  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. 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.
