Thermal Diffusion in Polar Crystals

Thermal diffusivity coefficient is measured directly using a spread of plane temperature waves excited by Peltier element. By this way, both self-ordering of polar bonds and their forced ordering in the bias electrical field are studied. The interaction of acoustic and optical phonons in the polar crystals manifests itself in two ways: (1) it gives an additional deposit into phonons scattering, observed as a deep minimum of diffusivity; (2) it makes contribution to heat transfer with an acute maximum of diffusivity seen in the antiferroelectrics, in which soft phonon mode vanishes in the middle of Brillouin zone and phase transition occurs with multiplication of lattice unit cell, so the optical phonons actively mix with the acoustic ones, participating in the heat transfer.