X-Ray Scattering and the Phase Transition of KMnF₃at 184°K

Abstract

The phase transition that KMnF 3 exhibits at T c =184°K has been studied in detail by X-ray scattering. The anomalous scattering that accompanies the transition near T c is found to be anisotropic for q ∼0.05Å -1 . For these wave vectors therefore, the Ornstein-Zernike approximation for the density-density correlation function cannot describe the critical properties. The temperature dependence of the square of the frequency of the soft \(\varGamma_{25}\) mode at R is found to be reasonably well described by a Curie-law divergence. The space group of the condensed phase of KMnF 3 has been reexamined. The space group is found to be the tetragonal I 4/ m c m ( D 4 h 18 ) rather than the orthorhombic P b n m ( D 2 h 16 ) proposed by Beckman and Knox. The space groups of the condensed phases of KMnF 3 and SrTiO 3 are therefore identical.