Pressure and Temperature Dependences of the Dielectric Properties of the Perovskites BaTiO3and SrTiO3

The dielectric properties of single-crystal and ceramic BaTi${\mathrm{O}}_3$ and of single-crystal SrTi${\mathrm{O}}_3$ were investigated as functions of hydrostatic pressure to 25 kbar and temperature to 150°C. The Curie point of BaTi${\mathrm{O}}_3$ decreased linearly with increasing pressure, but at a rate which varied from -4.6 to -5.9°C/kbar for different samples. Its spontaneous polarization $P_s$ decreased slowly (<2% per kbar) up to the transition pressure, where it dropped rapidly. This change in $P_s$ is primarily due to the displacement of the Curie point to lower temperatures. At constant temperature, the dielectric constant $\epsilon$ in the cubic phase of both compounds obeys the relationship $\epsilon =\frac{C^{*}}{(p-p_0)}$. At 25°C, $C^{*}=2.8\mathrm{}\times {10}^4$ kbar and $p_0=18.5\mathrm{}$ kbar for pure single-crystal BaTi${\mathrm{O}}_3$, and $C^{*}=1.2\mathrm{}\times {10}^4$ kbar and $p_0=-40\mathrm{}$ kbar for single-crystal SrTi${\mathrm{O}}_3$. Both $C^{*}$ and $p_0$ change with temperature. Similarly, the Curie-Weiss constant $C$ and temperature $T_0$ change with pressure. The temperature dependence of $\epsilon$ is separated into volume-dependent and volume-independent contributions, and it is shown that the change of the infrared polarizability with temperature at constant volume is the predominant factor in determining the change in $\epsilon$.