The effects of temperature and hydrostatic pressure on the real (${\epsilon }^{\prime }$) and imaginary (${\epsilon }^{\prime \prime }$) parts of the static dielectric constant of single crystals of Pb${\mathrm{F}}_2$, Ba${\mathrm{F}}_2$, Sr${\mathrm{F}}_2$, and Ca${\mathrm{F}}_2$ were investigated. In all cases ${\epsilon }^{\prime }$ decreases with increasing pressure. However, unlike the alkaline-earth fluorides and other normal ionic dielectrics where ${\epsilon }^{\prime }$ increases slowly with increasing temperature, ${\epsilon }^{\prime }$ of cubic Pb${\mathrm{F}}_2$ exhibits a relatively large decrease with increasing temperature and obeys a Curie-Weiss law over a substantial temperature range. It is suggested that this anomalous temperature dependence and the large value of ${\epsilon }^{\prime }$ (∼30) are associated with a soft longwavelength transverse optic phonon, i.e., a ferroelectric (FE) mode, although the crystal remains stable with respect to this mode down to the lowest temperatures. This is the first example of a soft FE mode in a crystal having the relatively simple cubic fluorite structure. The measured isobaric temperature dependence of ${\epsilon }^{\prime }$ is separated into its pure-volume and pure-temperature contributions. For the alkaline-earth fluorides both contributions are positive and additive, whereas for cubic Pb${\mathrm{F}}_2$ the pure-temperature contribution is negative and dominates the measured ${\epsilon }^{\prime }\mathrm{}\left(T\right)\mathrm{}$ response. This is attributed to the dominance of quartic anharmonicities in this crystal—a circumstance similar to that in other soft-FE-mode crystals. The Szigeti effective charges were calculated and the validity of the Lyddane-Sachs-Teller relationship was tested for all crystals. The effects of the pressure-induced cubic ($\mathrm{Fm}3\mathrm{}m-O_h^5$) to orthorhombic ($\mathrm{Pm}nb-V_{2h}^{16}$) phase transition on the dielectric properties of Pb${\mathrm{F}}_2$ and Ba${\mathrm{F}}_2$ were also investigated, and the possible lattice-dynamical origin of the transition is briefly discussed. In Pb${\mathrm{F}}_2$ the orthorhombic phase is recovered at ambient conditions after releasing the pressure, and the effects of temperature and pressure on the dielectric properties of this phase were investigated. Unlike cubic Pb${\mathrm{F}}_2$, the orthorhombic phase behaves as a normal ionic dielectric. At $T\ge 300$°K the dielectric loss in both phases of Pb${\mathrm{F}}_2$ becomes very large due to the high ionic conductivity of the material. The activation energies deduced from the loss data are in excellent agreement with those obtained from ionic conductivity measurements.

• Received 5 November 1975

DOI:https://doi.org/10.1103/PhysRevB.13.4529