Knowledge Bank

Bismuth oxide is monoclinic, space group $P21/c$, The sillenite compounds are impurity stabilized derivatives with general formula $MOX$ $6Bi2O3$, body-centered cubic with space group I23. Far infrared and Raman spectra have been measured on polycrystalline $Bi2O3$ and on sillenite structures in which the stabilizing oxide is $SiO2GeO2,TiO2,Ga2O3,ZnO$, or PbO. About half of the predicted 24 IR bands and band Raman bands are observed for the sillenite structures. Low frequency Raman bands are very sharp but high frequency bauds are greatly broadened. This is interpreted in terms of an orientational disorder of the lone-pair non-bonding orbital of $Bi3+$ in the bismuth-oxygen framework. Although an isolated regular tetrahedron, $MO4$, exists in the structure, internal modes from the tetrahedron are not identifiable, indicating a strong coupling with the frame-work.