Ab initio zone-center phonons in LiTaO3:: Comparison to LiNbO3 -: art. no. 224303

The four A(1)-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. Good agreement with the experimental data available is found for all modes; a reliable displacement pattern of different modes becomes available from the calculated eigenvectors. The Raman spectra recorded for A(1) modes in LiNbO3 exhibit a counterintuitive softening of the A(1)-TO3 mode frequency with respect to that measured in LiTaO3. We explain this behavior by a comparatively harder oxygen rotation in LiTaO3 and discuss other differences in the lattice dynamics of two materials, notably delocalization of Ta and Li contributions. over more that one corresponding mode in LiTaO3, differently from the situation in lithium niobate. The Li isotope shift is predicted in the calculation.