High-resolution (0.05 cm-') m R 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 (1x ) ~s o ~x s c o .and BTO silleaite single crystals (i.e. BillMeOar 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 xc determined. For the line peaking at 3442.84 cm'l in BSO both the fundamental and the first overtone vansitions 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 weal coupling theory the frequencies of coupled phonons were determined md found to be in agreement with those obmned by reflectance, Raman scattering measurements m d theoreucal calculations.

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