Stability of the Pb divalent state in insulating and metallic PbCrO3

PbCrO${}₃$ is a perovskite that undergoes a large volume collapse and concurrently an insulator-metal transition with pressure. While one may think that the volume collapse can be associated with conversion of Pb${}²⁺$ ions to the smaller Pb${}⁴⁺$, the spectroscopic measurements in this experiment reveal that the Pb ions remain divalent. Instead, the Cr ions undergo a transition from a charge-disproportionated 2Cr${}³⁺$/Cr${}⁶⁺$ configuration to a homogenous Cr${}⁴⁺$ with pressure. Thus, the authors can explain the insulating/metallic nature of PbCrO${}₃$ at low/high pressures, respectively.