Improved structural model of Pb-doped gamma-Bi2O3: (Bi23.68Pb0.32)(Bi1.28Pb0.72)O-38.48

Abstract

A polycrystalline single-phase sample with nominal composition Bi24PbO37 was synthesized from Bi2O3 and PbO by a high-temperature solid state reaction at 690 degrees C for 1.5 h. The compound adopts Bi12SiO20-type structure [cubic, space group I23 (No. 197); a = 10.24957(3) angstrom] and was refined to R-p = 7.96%, R-wp = 10.4%, R-exp = 8.43%, R-B = 3.06%, and S = 1.23. The distributions of Pb2+ and Bi3+ over cationic sites based on the X-ray powder diffraction data were determined using a combination of the Rietveld refinement and bond valence calculations. The results showed that the asymmetric unit contains two mixed cation sites: the fully occupied 24f site and the partly occupied 8c site, with the unit-cell content (Bi23.68Pb0.32)(Bi1.28Pb0.72)O-38.48. The structural constraints favor a preference of Pb2+ ion for the 8c site, i.e. only 1.3% of Bi3+ is substituted by Pb2+ at the 24f site and 36% at the 8c site. At the 24f site, the cations are surrounded by 5 + 2 or in a very small amount by 5 + 1 + 2 oxide ions, forming a base bicapped square pyramid or a bicapped highly deformed octahedron, respectively. At the 8c site, the cations with three oxide ions form a trigonal pyramid with the cations at the apex.