Structure optimization and frozen phonons in LiNbO3

The equilibrium ground-state structure of LiNbO3 in the paraelectric and fe rroelectric phases is fully optimized in a first-principles calculation usi ng the full-potential linearized augmented plane wave method. The equilibri um volume, cia ratio and all (four, in the ferroelectric phase) internal pa rameters are found to be in good agreement with the experimental data. Froz en 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 frequencie s for the A(1) modes is satisfactory (including the Li-6 isotope effect), a nd the displacement patterns are unambiguously attributed. For the (Raman i nactive) AZ modes, phonon frequencies and eigenvectors are predicted. (C) 1 999 Elsevier Science Ltd. All rights reserved.