Optical vibrational spectra of acoustooptical NaBi(MoO4)2 crystals

Polarized normal reflection IR (300-1000 cm(-1)) and Raman (0-1000 cm(-1)) spectra of acoustooptical NaBi(MoO4)(2) crystals are experimentally studied. A group-theoretic analysis of vibrational modes is performed, and frequencies and decay of IR- and Raman-active modes are determined using the model of weakly coupled oscillators. All the vibrational modes observed are assigned. It is found that in the region of internal vibrations (both in IR and Raman spectra) a greater number of modes is observed than predicted by the group-theoretic analysis. This was attributed to the presence of structural defects of two types: the MoO4 tetrahedra distorted because of the lattice disorder over Na and Bi, and also nonstoichiometry over Mo. Raman scattering cross sections are measured for a number of vibrational modes, in particular, for lattice modes, and are found to be comparatively high (by a factor of 10(3) higher than those for CaCO3). This was attributed to the high acoustooptic activity of the crystals studied.