Polymorphism in Bi5Pb3O10.5

Abstract

This paper describes the phase and structural relations between three polymorphs of Bi5Pb3O10.5. The low-temperature stable phase named beta(L) crystallizes in the triclinic system with a = 14.903(1) Angstrom, b = 14.184(1) Angstrom, c = 7.2115(7) Angstrom, alpha = 97.216(8)degrees, beta = 118.434(6)degrees, gamma = 80.647(8)degrees and Z = 4. The unquenchable high-temperature stable phase labeled beta forms a solid solution and has an anti-alpha-AgI-type structure with the space group Im3m; beta-Bi5Pb3O10.5 has a = 4.40 Angstrom at 600 degrees C and Z = 2 (Bi5/8Pb3/8O10.5/8). The metastable phase termed beta(2) also forms a solid solution and crystallizes in the tetragonal system with the possible space group P4(2)/n; beta(2)-Bi5Pb3O10.5 has a = 12.132(1) Angstrom, c = 20.059(2) Angstrom, and Z = 9. The beta(L) phase transforms to the beta phase at about 585 degrees C; on subsequent cooling, at about 560 degrees C, the beta phase changes to the beta(2) phase which is kept to room temperature. The beta(2) phase reverts to the beta(L) phase by annealing at low temperatures, e.g., at 500 degrees C. The structure of beta(L)-Bi5Pb3O10.5 is based on a pseudo-bcc beta-type subcell, and the transformation matrix is (-2, 2, 2)/(1, -1, 3)/(3/2, 1/2, -1/2). The structure of the beta(2) phase is connected to that of red tetragonal PbO-type structure according to (3, 0, 0)/(0, 3, 0)/(0, 0, 4). From the viewpoint of the lone pair of electrons on Bi3+ and Pb2+, the metastability of the beta(2) phase is discussed. (C) 1999 Academic Press.