Melting of orbital ordering in KMgxCu1-xF3 solid solution

The long range and short range structure of KMgxCu1-xF3 (0<x<1) has been investigated by means of XRPD and EPR. Two different solid solutions are present, based on the structure of KMgF3 (for x>0.42) and of KCuF3 (for x<0.25), respectively and they are separated by a biphasic zone. Positional disorder is induced by doping, due to the different Cu and Mg environments. In fact the EPR measurements have shown that the Cu environment is isotropic for x>0.8. It shows axial symmetry for 0.45<x<0.70 and orthorhombic symmetry for x=0.43. For x>0.42 the crystallographic structure is cubic and, in absence of local disorder, a fully isotropic octahedral undistorted environment is expected for Cu. In the tetragonal structure collective magnetic interactions arise and a progressive EPR signal symmetrisation is observed due to anisotropic exchange (AE) and to Dzialoshinsky – Moriya (DM) antisymmetric exchange processes. The mixing of triplet and singlet states induced by the above exchange mechanisms leads to the conclusion that the orbital order is melt in the x=0.1 sample, for which the cooperative Jahn-Teller distortion is still active and the 3D magnetic order is still antiferromagnetic as in KCuF3.