Unusual Mott transition associated with charge-order melting in BiNiO3 under pressure

We study the electronic structure, magnetic state, and phase stability of paramagnetic ${\mathrm{BiNiO}}₃$ near a pressure-induced Mott insulator-to-metal transition (MIT) by employing a combination of density functional and dynamical mean-field theory. We obtain that ${\mathrm{BiNiO}}₃$ exhibits an anomalous negative-charge-transfer insulating state, characterized by charge disproportionation of the Bi $6s$ states, with ${\mathrm{Ni}}²⁺$ ions. Upon a compression of the lattice volume by $\ensuremath{\sim}4.8$%, ${\mathrm{BiNiO}}₃$ is found to make a Mott MIT, accompanied by the change of crystal structure from triclinic $P\overline{1}$ to orthorhombic $Pbnm$. The pressure-induced MIT is associated with the melting of charge disproportionation of the Bi ions, caused by a charge transfer between the Bi $6s$ and O $2p$ states. The Ni sites remain to be ${\mathrm{Ni}}²⁺$ across the MIT, which is incompatible with the valence-skipping ${\mathrm{Ni}}²⁺$/${\mathrm{Ni}}³⁺$ model. Our results suggest that the pressure-induced change of the crystal structure drives the MIT in ${\mathrm{BiNiO}}₃$.