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BaBiO$_3$ is a mixed-valence perovskite which escapes the metallic statethrough a Bi valence (and Bi-O bond) disproportionation or CDW distortion,resulting in a semiconductor with a gap of 0.8 eV at zero pressure. Theevolution of structural and electronic properties at high pressure is, however,largely unknown. Pressure, one might have hoped, could reduce thedisproportionation, making the two Bi ions equivalent and bringing the systemcloser to metallicity or even to superconductivity, such as is attained atambient pressure upon metal doping. We address the high-pressure phase diagramof pristine BaBiO$_3$ by ab initio DFT calculations based on GGA and hybridfunctionals in combination with crystal structure prediction methods based onevolutionary algorithms, molecular dynamics and metadynamics. The calculatedphase diagram from 0 to 50 GPa indicates that pristine BaBiO$_3$ resistsmetallization under pressure, undergoing instead at room temperature structuralphase transitions from monoclinic \textit{I2/m} to nearly tetragonal\textit{P-1} at 7 GPa, possibly to monoclinic \textit{C2/m} at 27 GPa, and totriclinic \textit{P1} at 43 GPa. Remarkably, all these phases sustain and infact increase the inequivalence of two Bi neighboring sites and of their Bi-Obonds and, in all cases except semimetallic \textit{C2/m}, the associatedinsulating character. We then present high-pressure resistivity data whichgenerally corroborate these results, and show that the insulating characterpersists at least up to 80 GPa, suggesting that the \textit{C2/m} phase isprobably an artifact of the small computational cell.

High-pressure phase diagram, structural transitions, and persistent non-metallicity of BaBiO$_3$: theory and experiment