Vibrational modes and overlap matrix of LiNb1-xTaxO3 mixed crystals

The wave numbers and efficiencies of the first-order lattice vibrational modes of the isomorphous solid solutions of LiNbO3-LiTaO3 have been studied by Raman and infrared spectroscopies. The substitution of niobium by tantalum induces a continuous shift of wave numbers and intensities of phonon modes indicating a one-phonon behavior. In general, the wave numbers of vibrations at low frequencies (<325 cm(-1)) soften with the increased atomic mass. The frequencies of most of the high-frequency modes increase with the Ta/Nb ratio owing to enhanced force constants. A qualitative relation between the shift of transverse and longitudinal modes in the solid solutions has been found on the basis of an overlap matrix. The exceptional softening of several high-frequency modes with the replacement of Nb by Ta originates from the strong interaction of these modes with modes at low frequencies according to the overlap matrix. The intensities of the Raman modes are strongly modulated by the electronic properties with Ta/Nb substitution and this emphasizes their role in the physical properties of LiNbO3 and LiTaO3.