Pressure and temperature dependences of the ionic conductivities of cubic and orthorhombic lead fluoride (PbF2)

Abstract

A detailed study of the effects of temperature and hydrostatic pressure on the ionic conductivity of PbF2 in both the cubic (Fm3m-Oh5) and orthorhombic (Pmnb-V2h16) phases was performed with some emphasis on the changes in conductivity accompanying the transition between the two phases. Both phases exhibit relatively high conductivities and become supersonic conductors at high temperatures. As a function of temperature, the conductivity in both phases exhibits a number of activated regions (or stages). Measurements on a variety of samples with different impurities, along with earlier published data, have allowed a determination of the conduction mechanism in the various stages. The activation energies for the motion of F− ion vacancies and interstittals as well as the formation energies of Frenkel defects in the two phases were determined. These energies are relatively small, compared, e.g. with other crystals having the fluorite structure (Fm3m-Oh5), and this can be explained in terms of the large dielectric constants and relatively soft phonons in PbF2. The conductivity decreases with pressure in all stages, primarily as a result of the increase in activation energies. The pressure results allow a determination of the activation volumes associated with the various conduction mechanisms. The magnitudes of these volumes (∼2–7 cm3/mole) are consistent with values determined from either the strain energy model or an approximate dynamical model. Differences in the activation volumes for the different conduction mechanisms and between the two phases can be qualitatively understood on the basis of the details of the crystal structures.