QUANTUM-MECHANICAL DESCRIPTION OF THE FERROELECTRIC PHASE-TRANSITION IN LITAO3

We describe the dielectric properties and the ferroelectric phase transition of LiTaO3 using a self-consistent quantum-mechanical model in which the lowest-energy A1 mode is described with a potential in a single unit cell that is coupled to the mean field of the other unit cells. The calculated temperature dependence of the spontaneous polarization and the dielectric function agree very well with experimental results. Previously observed weak resonances at 1 and 3 THz are identified as a tunneling resonance and overtone absorption in the potential. We find that for LiTaO3 the divergence of the static dielectric constant at the phase-transition temperature is not due to mode softening.