High-Pressure-High-Temperature Polymorphism of the Oxides of Lead

The common oxides of lead, PbO, PbO₂, and Pb₃O₄, were examined at pressures of 0 to 60,000 atmospheres and temperatures of 100° to 600° C. The pressure-temperature curve for the litharge-massicot transition was measured, giving a ΔH of transition of 57 cal. per mole. Unusual meta-stability and inversion characteristics of the massicot phase were examined in detail. PbO₂ (rutile) transformed at pressures in excess of 13,000 atmospheres at 300°C. to an ortho-rhombic form. The univariant equilibrium curve for the transition gave a Δ H of 11 cal. per mole. Pb₃O₄ underwent a disproportionation reaction yielding PbO and "Pb₂O₃." The equilibrium curve for this reaction was measured, and Δ H was determined to be 4500 cal. per mole.