Jahn-Teller distortion, octahedra rotations and orbital ordering in perovskites: KScF as a model system

The KScF $$ perovskite has been used as a model for investigating the relative importance of the Jahn-Teller (JT) lift of degeneracy, the ScF $$ octahedra rotation (OR), and the quadrupole-quadrupole interaction linked to different occupancy of the Sc t $$ subshell in various sites of the unit cell (orbital ordering, OO). The group-subgroup sequence $$ , $$ , $$ , and $$ , supplemented by $$ and $$ , has been explored by using an $$ Gaussian type basis set, hybrid functionals, and the CRYSTAL17 code. The JT lift of degeneracy provides a stabilization about 5 times larger than the sum of the OO and OR effects. The energy gained in the transition from $$ to $$ , consisting in a rotation of the octahedra around the $$ axis, is 1077 $$ E $$ . From $$ to $$ , additional rotations around the $$ and $$ axes are possible, and the $$ Sc electron can occupy a different t $$ orbital, with a total energy reduction of 2318 $$ E $$ . The rotation of the octahedra reduces the strength of superexchange: in going from $$ to $$ the G-AFM stabilization with respect to FM shrinks by a factor 4.