Abstract

Density functional theory calculations of the vibrational modes of Bi12SiO20 are presented, with a detailed assignment and geometry investigation of the vibrational modes in the sillenite structure. We show that the symmetry and geometry of the strong Raman peaks allow direct probing of the Bi-O(1) and Bi-O(2) bonds. The physical significance of the vibrational modes is considered to discuss experimental data on the trivalent sillenites and show evidence of O(1) and/or O(2) vacancies in the Bi-O framework. The infrared modes are also discussed to motivate future systematic studies of sillenites. The Born effective charges of sillenites are studied and the results show the existence of large and anisotropic charges. The magnitude of the LO-TO splits is also calculated and compared with experiment.