Long range structure and local Cu environment in KMg1-xCuxF3 solid solution probed by means of XRPD and EPR

KCuF3 is a Mott-Hubbard insulator with a pseudocubic perovskite structure. In its most common crystalline form, it belongs to the I4/mcm space group. The structural distortion is due to orbital ordering associated with cooperative Jahn-Teller effect [1]. CuF6 octahedra are elongated in the ab plane along a or b axis, in an antiferrodistorsive pattern. The distortion corresponds to an alternate occupation of Cu-3dy2-z2 and Cu-3dx2-z2 hole states on Cu(3d9) ion. The orbital configuration results in quasi one-dimensional magnetic properties. Nearest-neighbour superexchange (NN-SE) interactions are strong and antiferromagnetic (AF) along the c axis and, for T>TN=38 K, weak and ferromagnetic in the ab plane; the ratio between NN-SE along and perpendicular to c is |Jc|/Ja100 [2]. For T<38 K KCuF3 shows three-dimensional antiferromagnetic order. Melting of orbital order in KCuF3 is expected to occur well above room temperature [1], where the cooperative structural distortion should disappear and the undistorted cubic perovskite structure (space group Pm3m) should be favoured. Preliminary XRPD measurements carried out at beamline ID31 at ESRF did not detect any phase transition up to 730 °C. A system where melting of the orbital order could be more easily observed is the complete solid solution KMg1-xCuxF3 (0<x<1). At room temperature, the structure of this system is cubic (space group Pm3m) for low Cu concentration, while it is isomorphic with KCuF3 for high Cu concentration [3]. The cubic to tetragonal phase transition with increasing x is accompanied by the onset of a cooperative Jahn-Teller distortion [3]. In the present paper we present a combined XRPD and EPR study on the relation between long range structure, local Cu environment and magnetic properties of KMg1-xCuxF3 (0<x<1) solid solution as a function of x and T. KMg1-xCuxF3 is cubic (space group Pm3m) for x<0.63 while it is tetragonal (space group I4/mcm) for x>0.63. A miscibility gap is present for 0.63<x<0.73. As shown by EPR spectroscopy, Cu environment is isotropic for x0.2 in the cubic domain. Its symmetry becomes axial and then orthorhombic with increasing Cu concentration. Collective orbital ordering becomes more and more important increasing x in the tetragonal domain, causing a symmetrisation of the EPR lineshape.