Unusual Mott transition associated with charge-order melting in ${\mathrm{BiNiO}}_3$ 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 $\sim 4.8$%, ${\mathrm{BiNiO}}_3$ 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 ${\mathrm{BiNiO}}_3$.

Figure 1

Total energy (top) and local magnetic moments (bottom) of paramagnetic ${\mathrm{BiNiO}}_3$ obtained by $\mathrm{DFT}+\mathrm{DMFT}$ for the ambient-pressure $P\overline{1}$ (AP) and high-pressure $Pbnm$ (HP) phases as a function of the unit cell volume at a temperature $T=387$ K.

Figure 2

Orbitally resolved spectral functions of ${\mathrm{BiNiO}}_3$ calculated within $\mathrm{DFT}+\mathrm{DMFT}$ using the maximum entropy method for the ambient-pressure $P\overline{1}$ (left panel) and high-pressure $Pbnm$ (right panel) structures for different unit cell volumes at a temperature $T=387$ K.

Figure 3

Bi $6s$ occupations for the ambient-pressure (AP) $P\overline{1}$ phase of ${\mathrm{BiNiO}}_3$ calculated by $\mathrm{DFT}+\mathrm{DMFT}$ as a function of lattice volume (top). $\mathrm{\Delta }{N}_{\text{Bi}}$ denotes the corresponding Bi $6s$ charge disproportionation ($\mathrm{\Delta }{N}_{\text{Bi}}=N_{\text{Bi1}}-N_{\text{Bi2}}$). Bottom: the Ni $e_g$ and O $2p$ crystal field levels for the AP and high-pressure $Pbnm{\mathrm{BiNiO}}_3$ [35].

Figure 4

Local spin correlation function $\chi \left(\tau \right)=\langle {\widehat{m}}_z\left(\tau \right){\widehat{m}}_z\left(0\right)\rangle$ of the Ni $3d$ states calculated by $\mathrm{DFT}+\mathrm{DMFT}$ for the ambient-pressure $P\overline{1}$ (AP) and the high-pressure $Pbnm$ (HP) structures of ${\mathrm{BiNiO}}_3$ for different volumes.