evidence for this

long range directional ordering of the CoQ chains and therefore a tetragonal str~lcture. This struct~re is a direct relation of YSr2Cu2CoO 7 with yttrium having been replaced by a fluorite block of composition (Nd,Ce)~.Q. The single phase synthesis of this mat, erial t~as at present not been reported therefore a synthetic study of materials of the type (Li~,Ce)~Sr2Cu2CoO~ (with Ln=lanthanide, Y) was conducted, with a subsequent detailed structural investigatior: performed using time-of-flight neutron diffraction.

2. EXPERIMENTAL

Appropriate stoichiometric mixtures of high purity Ln203, CeO~, SrCO:,, c u e and Co304 were intimately ground and fired twice at iiUu~; ~or 24 hours i~ ~ w ~ ; ~ . . . . . . . . . . . . w y ~ , , with an intermediate r e ~ n d . Powder X-ray diffraction (Siemens D5000 ~sing Cu K(~ radiation) was used for initial characterisation. Time-of-flight neutron diffiactio,~ data were coilected using the diffractometer Polaris at ISIS, Rutherford App!et.on Laboratory, Oxford, UK Structural refi~lements were achieved usin~ the program TF15][~S which is based on

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886 A.J. Wright. C. Greaves/Physica C 235-240 (1994) 885-886

Table 1 Structural parameters for Y,.sCeo.sSr~CoCu20~ i i

ATOM POS X Y Z B,so ii , , , i r a

¼ 0.37(2) ¼ 0.37(2) ¼ 0.26(2) ¼ 0.64(2)

0.285(2) 0.15(13) 0 O.55(3) 0 O.36(5) 0 0.74(6)

0.233(2) 1.08(6) 0.857(2) 1.36(8)

8g 0.2939(I) . . . . . . . . ¼ Ce 8g 0.2939(1) ¼ Cu 8g 0.1436(1) ¼ Sr 8g 0.0809(1) 0.7471(9) Co 8f 0 0.191(2) O1 8e 0.2520(2) 0 02 8e 0.1477(1) 0 03 8e 0.1513(1) ½ O4 16h 0.0613(1) 0.2788(8)

..... O5 8f 0 0.133(2) a=28.1572(3)A, b=5.4322(1)A, c=5.4075(1)A; Cmcm

R~=2.91%, R~=1.11%, R,=5.19%

the Cambridge Crystallographic Subroutine Library 4.

3. RESULTS AND DISCUSSION

The synthesis of single phase products of the type (Ln,Ce)2Sr2Cu2CoO9 was successfully achieved with Ln=Y. As previously reported 3, large scale impurities were pre~ent with Ln=Nd and these were assigned to the perovskite NdCoOa. The time-of-flight neutron diffraction performed on (Y,Ce)2Sr~Ct~CoO 9 indicated similar ordering of the corner linked tetrahedra as was reported for the non-fluorite type block analogue YSr2Cu2CoO 7 with the results shown in Table 1. The presence of equal quantities of the two chain types (i.e. a lack of ordering or" the chain type) was indicated by the significantly better R-factors observed for the model based on the space group Cmcm as compared to those for the single chain type model based on the space group Cmc2, (R.=3.19%, l~p=l.11%, R,=5.17%). The presence of long range ordering of the chain directions is in contrast to the purely random arrangement reported for the (Nd,Ce)2Sr2Cu2CoO 9 material It seems likely that this discrepancy can be attributed to the increased impurities present in the Nd material. The cobalt rich nature of these impurities may lead to the presence of some copper replacing cobalt on the "Cul" sites

UNIT CELL OCCUPANCY

6 2 8 8 4 8 8 8 8 4

and therefore a loss in the long range chain directional order. No significant evidence was found from the refinement of the presence of copper on the cobalt sites in (Y, Ce)2Sr~Cu~CoO 9. However, the presence of small quantities of copper on these sites may lead to ~he loss of long range order of the chain types.

ACKNOWLEDGEMENTS

We would like to umnk EPSRC for funding and providing a studentship for AJW and to S. Hull and R. Smith for experimental ;assistance during the collection of neutron diffraction data.

REFERENCES

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