X-Ray Scattering and the Phase Transition of KMnF₃ at 184°K

Abstract

The phase transition that KMnF₃ 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 Γ₂₅ mode at R is found to be reasonably well described by a Curie-law divergence. The space group of the condensed phase of KMnF₃ has been reexamined. The space group is found to be the tetragonal I4⁄mcm (D_4h¹⁸) rather than the orthorhombic Pbnm (D_2h¹⁶) proposed by Beckman and Knox. The space groups of the condensed phases of KMnF₃ and SrTiO₃ are therefore identical.