Structure optimization and frozen phonons in LiNbO3

The equilibrium ground-state structure of LiNbO3 in the paraelectric and ferroelectric phases is fully optimized in a first-principles calculation using the full-potential linearized augmented plane wave method. The equilibrium volume, cia ratio and all (four, in the ferroelectric phase) internal parameters are found to be in good agreement with the experimental data. Frozen phonon calculations are performed for TO-Gamma phonons corresponding to the A(1) and A(2) irreducible representations of the R3c space group in the ferroelectric phase. The comparison with available experimental frequencies for the A(1) modes is satisfactory (including the Li-6 isotope effect), and the displacement patterns are unambiguously attributed. For the (Raman inactive) AZ modes, phonon frequencies and eigenvectors are predicted. (C) 1999 Elsevier Science Ltd. All rights reserved.