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, ±0.22Â from the high-symmetry site. This result provides quantitative understanding of the incomplete softening of the phonon mode. © 1979 The American Physical Society.