Magnetic Superexchange and Mott Insulator Mechanisms in Cubic Perovskites: From First-Principles to Canonical Models.

The ground state of many insulating, open-shell transition-metal perovskites with a 180° metal-ligand-metal bridge is antiferromagnetic (AFM), as predicted by Anderson's superexchange interaction or Hubbard's model. These well-established, standard models show how these systems are insulators due to the minimization of the interactions between electrons, at the cost of localizing the electrons on the metal ions. In this work, we carry out first-principles simulations on the cubic perovskites KNiF and KVF, analyzing electron densities, energies and bond indices. Although our calculations predict an antiferromagnetic ordering (AFM), in agreement with canonical superexchange models, we show through various indicators that the stabilization of this phase is not mainly associated with the antibonding magnetic orbitals but rather with bonding orbitals not included in the models. In particular, these traditional descriptions of superexchange do not adequately describe the ligand-to-metal electronic backdonation, which is an important element for stabilizing the insulating state of the two studied perovskite fluorides, albeit by diametrically different mechanisms: (1) reducing electron-electron repulsion in KNiF, as proposed by Hubbard, whereas (2) enhancing electron-nuclear attraction in KVF. Our findings highlight some of the limitations of these foundational models and offer a novel perspective on the understanding of magnetism.