Crystal structure and phase transitions of Sr1 +/- xBi2 +/- yTa2O9 ceramics

Crystal structural parameters, such as space group, lattice parameters, app arent valences (AV), and site occupancies were analyzed in Sr1 +/-xBi2 +/-y Ta2O9 ceramics with varying ceramic compositions and diffraction temperatur e. The crystal structure was refined by Rietveld method using neutron and X -ray powder diffraction data. The phase transition temperature T-c was meas ured on these ceramics. The spontaneous polarization P-s was calculated fro m the refined parameters, The measured T-c decreased with increasing Sr-con tent. The crystal structure of Sr1Bi2Ta2O9 changed from A21am at room tempe rature to B2cb at temperature above 400 degreesC and to a higher symmetry o ne eventually. The cation place exchanges were observed between Sr2+ and Bi 3+ and the calculated AVs were in accordance with the cation exchanges. (C) 2001 Elsevier Science Ltd. All rights reserved.