Order-Disorder Behavior at Displacive Structural Phase Transitions

Abstract

We show that a suitable electron paramagnetic resonance experiment can discriminate sufficiently against the predominantly harmonic fluctuations characterizing a system undergoing a displacive phase transition, that a cluster-induced order-disorder component of the local behavior is exposed. We find evidence that, at ${T}_{c}$, the oxygen octahedra in SrTi${\mathrm{O}}_{3}$ oscillate about quasiequilibrium positions displaced by, typically, \pm\else\textpm\fi{}0.22^\circ\else\textdegree\fi{} from the high-symmetry site. This result provides quantitative understanding of the incomplete softening of the phonon mode.