STUDIES ON THE BEHAVIOR OF THE INTERNAL VIBRATIONAL MODES IN (NH₄)₂Te_xSn_1-xCl₆ MIXED CRYSTALS

The behavior of the internal lattice vibrational modes in (NH₄)₂Te_xSn_1-xCl₆ mixed crystals has been investigated by Raman scattering. All of the Raman-active MX₆^2- internal modes show two-mode property in the mixed crystals with compositions from x=0 to 1, and apart from the sample with x=0.5, their energy positions are composition-independent. Space group calculations including site symmetry considerations and frequency fittings describe a good force field model. It has been demonstrated that the composition-independent two-mode behavior may come from the strong clustering effect, rather than from the character of the gap and local modes of impurities in mixed crystals. For the sample with x=0.5, the two component elements seem to form a pseudo-periodic lattice structure.