High temperature orbital order melting in KCrF3 perovskite

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 3d3x2−r2 and 3d3y2−r2 orbitals in the ab plane. At TJT ≈ 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.