ELSEVIER April 1994 MaterialsLetters 19 (1994) 177-183 On the transformation of Y2Cu205 into YCuO, A.J.S. Machado zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQP Faculdade de Engenharia Q uhica de Lorena, CF 16, 12600 Lorena, SP, Brazil R.F. Jardim Institute de Fisica, Universidade de Sfio Pauio, CP 20516, 01452-990 SGo Paula, SP, Brazil Received 6 January 1994; accepted 11 January 1994 zyxwvutsrqponmlkjihgfedcbaZYXWVUTS Abstract The pseudo-binary system YOI,s-CuOx was investigated in the range Y,,,Cur +O, (0.1 d w c 0.9) by X-ray powder diffraction, thermal analysis and iodometric titration. From X-ray diffraction results obtained on ~ly~stailine samples prepared at am- bient pressure and from different starting materials, sintering tem~mt~es, and reducing a~ospher~, two stoichiometric phases were confirmed: Y2Cu105 and YCu02. A combined analysis of the results from thermal analysis and from iodometric titration reveals that the YCu02 phase is retained at higher temperatures and reducing atmospheres, which is consistent with the presence of Cu+‘. The transformation of Y,Cu205 into YCuOz with increasing temperature and the formation kinetics of the YCuO, phase are discussed. Also, a YO,.,-CuO, phase diagram is proposed. 1. Introduction After the discovery of superconducting properties above 90 K in the Y-Ba-Cu-0 system [ 1 ] several efforts have been made to determine the phase rela- tions in the pseudo-bina~ phase diagrams Y01.5- CuO,, BaO-CuO,, and YO,,,-BaO f 2-6 J. Among these phase diagrams the one which has attracted some attention is the YO,.s-CuO,. This has occurred because of several discrepancies regarding the consti- tution of this phase diagram. First of all, the presence of two stoichiometric phases Y2Cu205 and YCuOz is still object of some controversy. Earlier studies in the YO,.s-CuO, phase diagram by Schmitz-DuMont and Kasper [ 2 ] revealed the presence of only one stoichi- ometric phase, Y2Cuz05. This phase was confirmed by Arjomand and Mach [ 3 1, based on X-ray dif- fraction measurements made in single crystals. It was proposed that the Y,Cu,O, phase crystallizes in an orthorhombic structure (space group Pna2 1) with lattice parameters a=10.799 A, b=3.496 A and c= 12.456 A. More recently, an additional stoichio- metric phase in the YO1.s-CuOx phase diagram was proposed by Ishiguro et al. [ 41. This new phase was found to have the stoichiomet~ YCu02, with Dela- fossite-type c~stallographic structure, isost~ctural with AlCu02. The hexagonal crystallographic struc- ture, space group Pb3/mmc, was met to have lattice parametersaz3.521 Aandc=11.418 A. On the other hand, the existence of the YCu02 phase was not confirmed by several groups. Roth and collaborators [ 5 1, in their preliminary X-ray diffrac- tion results, confirmed only the presence of the Y2CuzOs phase. Similar results were observed exper- imentally by Zhang and Osamura [ 61, and through the~odynamic calculations of the YOI.s-CuOx phase diagram by Lee and Lee [ 71. Contmdicto~ly, the presence of the YCu02 phase in the YO,.J-CuO, phase diagram was proposed by Wiesner et al. [ 8 ] mostly based on coulometric titration results. 0167-577x/94/%07.00 0 1994 Elsevier Science B.V. All rights reserved Z%SDIO167-577x(94)00032-1