Cation disorder in ferroelectric PbBi₂Nb₂O₉

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, λ = 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.

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