Colossal Thermal Expansion in Ca-Substituted PbCrO3

Colossal thermal expansion, 11.9% volume expansion on heating, was observed in Pb_0.7Ca_0.3CrO₃. The parent compound, PbCrO₃, had been known to have a characteristic Pb²⁺_0.5Pb⁴⁺_0.5Cr³⁺O₃ charge distribution with a glassy distribution of Pb²⁺ and Pb⁴⁺ in ambient conditions and undergoes a charge transfer transition to the Pb²⁺Cr⁴⁺O₃ high-pressure phase at 2.5 GPa. It decomposes at 720 K without such a transition when heated at ambient pressure. Our in-situ synchrotron X-ray diffraction study revealed a temperature-induced phase transition from Pb²⁺Cr⁴⁺O₃ to Pb_²⁺0.5Pb⁴⁺_0.5Cr³⁺O₃ at moderate pressures between 2 and 3.5 GPa. Ca substitution for Pb shifted the phase boundary in the pressure–temperature space, and essentially the same charge transfer transition was caused by heating at ambient pressure. A remarkable volume change of 11.9% occurred, corresponding to a colossal linear thermal expansion coefficient of 550 ppm/K─an unprecedented value in oxide ceramics.