Multimode transitions of the tetrahedralMO4units(M=Si,Ge,Ti)in sillenite single crystals

Abstract

Multimode transitions of the tetrahedral unit modes in ${\mathrm{Bi}}_{12}{\mathrm{SiO}}_{20}$ (BSO), ${\mathrm{Bi}}_{12}{\mathrm{GeO}}_{20}$ (BGO), ${\mathrm{Bi}}_{12}{\mathrm{TiO}}_{20}$ (BTO), and mixed $(1\ensuremath{-}x)\mathrm{BSO}\ensuremath{\cdot}x\mathrm{BGO}$ sillenite crystals have been studied using Fourier transform infrared absorption spectroscopy, with a resolution as fine as 0.04 ${\mathrm{cm}}^{\ensuremath{-}1}$ in the 600--3500-${\mathrm{cm}}^{\ensuremath{-}1}$ wave number and 9--300-K temperature ranges. Up to four-mode transitions (overtones of the F mode and combinations of F and A modes) were detected in BGO, BSO, and BTO, and up to three-mode transitions in the mixed crystals. The F-mode anharmonicity parameter, evaluated in the framework of the Morse oscillator model, was in the range 0.003--0.006. The A-mode anharmonicity was found to be still lower. Fine structures of the F-mode absorption were observed and attributed to the LO-TO splitting of the F mode; in the case of BSO, additional structures due to the natural abundance of Si isotopes were detected. The transitions monitored in the mixed crystals were well described by a two-mode behavior. The temperature dependence of the line position and width was studied as well. ``Red'' and ``blue'' shifts were observed for different lines: the results were analyzed by considering thermal expansion and anharmonicity effects.