Inert gas pressure effects on phase stability of oxides

Phase stability of Y-Ba-Cu-O superconducting phases was evaluated as a function of temperature, oxygen partial pressure and total gas pressure by the hot isostatic pressing method using argon and oxygen mixtures as a surrounding atmosphere. The phase transformation temperature from Y2Ba4Cu7Oy to YBa2Cu4O8 increased with increasing the total gas pressure, even if the oxygen partial pressure was kept constant. It indicates that the equilibrium partial pressure of a gas involving the chemical reaction changes if a solid-gas reaction is progressed under high total gas pressure. Dependence of chemical potential of the reactant gas on the inert gas pressure was thermodynamically discussed on the basis of Lewis-Randall's empirical scaling law and Ishizaki's approximation. These two approximations resembled each other at pressures less than the usual HIPing pressure.