Polymorphism in Bi5Pb3O10.5

This paper describes the phase and structural relations between three polym orphs of Bi5Pb3O10.5. The low-temperature stable phase named beta(L) crysta llizes in the triclinic system with a = 14.903(1) Angstrom, b = 14.184(1) A ngstrom, 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-tempera ture 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 = 4.4 0 Angstrom at 600 degrees C and Z = 2 (Bi5/8Pb3/8O10.5/8). The metastable p hase termed beta(2) also forms a solid solution and crystallizes in the tet ragonal system with the possible space group P4(2)/n; beta(2)-Bi5Pb3O10.5 h as a = 12.132(1) Angstrom, c = 20.059(2) Angstrom, and Z = 9. The beta(L) p hase transforms to the beta phase at about 585 degrees C; on subsequent coo ling, at about 560 degrees C, the beta phase changes to the beta(2) phase w hich 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 structu re of beta(L)-Bi5Pb3O10.5 is based on a pseudo-bcc beta-type subcell, and t he transformation matrix is (-2, 2, 2)/(1, -1, 3)/(3/2, 1/2, -1/2). The str ucture 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 o f the lone pair of electrons on Bi3+ and Pb2+, the metastability of the bet a(2) phase is discussed. (C) 1999 Academic Press.