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.