X-ray scattering study of the R-point instability in SrTiO₃

The wave-vector and temperature dependence of the X-ray critical scattering from nominally pure SrTiO 3 has been investigated above the 105K phase transition. Far above T c the form of the scattering is consistent with the expected anisotropic dispersion at the R-point although the correlation length has a slightly stronger temperature dependence than expected for a perfect crystal. Within 10K of T c , however, there is a marked qualitative change in the form of the critical scattering which shows simultaneously two different length scales for the fluctuations, suggesting inhomogeneity in the critical behaviour either as a result of dislocations or clusters of defects or of the presence of a surface layer having the distorted low-temperature structure above the bulk transition temperature. The experimental evidence appears to favour the extended defect model and suggests a connection with the central peak observed in inelastic neutron scattering experiments.