Determination of the critical exponents at the R-point instability in KMnF3

Abstract
X-ray scattering measurements have been performed on the perovskite KMnF3 to investigate the critical behaviour at the cubic-to-tetragonal structural phase transition at 186 K. The critical exponents describing the temperature dependence of the order parameter, the correlation length and the susceptibility have been determined. An anomalous second component was observed in the scattering cross section close to the transition temperature, in addition to the anisotropic diffuse scattering from the softening R25 normal mode. The anomalous component was well described by an isotropic Lorentzian-squared lineshape.