The Superexchange Mechanism in Crystalline Compounds: The Case of KMF3 (M=Mn, Fe, Co, Ni) Perovskites

The ferromagnetic and antiferromagnetic wavefunctions of four KMF₃ (M= Mn, Fe, Co and Ni) perovskites have been obtained quantum-mechanically with the CRYSTAL code, by using the Hartree-Fock (HF) Hamiltonian and three flavours of DFT (PBE, B3LYP and PBE0) and an all- electron Gaussian type basis set. In the Fe and Co cases, with d⁶ and d⁷ occupation, the Jahn Teller distortion of the cubic cell is as large as 0.12 ˚A. Various features of the superexchange interaction energies (SIE), namely additivity, dependence on the M-M distance, on the M^F M angle, and on the adopted functional, are explored. The effect of SIE on the equilibrium geometry and volume of the unit cell is discussed, and it is shown that the key quantity is the spin polarization of the (closed shell) F ions along the M-F-M path. The effect of this magnetic pressure is evaluated quantitatively for the first time.