Abstract
A polycrystalline single-phase sample with nominal composition Bi24PbO37was synthesized from Bi2O3and PbO by a high-temperature solid state reaction at 690 °C for 1.5 h. The compound adopts Bi12SiO20-type structure [cubic, space groupI23 (No. 197);a = 10.24957(3) Å] and was refined toRp = 7.96%,Rwp = 10.4%,Rexp = 8.43%,RB = 3.06%, andS = 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 24fsite and the partly occupied 8csite, with the unit-cell content (Bi23.68Pb0.32)(Bi1.28Pb0.72)O38.48. The structural constraints favor a preference of Pb2+ion for the 8csite, i.e. only 1.3% of Bi3+is substituted by Pb2+at the 24fsite and 36% at the 8csite. At the 24fsite, 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 8csite, the cations with three oxide ions form a trigonal pyramid with the cations at the apex.
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