Multimode transitions of the tetrahedral MO4 units (M=Si,Ge,Ti) in sillenite single crystals -: art. no. 174307

Multimode transitions of the tetrahedral unit modes in Bi12SiO20 (BSO), Bi12GeO20 (BGO), Bi12TiO20 (BTO), and mixed (1-x)BSO.xBGO sillenite crystals have been studied using Fourier transform infrared absorption spectroscopy, with a resolution as fine as 0.04 cm(-1) in the 600-3500-cm(-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.