Thermochemistry of the U─O and Zr─O Systems

The thermodynamic constraint relating the standard free energy of formation of an oxide (MO c ) to its oxygen potentials by Δ G f 0 (MO c ) =½ƒ o c Δ Ḡ (O 2 ) d C ′ has been used to develop the full spectrum of oxygen potentials of the U─O and Zr─O systems at high temperatures. Henry's law is applied to the metal region in the U─O system. Oxygen dissolution in liquid Zr has been shown to deviate only slightly from Henry's law, despite the high solubility. Minor modifications of the previously reported oxygen pressures in the two‐phase regions of both of the U─O and Zr─O systems result from the requirement of satisfying the integral constraint. Oxygen pressures in the oxygen‐containing metal regions where no data are available have also been estimated. The ensemble of the results is displayed in the form of oxygen isobars superimposed on the phase diagrams of the U─O and Zr─O systems. The oxygen pressure‐temperature–composition relation of tetragonal (β) ZrO 2± x has also been established.