Pressure as a probe of dielectric properties and phase transition of doped quantum paraelectrics:Sr1−xCaxTiO3(x=0.007)

Abstract

Pressure is shown to have a very strong influence on the glass-like (relaxor) dielectric response of ${\mathrm{Sr}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{TiO}}_{3}$ $(x=0.007),$ or SCT (0.007), reducing the dielectric susceptibility and lowering the glass transition temperature, ${T}_{m},$ at the very rapid rate of \ensuremath{\sim}35 K/kbar. Above 0.5 kbar there is complete suppression of the transition and relaxor state and evolution of a quantum paraelectric state at low temperatures. These effects can be understood in terms of the increase of the ferroelectric soft mode frequency, ${\ensuremath{\omega}}_{s},$ with pressure which strongly reduces the polarizability, and thereby the correlation length, ${r}_{c},$ for dipolar interactions. It is estimated that above 0.5 kbar ${r}_{c}$ becomes significantly smaller than the average separation between neighboring ${\mathrm{Ca}}^{2+}$ ions (2.1 nm) leading to loss of overlap and correlations between adjacent nano-domains and loss of the relaxational character of the dielectric response. While pressure lowers ${T}_{m},$ the application of a dc biasing electric fields raises ${T}_{m}$ and can reinstate a ferroelectric state at modest pressures. The interplay between pressure and dc field provides insights into the physics. The very large pressure and field effects observed are attributed to the small characteristic energies of the crystal in the quantum regime.