Crystal structure and phase transitions of Sr1±xBi2±yTa2O9 ceramics

Crystal structural parameters, such as space group, lattice parameters, apparent valences (AV), and site occupancies were analyzed in Sr1 +/-xBi2 +/-yTa2O9 ceramics with varying ceramic compositions and diffraction temperature. The crystal structure was refined by Rietveld method using neutron and X-ray powder diffraction data. The phase transition temperature T-c was measured on these ceramics. The spontaneous polarization P-s was calculated from the refined parameters, The measured T-c decreased with increasing Sr-content. The crystal structure of Sr1Bi2Ta2O9 changed from A21am at room temperature to B2cb at temperature above 400 degreesC and to a higher symmetry one eventually. The cation place exchanges were observed between Sr2+ and Bi3+ and the calculated AVs were in accordance with the cation exchanges. (C) 2001 Elsevier Science Ltd. All rights reserved.