Publications

Pressure as a probe of the physics of 18O - substituted SrTiO3

Venturini, Eugene L.; Venturini, Eugene L.; Samara, George A.

Studies of the dielectric properties and phase behavior of an {sup 18}O-substituted SrTiO{sub 3} (>97% {sup 18}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 {sup 18}O-induced ferroelectric transition in this material. Dielectric measurements reveal an equilibrium phase transition (T{sub c} {approx_equal} 24K at 1 bar) and an enhancement of the static dielectric constant {var_epsilon} over that of normal (i.e., {sup 16}O) SrTiO{sub 3}, or STO-16, over a large temperature range above T{sub c}. This enhancement is quantitatively shown to be attributed to additional softening of the ferroelectric soft-mode frequency ({omega}{sub s}) of STO-16, in agreement with lattice dynamic calculations. Thus, in STO-18, two effects due to the heavier mass of {sup 18}O conspire to induce the transition: (i) this additional softening of {omega}{sub s} and (ii) damping of quantum fluctuations. Pressure lowers T{sub 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 {var_epsilon} 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.