High-temperature thermodynamic properties of the thorium-oxygen system

Thermodynamic properties of the Th-O system at high temperatures were investigated by means of mass effusion, mass spectrometric, and transpiration techniques over compositions ranging from the congruently vaporizing composition to the lower phase boundary composition. A very sharp decrease in O potential occurs when stoichiometric thoria is reduced but slightly from the stoichiometric composition. The small extent of reduction over a wide range of O potentials measured in the temperature range 2400 to 26550K is a clear illustration of the higher stability of the ThO/sub 2-x/ phase compared with actinide oxides such as UO/sub 2-x/, PuO/sub 2-x/, as well as other metal oxide systems. Estimates of ΔH-bar/sub O2/, ΔS-bar/sub O2/, and the standard free energy of formation of bivariant ThO/sub 2-x/ compositions were made. A substantial increase in the total pressure of Th-bearing species occurs when stoichiometric thoria is reduced toward the lower phase boundary. The increase in pressure can be attributed primarily to the increasing contribution of the ThO gaseous species. Total pressures of Th-bearing species above ThO/sub 2-x/ compositions calculated by means of experimental O potentials and the known or estimated free energy of formation values of the gaseous species and condensed phase are in reasonable agreement with experimental values