Ab initio structure and zone-center phonons in LiNbO3

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 Li-6 isotope shift.