Pressure as a probe of the physics of18O-substitutedSrTiO3

Abstract

Studies of the dielectric properties and phase behavior of an ${}^{18}\mathrm{O}$-substituted ${\mathrm{SrTiO}}_3$ (>97% ${}^{18}\mathrm{O}),$ or STO-18, crystal at 1 bar and as functions of hydrostatic pressure and applied dc biasing electric field have shed much light on the mechanism of the ${}^{18}\mathrm{O}$-induced ferroelectric transition in this material. Dielectric measurements reveal an equilibrium phase transition ${(T}_c\simeq 24\mathrm{K}$ at 1 bar) and an enhancement of the static dielectric constant ${\epsilon }^{\prime }$ over that of normal (i.e., ${}^{16}\mathrm{O})$ ${\mathrm{SrTiO}}_3,$ or STO-16, over a large temperature range above $T_c.$ This enhancement is quantitatively shown to be attributed to additional softening of the ferroelectric soft-mode frequency $\left({\omega }_s\right)$ of STO-16, in agreement with lattice dynamic calculations. Thus, in STO-18, two effects due to the heavier mass of ${}^{18}\mathrm{O}$ conspire to induce the transition: (i) this additional softening of ${\omega }_s$ and (ii) damping of quantum fluctuations. Pressure lowers $T_c$ at the large initial rate of 20 K/kbar and completely suppresses the ferroelectric state leading to a quantum paraelectric state at ⩾0.7 kbar, confirming earlier results. Very large effects of a biasing dc electric fields on the peak temperature and ${\epsilon }^{\prime }$ are also observed in the quantum regime reflecting the small characteristic energies of the system. The results also reveal a dielectric relaxation process near 10 K with interesting properties. The implications of all the results on our understanding of the physics of STO-18 are discussed.