The structural stability, magnetic properties and electronic structure of tetragonal BiCoO(3) under pressure have been studied by first-principles density functional calculations. The calculated results reveal that no tetragonal-to-cubic and ferroelectric-to-paraelectric phase transitions occur up to 30 GPa with a volume compression of about 25%. An electronic spin crossover transition of the Co(3+) ion from the high-spin to nonmagnetic low-spin configuration (magnetic moment collapse) occurs at 4 GPa by about 4.87% volume compression, which is concomitant with a first-order isosymmetric transition and an insulator-to-semimetal transition. The metallization in BiCoO(3) is driven by the spin-state transition at high pressure. Coexistence of the structural, spin-state and insulator-to-semimetal transitions implies that there is a strong coupling among the lattice, spin and charge degrees of freedom in BiCoO(3).

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The electronic structure of room-temperature (RT) phase α -NaV 2 O 5 has been investigated by fully self-consistent first-principles calculations based on density functional theory (DFT). For the crystallographic unit cell, a nonmagnetic (NM) metallic solution is obtained by spin-restricted generalized gradient approximation (GGA) calculations, whereas a ferromagnetic (FM) insulating solution is successfully simulated within the spin-polarized GGA. An insulating antiferromagnetic (AFM) state with lower energy is obtained for the 1 × 2 × 1 crystallographic supercell. The magnetic S = 1/2 electrons are fully spin-polarized and delocalized on the V-O-V molecular orbitals (along the rung), where the net spin magnetic moments amount to 0.96 μ B on the V-O-V rungs of the ladder derived from Mulliken population analysis. The intra-rung vanadium d xy orbitals form the bonding-antibonding orbitals split by inter-orbital interactions. It is not the on-site Coulomb interaction, but the AFM spin exchange couplings that lead to the half-filled bonding orbitals splitting and forming a magnetic insulating gap. The present spin-polarized DFT calculations reveal that α -NaV 2 O 5 (RT) is a Slater insulator. The calculated electronic structure explains the controversial topics of the absorption peak in the optical spectra and the energy loss peak in the resonant inelastic x-ray scattering (RIXS).

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