The four A1-TO Gamma phonon frequencies in lithium tantalate are calculated in the frozen-phonon approach from first principles using the full-potential linearized augmented plane wave method. A good agreement with the experimental data available is found for all modes; reliable displacement pattern of different modes becomes available from the calculated eigenvectors. The Raman spectra recorded for A1 modes in LiNbO3 exhibit a counter-intuitive softening of the A1-TO3 mode frequency with respect to that measured in LiTaO3. We explain this behaviour by a comparatively harder oxygen rotation in LiTaO3 and discuss other differences in lattice dynamics of these two materials, namely a notably delocalization of Ta and Li contributions over more that one corresponding mode in LiTaO3, which is different from the situation in lithium niobate. The Li isotope shift is predicted in the calculation.
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