Ab initio structure and zone-center phonons in LiNbO3

The TO Gamma phonon frequencies are calculated in the frozen phonon approxi mation from first-principles calculations using the full-potential lineariz ed 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) mode s, are shown to be due to anharmonic effects. The displacement pattern of d ifferent modes becomes available from the calculated eigenvectors. For the silent A(2) mode, a full set of phonon frequencies and eigenvectors is pred icted from the calculation. The attribution of experimentally measured E mo des is proposed on the basis of their different behavior with respect to th e Li-6 isotope shift.