Stabilization of the paraelectric phase of KTaO3and SrTiO3by strong quartic anharmonicity

The isobaric temperature dependence of the low frequency dielectric response of KTaO 3 and SrTiO 3 is reported for the range 2-300 K together with its isothermal pressure dependence for hydrostatic pressures up to 12 kbar in the range 77-300 K. The results are used to show that the paraelectric phase of these materials is stabilized by strong interactions of the soft mode through the quartic anharmonicity. The low frequency anharmonic self energy of the soft mode in both materials dominates both the thermal strain and the harmonic contribution, this latter being imaginary. At temperatures above 290 K it appears that second order quartic interactions may be becoming important. A comparison with the theoretical calculations of Cowley for SrTiO 3 shows reasonable agreement with the rate of increase with temperature of both the thermal strain and the anharmonic self energy contributions to the soft mode.