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

We describe the dielectric properties and the ferroelectric phase tran sition of LiTaO3 using a self-consistent quantum-mechanical model in w hich the lowest-energy A1 mode is described with a potential in a sing le unit cell that is coupled to the mean field of the other unit cells . The calculated temperature dependence of the spontaneous polarizatio n and the dielectric function agree very well with experimental result s. Previously observed weak resonances at 1 and 3 THz are identified a s a tunneling resonance and overtone absorption in the potential. We f ind that for LiTaO3 the divergence of the static dielectric constant a t the phase-transition temperature is not due to mode softening.