Stress-induced ferroelectricity and soft phonon modes in SrTiO3

Abstract

By means of dielectric measurements and a Raman-scattering experiment, the uniaxial stress dependence of the ferroelectric and structural phonon modes in SrTi${\mathrm{O}}_{3}$ crystal has been studied at liquid-helium temperature. The ferroelectric phase transitions were induced by a stress normal to the (100) or (110) face. The inverse dielectric susceptibilities were found to change linearly with applied stress, and the phonon frequencies of corresponding ferroelectric modes were found to vary following the Lyddane-Sachs-Teller relation. These characteristics were analyzed by using the phenomenological free energy which contains as interaction terms ${Q}_{\ensuremath{\lambda}\ensuremath{\mu}}{X}_{\ensuremath{\lambda}}{(\stackrel{\ensuremath{\rightarrow}}{\mathrm{P}}\stackrel{\ensuremath{\rightarrow}}{\mathrm{P}})}_{\ensuremath{\mu}}+{R}_{\ensuremath{\lambda}\ensuremath{\mu}}{X}_{\ensuremath{\lambda}}{(\stackrel{\ensuremath{\rightarrow}}{\mathrm{\ensuremath{\Phi}}}\stackrel{\ensuremath{\rightarrow}}{\mathrm{\ensuremath{\Phi}}})}_{\ensuremath{\mu}}+{t}_{\ensuremath{\lambda}\ensuremath{\mu}}{(\stackrel{\ensuremath{\rightarrow}}{\mathrm{P}}\stackrel{\ensuremath{\rightarrow}}{\mathrm{P}})}_{\ensuremath{\lambda}}{(\stackrel{\ensuremath{\rightarrow}}{\mathrm{\ensuremath{\Phi}}}\stackrel{\ensuremath{\rightarrow}}{\mathrm{\ensuremath{\Phi}}})}_{\ensuremath{\mu}}$. From the behavior of the dielectric constants below the critical stress and that of the soft-phonon-mode frequencies above the critical stress, the coupling coefficients ${Q}_{\ensuremath{\lambda}\ensuremath{\mu}}$, ${R}_{\ensuremath{\lambda}\ensuremath{\mu}}$, ${t}_{\ensuremath{\lambda}\ensuremath{\mu}}$ and other parameters in the free energy have been determined consistently. Anticrossing between the ferroelectric and structural modes was observed for an oblique-wave-vector phonon. Anomalous increase of the damping of the total symmetric ferroelectric mode near the transition stress has been found and discussed.