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Oxygen plays a critical role in strongly correlated transition metal oxidesas crystal field effect is one of the key factors that determine the degree oflocalization of the valence d/f states. Based on the localization, a set ofconventional mechanisms such as Mott-Hubbard, Charge-transfer and Slater wereformulated to explain the antiferromagnetic and insulating (AFI) phenomena inmany of these correlated systems. From the case study on LiFePO4, throughdensity-functional calculations, we demonstrate that none of these mechanismsare strictly applicable to explain the AFI behavior when the transition metaloxides have polyanions such as (PO4)3-. The symmetry-lowering of themetal-oxygen complex, to stabilize the polyanion, creates an asymmetric crystalfield for d/f states. In LiFePO4 this field creates completely non-degenerateFe-d states which, with negligible p-d and d-d covalent interactions, becomeatomically localized to ensure a gap at the Fermi level. Due to large exchangesplitting, high spin state is favored and an antiferromagnetic configuration isstabilized. For the prototype LiFePO4, independent electron approximation isgood enough to obtain the AFI ground state. Inclusion of additional correlationmeasures like Hubbard U simply amplifies the gap and therefore LiFePO4 can bepreferably called as weakly coupled Mott insulator.

Unconventional Magnetism and Band Gap Formation in LiFePO4: Consequence of Polyanion Induced Non-planarity