Impossibility of pressure-induced crossover from ferroelectric to nonergodic relaxor state in aPb(Mg1∕3Nb2∕3)0.7Ti0.3O3crystal: Dielectric spectroscopic study

Abstract

Relaxor behavior induced by hydrostatic pressure up to $0.95\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ in the $\mathrm{Pb}({\mathrm{Mg}}_{1∕3}{\mathrm{Nb}}_{2∕3}{)}_{0.7}{\mathrm{Ti}}_{0.3}{\mathrm{O}}_{3}$ (PMN-30PT) ferroelectric crystal was studied using dielectric spectroscopy. With increasing pressure we observed the decrease of the ferroelectric phase transition temperature $({T}_{C})$, the suppression and smearing of the dielectric anomaly at ${T}_{C}$, and the appearance of strong relaxorlike dielectric dispersion below the temperature of the permittivity maximum $({T}_{m})$. Such kinds of pressure-induced alteration are inherent in compositionally disordered perovskite ferroelectrics. It is usually believed to signify a crossover from the ferroelectric ground state to the nonergodic relaxor ground state in which the dipole moments of polar nanoregions (PNRs) are frozen in a way characteristic of dipole glasses. Surprisingly, our analysis of the dielectric spectra in PMN-30PT at high pressure did not reveal any glassy freezing of dipole dynamics. This means that the nature of the high-pressure-induced ground state is different from the nonergodic relaxor state observed in canonical relaxors at ambient pressure. At $T>{T}_{C}$ the dielectric spectra measured in PMN-30PT under different pressures are qualitatively similar. They are composed of two contributions that follow the Kohlrausch-Williams-Watts (KWW) and the Curie--von Schweidler (CS) relaxation patterns, respectively. The dielectric susceptibility related to the KWW relaxation provides the major contribution to the total dielectric constant. The shapes of the frequency and temperature dependences of this susceptibility remain practically unaffected by pressure. Contrary to the canonical relaxors the KWW relaxation time does not obey the Vogel-Fulcher law. On the other hand the CS-related susceptibility, which is significant only at low frequencies, considerably increases with increasing pressure and the shapes of its frequency and temperature dependences change radically. At $T<{T}_{C}$ the KWW and CS relaxation processes are not observed at ambient pressure, but persist at $0.8\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. The KWW characteristic relaxation time varies with temperature according to the Arrhenius law. We propose that the observed variation of properties results from the pressure-induced crossover from the sharp order-disorder-type ferroelectric phase transition, which is triggered by the cooperative interactions among dynamic (in the high-temperature phase) PNRs to the diffuse displacive-type ferroelectric transition, which is related to the growth of PNR dimensions.