Jahn-Teller distortion, octahedra rotations and orbital ordering in perovskites: KScF   3 as a model system.

The KScF   3 perovskite has been used as a model for investigating the relative importance of the Jahn-Teller (JT) lift of degeneracy, the ScF   6 octahedra rotation (OR), and the quadrupole-quadrupole interaction linked to different occupancy of the Sc t   2 g subshell in various sites of the unit cell (orbital ordering, OO). The group-subgroup sequence P m 3 ¯ m , P 4 m m m , P 4 m b m , and P n m a , supplemented by C m m m and I 4 m c m , has been explored by using an all electron 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 P 4 m m m to P 4 m b m , consisting in a rotation of the octahedra around the c axis, is 1077 μ E   h . From P 4 m b m to P n m a , additional rotations around the a and b axes are possible, and the d Sc electron can occupy a different t   2 g orbital, with a total energy reduction of 2318 μ E   h . The rotation of the octahedra reduces the strength of superexchange: in going from P 4 m m m to P n m a the G-AFM stabilization with respect to FM shrinks by a factor 4.