Vibrational spectra of bismuth oxide and the sillenite-structure bismuth oxide derivatives

Abstract

Bismuth oxide is monoclinic, space group P2 1/c. The sillenite compounds are impurity stabilized derivatives with general formula MO x·6Bi 2O 3, body-centered cubic with space group 123. Far i.r. and Raman spectra have been measured on polycrystalline Bi 2O 3 and on sillenite structures in which the stabilizing oxide is SiO 2, GeO 2, TiO 2, Ga 2O 3, ZnO, or PbO. About half the predicted 24 i.r. bands and Raman bands are observed for the sillenite structures. Low frequency Raman bands are very sharp, but high frequency bands are greatly broadened. This is interpreted in terms of an orientational disorder of the lone-pair non-bonding orbital of Bi 3+ in the bismuth—oxygen framework. Although an isolated regular tetrahedron, MO 4, exists in the structure, internal modes from the tetrahedron are not identifiable, indicating a strong coupling with the framework. A cluster of relatively narrow bands in the i.r. spectra in the range of 400–650 cm −1 can be correlated with the various BiO bonds in the highly distorted polyhedra that make up the bismuth—oxygen framework.