Ab initiostructure and zone-center phonons inLiNbO3

Abstract

The TO $\Gamma$ phonon frequencies are calculated in the frozen phonon approximation from first-principles calculations using the full-potential linearized augmented plane-wave method. A good agreement with the experimental data available for the $A_1,$ $A_2,$ and E modes is found. The largest deviations from experiment, obtained for the softest and the hardest of the $A_1$ modes, are shown to be due to anharmonic effects. The displacement pattern of different modes becomes available from the calculated eigenvectors. For the silent $A_2$ mode, a full set of phonon frequencies and eigenvectors is predicted from the calculation. The attribution of experimentally measured E modes is proposed on the basis of their different behavior with respect to the ${}^6\mathrm{Li}$ isotope shift.