Study of YBa 2 Cu 3 0 7 _ x reaction kinetics by Rietveld method Maria Cristina Comunian Ferraz Faculdades Integradas de Jau, Jau, S.P., Brazil Heitor Cury Basso a) Departamento de Engenharia Eletrica—EESC—USP, C.P. 359, CEP 13560-970 S. Carlos, S.P., Brazil Yvonne P. Mascarenhas Institute de Fisica de Sao Carlos, USP, S. Carlos, S.P., Brazil (Received 12 February 2000; accepted 29 November 2000) Using a recent proposed analysis procedure for quantitative phase determination by X-ray powder diffraction, YBa 2 Cu 3 O 7 _^ solid state formation reaction kinetics at 900 °C was studied. Although there was the presence of partial amorphous components, it was possible to determine a reaction route for the synthesis of the title compound from X-ray powder diffraction data collected at various stages of the thermal treatment and using the Rietveld method for the quantitative determination of the phase composition. © 2007 International Centre for Diffraction Data. INTRODUCTION The study of a compound reaction kinetics is one of the fundamental points in the optimization of its production. To achieve it, it is very important to have the exact determina- tion of the relative amount of each present phase during the reaction process. For quantitative analysis, the X-ray powder diffraction is the most used technique. In a simple analysis, from the com- parison of the relative intensities of the diffraction peaks, the variation of the relative amounts of the present phases can be determined in the sample. With the use of more sophisticated mathematical analyses, like the Rietveld Method (Young, 1992), it is possible to do the determination of the percent- ages in mass of the different present phases (Hill and Howard, 1987) with better precision, since even preferred grain orientation can be corrected. However, X-ray powder diffraction has a limitation: the amorphous part present in the sample cannot be quantified. That limitation can be eliminated by the application of a new procedure (Ferraz and Basso, 1996), using as a constraint the mol number conservation of the chemical elements present in the reaction. We applied here this new procedure to study the reaction kinetics of YBa2Cu 3 O 7 __,. at 900 °C in oxygen atmosphere, starting with Y 2 Cu 2 O 5 , Y 2 O 3 , BaCuO 2 , and CuO. The results are used to the reaction mechanism deter- mination. It is well known (Wu et al, 1987; Hazen et al, 1987) that YBa 2 Cu 3 O 7 _j can be obtained by heating finely pow- dered stoichiometric amounts of Y 2 O 3 , CuO, and BaCO 3 . However, the slow decomposition of BaCO 3 results in long calcination times (Ruckenstein et al., 1989). Another way to produce YBa 2 Cu 3 0 7 _^ is using already reacted BaCO 3 and CuO as BaCuO 2 (Fraser and Clarke, 1987; Roth, Davis, and Dennis, 1987), together with Y 2 O 3 , Y 2 Cu 2 O 5 , Y 2 BaCuO 5 , and CuO, where the BaCO 3 decomposition is avoid. Ruck- enstein et al. (1989) have even studied the kinetics of these reactions with X-ray powder diffraction quantitative charac- terization, proposing a series of reaction mechanisms; yet, only peak-intensity characterization was used, which results, even with standards, in a poor absolute quantitative analysis of samples. Particularly, they reported a reaction involving the direct transformation of Y 2 Cu 2 O 5 and BaCuO 2 in YBa 2 Cu 3 0 7 _,: Y 2 Cu 2 0 5 +4BaCuO 2 + x jO 2 =>2 YBa 2 Cu 3 O 7 _ x . Later, however, Flor et al. (1990) reported the appear- ance of 211 (Y 2 BaCuO 5 ) during Y 2 Cu 2 O 5 and BaCuO 2 re- action, as a intermediary product: Y 2 Cu 2 O 5 +4BaCuO 2 =>Y 2 BaCuO 5 a 'Author to whom correspondence should be addressed. Electronic mail: hcbasso@sel.eesc.sc.usp.br As far as we know, no further work has been published about this point. Due to the presence of semi-amorphous phases, it is a good problem to be solved with our procedure. EXPERIMENTAL AND RESULTS The synthesis of YBa 2 Cu 3 0 7 was performed with the following start molar composition: 0.47Y 2 Cu 2 O 5 +0.53(Y 2 O 5 +2CuO)+4BaCuO 2 , where Y 2 Cu 2 O 5 was already prepared reacting Y 2 O 3 with CuO, and BaCuO 2 reacting BaCO 3 with CuO at the proper amount, temperature, and reaction time according to the pro- cedure described by Kwetroo et al. (1974). The products were analyzed by XRD and showed no residual peaks. In all reactions the reagents were of high purity [Y 2 O 3 99.9% from SIGMA, BaCO 3 99% from MERCK, and CuO obtained by thermal decomposition of Cu 2 (OH) 2 CO 3 99% from Carlo Erba], ground and screened through a mesh 200 sieve. The mixture was then thoroughly homogenized and pressed to form pellets and subsequently taken to a horizon- tal oven in alumina crucibles. The samples were then treated in an oxygen atmosphere at 900 °C for 0.5, 4.0, 8.8, 13.0, and 23.2 h. As will be shown later, the maximum reaction time has been chosen to keep BaCuO 2 as the major phase. All X-ray diffractograms were obtained in the following conditions: the samples were ground down to powder, sifted through a mesh 200 sieve, spread over glass plates previ- ously covered with a thin sheet of silicone grease and taken 25 Powder Diffraction 16 (1), March 2001 0885-7156/2001/16(1 )/25/5/$18.00 ) 2001 JCPDS-ICDD 25