Order-Disorder Behavior at Displacive Structural Phase Transitions

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 Tc, the oxygen octahedra in SrTiO₃ oscillate about quasiequilibrium positions displaced by, typically, ±{}0.22^∘{} from the high-symmetry site. This result provides quantitative understanding of the incomplete softening of the phonon mode.