Physica C 235-240 (1994) 883-884

North-Holland PHYSRCA g

The Format ion of Strontium analogues of YBazCu OT., A.J. Wright and C. Greaves

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

The existence of materials of the type (Ln,Ce)2Sr2CuaOs., (Ln=lanthanide, Y), analogues of the unstable YSr~CuaO,.~, with yttr ium replaced by a fluorite block, (Ln,Ce)202, have yet to confirmed by detailed structural studies. Reported here are the results of an investigation into their formation and structure which indicate the partial substitution of copper on the "Cul" sites by cerium to be necessary to stabilise the YBCO-type structure.

1. I N T R O D U C T I O N

Since initial studies on YBaaCuaO7.~, it has been known that the replacement of barium by the smaller strontium leads to instability in the structure at all but very high pressures ~. However, stability at ambient pressures can be achieved by the partial substitution of copper on the chain "Cul" sites by various cations ~. This stability is thought to be a consequence of the increased oxygen displacements around the dopant ions stabilising the lattice containing the smaller strontium. However, the reported synthesis of (Ho,Ce)2Sr2CuaO~.. 3, a structural analogue of YSraCuaOT.., with yttrium replaced by a fluorite block, appeared tq question this explanation.

Subsequent research in this area has drawn differing conclusions with Ono et a l ~

claiming tha t stability at moderate and ambient pressures is a :onsequence of the presence of some cerium on the Cul sites, while Kopnin e t a l s cla:.m the existence of two distinct structure types depending on the size of the lanthanide ion in the fluorite block. They suggest that for the larger cations (Ln=Pr-Gd) copper is the sole occupier of ~he . . . . . . . . . . . 1- -"-- "~'^ k J U k . b l l U ~ l L e ~ W i l ~ I - ~ i z l b ItUI

smaller cations (Ln=Dy-Yb and Y) cerium is also present.

Therefore, in order to clarify the synthesis and structure of these materials a detailed study was performed on phases of the type (Ln,Ce)2Sr,Cu2(Cu~..Ce=)O~. . (where Ln= Nd, Ho, Y) and its findings are reported here.

2. EXPERIMENTAL

Standard solid state methods were employed in the synthesis of these materials. High purity Ln203, CeO2, SrCOa and CuO were intimately ground and fired at 1050°C for 24 hours in flowing oxygen. Initial characterisation was achieved using X-ray diffraction (Siemens D5000) with subsequent time-of-flight neutron diffraction performed on the purest samples. Neutron diffraction data were collected on the diffractometer Polaris at the Rutherford Appleton Laboratory, Oxford, UK. Structural refinements were achieved using the TF15LS program based on the Cambridge Crystallographic Subroutine '~.

3. RESULTS AND DiSCUSSiON

The attempted synthesis of a phase pure product of stoichiometry (Ln,Ce)~Sr2CuaO~. ~ proved unsuccessful. However a sample of composition (Ho,Ce)2SraCu~O~.~ was synthesised with a high level of purity (<10% impurities) comparable to that reported by Ichinose e t a l 3. Neutron diffraction refinements performed on this phase, although not satisfactory because of the impurities present, did indicate ~40% occupancy oft he Cul sites by cerium. Of the larger cations (Ln=Pr-Gd), necJymium was the only one suitable for neutron diffraction yet the phase could not be made with impurities of less than 59% (as observed by Kopnin et a l 5) and hence no neutron diffraction was possible which could have substantiated the fi~dings of Xopnin e t a ! .

0921-a534/94/S07.00 © 1994 - Elsevier Science B.V. All rights reserved. SSDI 0921-4534(94)01005-6

884 A.J Wright, C. Greaves/Physica C 235-240 (1994) 883-884

Table 1 Structural parameters for (Y~.,Ceoa)Sr~Cu~(Cuo6Ceo4)Os. ,

ATOM POS' X ' Y Z ..... B,so/ik 2 uNIT CELL OCCUPANCY

Y 4 e ½ ' h 0'20618(4) 0.50(2) 2.4 Cel 4e ½ ½ 0.20618(4) 0.50(2) 1.6 Cu2 4e 0 0 0.14584(4) 0.24(1) 4 Sr 4e ½ ½ 0.08348(4) 1.09(2) 4

Ce2 8h 0.0432(9) 0.0432(9) 0 1.40(8) 0.8 Cul 8h 0.0432(9) 0.0432(9) 0 1.40(8) 1.2 Ol 8j 0.319(1) ½ 0 § 1.88(5) 02 4e 0 0 0.0729(2) 1.19(3) 1.6 O3 8g 0 ½ 0.1576(1) 0.36(2) 4 04 4d 0 ½ ¼ 0.71(2) 4 O5 4e 0 0 0.0637(1) 1.19(3) 2.4 06 8g 0 __~ 0.1488(1) 0.36(2) 4 ....

§ Anistropic temperature factor (A 2) for atom O1: B~=3.1(2), B~=1.6(2), B~3=0.0(2) o a

a=b=3.81696(4)A, c=29.1139(3)A; I4/mmm R~=2.43%, R~=1.32%, R,=4.93%

The only single phase material obtained in this study was for the stoichiemetry Y~.~CeL~Sr~Cuz60 ~ with refined structural parameters obtained from neutron diffraction listed in Table 1. Initial refinements clearly indicated the joint occupancy of the Cul site by copper and cerium. Refinement of all cation site occupancies showed no significant change from their initial values assuming the composition Y~2Ceo~Sr~Cu2~Ceo~O~ and these occupancies were subsequently fixed to aid stability.

The results also indicate that the oxygens which surround the Cul sites are sinificantly displaced in order to accommodate the cerium. The O1 is in the equatorial position and the 02 and 05 are in the apical positions which are bonded to cerium and copper respectively. An oxygen content of 8.94 is implied and is consistant with an average copper oxidation state of +2.11, but no superconQuc~w~y has as yet ' . . . . . ~ .. . . . u ~ e ~ u u ~ r v e u .

4. CONCLUSIONS

Results from this study indicate that under ambient pressure conditions, syhtl~esis of fluorite block materials of the type (Ln,Ce)2Sr2Cu3Oo. ~ is not possible. However,

single phase products can be synthesised by assuming partial occupancy of the chain copper sites by cerium and this has been confirmed by neutron powder diffraction.

ACKNOWLEDGEMENTS

The authors would like to thank EPSRC for funding and for a studentship for AJW and to S. Hull and R. Smith for their assistance in collecting the neutron data.

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5) ~. M. Kopnin. A. L. Kharlanov, I. Bryntse al~d E. V. Ap.tipov. Phy:~ica C, 219 (1994) 47

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