Acta Crystallographica Section B

Structural Science

Volume 52, Part 3 (June 1996)

research papers

Acta Cryst. (1996). B52, 432-439 [ doi:10.1107/S0108768195015072 ]

Cation disorder in ferroelectric PbBi₂Nb₂O₉

V. Srikanth, H. Idink, W. B. White, E. C. Subbarao, H. Rajagopal and A. Sequeira

Abstract: The ferroelectric Curie temperature, T_c, of PbBi₂Nb₂O₉ was found to shift from 833 K in samples slowly cooled from 1373 K to T_c's up to 883 K when the samples were quenched from various temperatures in the range 973-1373 K. The variation of T_c with heat treatment was fully reversible. Extra Raman modes appear and the width of some Raman bands change as a function of heat treatment. These results are interpreted in terms of cation order-disorder involving the Pb²⁺ and Bi³⁺ ions in the perovskite units and the Bi₂O₂ layer. Neutron Rietveld analyses of the structures (orthorhombic, A2₁am, Z = 4, [lambda] = 1.216) indicate a possible change in cationic order-disorder, with Pb²⁺ ions preferentially occupying the perovskite A-sites to the extent of 91 (7) and 80 (7)%, respectively, in the quenched and slowly cooled samples.