Thermal Conduction Across Ferroelectric Phase Transitions: Results on Selected Systems

A ferroelectric phase transition represents a special class of structural phase transition characterized by the appearance of a spontaneous polarization in the material. Above the Curie temperature the transition often follows a diverging differential dielectric response or permittivity , which varies with temperature in an approximate Curie-Weiss manner = C/(T-T0), where T0 is the Curie-Weiss temperature, which is equal to the Curie temperature Tc for a continuous transition. The crystalline phase which undergoes transformation to the ferroelectric form at Tc is the paraelectric one. Below Tc, in the absence of an applied field, there are at least two directions along which a spontaneous polarization can develop. To minimize the depolarizing fields different regions of the crystal polarize in each of these directions, each volume of uniform polarization being called a domain. The resulting domain structure usually results in a near complete compensation of polarization and the crystals consequently exhibit very small pyroelectric effects until they are poled by the application of a field. A ferroelectric transition is usually associated with the condensation of a soft (or lowfrequency) mode of lattice motion at the Brillouin-zone centre. Structural transitions triggered by zone-centre soft modes are generally termed ferrodistortive, and in this sense ferroelectrics constitute a subgroup of the class of ferrodistortive transitions. This subgroup involves the condensation of a polar or optically active mode whose condensation causes the appearance of a long rage polar order. If the transition is strongly first order then mode softening may not occur to a significant degree, and in this situation, there is also a possibility that the large polarization which sets in discontinuously at Tc may not be reversible, or the low temperature phase may be pyroelectric only. Ferroelectric transitions are categorized as being either displacive or order-disorder in character. This distinction is generally made in terms of whether the paraelectric phase is microscopically nonpolar (displacive) or only nonpolar in a macroscopic or thermally averaged sense (order-disorder). The displacive or order-disorder character is often defined in terms of the dynamics of the phase transition, as to whether the soft mode is a propagating or diffusive type respectively. The displacive or propagating soft mode is a damped optic phonon, representing small quasi-harmonic motion about the mean position, while the diffusive soft mode represents large amplitude thermal hopping motion between the domain wells. Although most ferroelectrics are ferrodistortive (common examples being barium titanate, sodium nitrite, and triglycine sulphate) some are not. To understand this it is necessary to recognize that, because of the existence of coupling between modes, it is not a necessary