Vibrational frequencies of the impurity-centred oxygen tetrahedra in sillenites

Fundamental and multi-mode transitions of the tetrahedral unit modes (A1 and F2) in Bi12SiO20 (BSO), Bi12GeO20 (BGO), and Bi12TiO20 (BTO) sillenite crystals have been studied by measuring Raman scattering and FTIR absorption spectra. The ν1 and ν3 frequencies related to the A1 symmetric and F2 asymmetric stretching modes, respectively, were obtained for MO4 and ImO4 tetrahedral complexes, where M is the host matrix metal, i.e. Si4+, Ge4+, and Ti4+ and Im is an impurity substituting for M. The Im belong to three different isoelectronic series as (a) Al3+, Si4+, P5+, S6+, (b) Ga3+, Ge4+, As5+, Se6+, and (c) Ti4+, V5+, Cr6+. Ions with different valence charges, as Cr4+, Cr5+, Cr6+, and Mn4+, Mn5+, were also considered. Systematic trends were put in evidence for the ν1 and ν3 frequency pairs as a function of M (or Im) atomic mass and charge. They are discussed also in the framework of some general regularities found for metal tetra-oxo-complexes in a variety of materials.