Rearrangement of orbitals in KAgF3 due to the Kugel-Khomskii mechanism: A neutron diffraction and density functional theory study

The crystal structure of KAgF3 was studied by powder neutron diffraction. KAgF3 exhibits at all temperatures an orthorhombic symmetry in space group Pnma that allows for several distortions with respect to the ideal cubic perovskite structure. At all temperatures there is a strong splitting of Ag-F distances parallel to the a,c planes that documents alternating occupation of holes in x2-y2 and z2-y2 orbitals. The orientation of the octahedron elongation and thereby the orbital order flips at a structural phase transition occurring around Ts=240K which is accompanied by a suppression of magnetic susceptibility. The orbital ordering is further enhanced in the low-temperature phase and the twisting of the AgF4 plaquettes forming the antiferromagnetic chains changes. Density functional theory (DFT) calculations show an enhancement of the magnetic interaction in the low temperature phase indicating that the transition and the orbital order are partially driven by the Kugel-Khomskii mechanism.