Structural transition in KMnCrF6 – a chemically ordered magnetic ferroelectric

Abstract

The structure of the tetragonal tungsten bronze fluoride KMnCrF₆ was investigated via high-resolution synchrotron powder diffraction at variable temperatures (T = 5–950 K). A structural transition is observed at T = 400 K from a high-temperature centrosymmetric phase (P4₂/mbc) to an orthorhombic non-centrosymmetric phase (Ccc2). The phase transition is driven by the cooperative tilt of the MF₆ octahedra and displacements of Mn²⁺ and Cr³⁺ along the polar c-axis. The polar structure and the low temperature ferrimagnetic behaviour (T_N = 23 K) make KMnCrF₆ a potential multiferroic material. The room temperature orthorhombic structure is robust upon application of high pressures up to 18 GPa with a bulk modulus K = 49.2(7) GPa.