Melting of Pb Charge Glass and Simultaneous Pb–Cr Charge Transfer in PbCrO₃ as the Origin of Volume Collapse

A metal to insulator transition in integer or half integer charge systems can be regarded as crystallization of charges. The insulating state tends to have a glassy nature when randomness or geometrical frustration exists. We report that the charge glass state is realized in a perovskite compound PbCrO₃, which has been known for almost 50 years, without any obvious inhomogeneity or triangular arrangement in the charge system. PbCrO₃ has a valence state of Pb²⁺_0.5Pb⁴⁺_0.5Cr³⁺O₃ with Pb²⁺–Pb⁴⁺ correlation length of three lattice-spacings at ambient condition. A pressure induced melting of charge glass and simultaneous Pb–Cr charge transfer causes an insulator to metal transition and ∼10% volume collapse