High resolution FTIR absorption spectroscopy in the 600-3500 cm(-1) wa
venumber and 9-300 K temperature ranges was applied to study multi-pho
non transitions of the tetrahedral unit modes in BSO, EGO, BTO and mix
ed BSO-BGO sillenite crystals. Up to four-phonon (overtones of the F m
ode and combination of F and A modes) transitions were detected in EGO
, BSO, and BTO and up to three- in the mixed crystals. The F mode anha
rmonicity, evaluated in the framework of the Morse oscillator model, w
as in the range 0.0024-0.0056. Fine structures due to Si-isotopes were
observed in BSO. The two- and three-phonon transitions in the mixed c
rystals were well described by a two-mode behaviour.