FTIR SPECTROSCOPY OF OH STRETCHING MODES IN BSO, BGO AND BTO SILLENITES

High-resolution (0.05 cm-1) FTIR spectroscopy in the temperature range 9-320 K has been used to monitor the narrow line absorption spectra. induced by the stretching modes of OH and OD impurities in as-grown and in-deuterated samples of BSO, BGO, mixed (1 - x)BSO.xBGO, and BTO sillenite single crystals (i.e. Bi12MeO20 with Me=Si, Ge and Ti). The model of the anharmonic oscillator in the Morse potential was applied to interpret the observed spectra and the anharmonicity parameter x, determined. For the line peaking at 3442.84 cm-1 in BSO both the fundamental and the first overtone transitions were detected: in this way the complete set of the Morse parameters could be determined. A model for the OH centre is proposed on the basis of the dependence of the line position against lattice constant in all the systems examined. The phonon coupling of the stretching modes observed was studied by analysing the detailed temperature dependence of the line position and width. In the framework of weak coupling theory the frequencies of coupled phonons were determined and found to be in agreement with those obtained by reflectance, Raman scattering measurements and theoretical calculations.