Phase Transitions Studied by High-Pressure Dielectric Spectroscopy and Calorimetry

Abstract

The application of an electric field to a capacitor containing a dielectric material induces displacements of the atomic and molecular charges leading to polarisation of the medium. There are several mechanisms responsible for this effect. The so called electronic polarisation is due to the displacements of the electron clouds centres. This contribution is extremely fast with resonant frequencies in the ultraviolet or visible range of the electromagnetic spectrum. The other type of polarisation, named atomic or ionic polarisation, occurs when the oppositely charged ions are displaced from their equilibrium sites creating induced dipole moments. The resonant frequencies of these effects are observed in the infrared and short microwave ranges. The electronic and atomic contributions result from field-induced dipole moments, but there are many materials containing permanent electric dipoles. The alignment of these dipoles in the direction of the applied field gives rise to the dipole polarisation. While the electronic and atomic polarisation can be considered as instantaneous, the orientations of the permanent dipoles are time-dependent. Thus, the phenomena related to the dipole polarisation have non-resonant, relaxational nature. Most often they occur in the radiofrequency range that is convenient for experimental studies. In general, the polarisation vector due to all contributions may be unable to follow the change of the ac electric field.