  Oxygen plays a critical role in strongly correlated transition metal oxides
as crystal field effect is one of the key factors that determine the degree of
localization of the valence d/f states. Based on the localization, a set of
conventional mechanisms such as Mott-Hubbard, Charge-transfer and Slater were
formulated to explain the antiferromagnetic and insulating (AFI) phenomena in
many of these correlated systems. From the case study on LiFePO4, through
density-functional calculations, we demonstrate that none of these mechanisms
are strictly applicable to explain the AFI behavior when the transition metal
oxides have polyanions such as (PO4)3-. The symmetry-lowering of the
metal-oxygen complex, to stabilize the polyanion, creates an asymmetric crystal
field for d/f states. In LiFePO4 this field creates completely non-degenerate
Fe-d states which, with negligible p-d and d-d covalent interactions, become
atomically localized to ensure a gap at the Fermi level. Due to large exchange
splitting, high spin state is favored and an antiferromagnetic configuration is
stabilized. For the prototype LiFePO4, independent electron approximation is
good enough to obtain the AFI ground state. Inclusion of additional correlation
measures like Hubbard U simply amplifies the gap and therefore LiFePO4 can be
preferably called as weakly coupled Mott insulator.
