Pressure and temperature dependence of the dielectric properties and phase transitions of the ferroelectric perovskites: PbTiO3 and BaTiO3

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Abstract
The effects of temperature and hydrostatic pressure on the ferroelectric (FE) properties of PbTiO3 and on certain properties of BaTiO3 which have not been treated or emphasized earlier were investigated. For PbTiO3, the FE-paraelectric (PE) transition is of first-order, and the transition temperature, Tc, decreases with pressure with an initial slope of -8.4 ± 0.3°K/kbar. It is estimated that the latent heat associated with the transition is 350 ± 45 cal/mole, with a corresponding entropy change of 0.46 ± 0.05 cal/mole°K. In the PE phase, the static dielectric constant, ε, obeys the Curie-Weiss law ε = C/(T- T0) over a wide temperature range. Both C and T0 decrease with pressure. At constant % e varies with pressure according to ε = C*/(p-po), where p0 decreases strongly with increasing T while C* is very weakly T-dependent. The results can be explained in terms of an increase of the frequency of the soft FE mode with pressure, and the Gruneisen parameter and its Tdependence are determined for this mode. The pressure dependence of the spontaneous polarization is estimated. For BaTiO3, the orthorhombic-rhombohedral transition temperature decreases with pressure at a rate of -1.2°K/kbar. The pressure dependence of the a-axis dielectric constant in the tetragonal phase was determined. The change, with increasing pressure, of the nature of the FE-PE transition from first- to second-order in both PbTiO3 and BaTiO3 is discussed.