High temperature orbital order melting in KCrF3 perovskite

Abstract

The effect of high temperatures on the structural properties of the ternary transition metal fluoride KCrF₃ has been investigated by high-resolution synchrotron X-ray powder diffraction. The tetragonal room temperature structure (a>c/√2) exhibits large cooperative Jahn–Teller distortions of the CrF₆ octahedra which are driven by the antiferrodistortive orbital ordering of the 3d_3x²−r² and 3d_3y²−r² orbitals in the ab plane. At T_JT ≈ 973 K, KCrF₃ undergoes a first-order structural phase transition to an orbitally disordered cubic phase comprising equivalent Cr–F bond lengths and regular CrF₆ octahedra. The onset of the orbitally disordered state is accompanied by an abrupt volume collapse, δV/V = 0.44%, which arises from the more efficient packing of the CrF₆ octahedra in the orbital liquid state.