A thermodynamic analysis of the vaporization of the substoichiometric plutonium dioxide phase [~1.6 O/Pu 5$ 2.0] is presented. The results are based on measurements of the vapor pressure of known compositions in tungsten, rhenium, and tantalum effusion cells over the range of temperature 1650 to 2100°K. The vapor pressures are interpretable in terms of Pu02(g), PuO(g), and oxygen as the important vapor species; the partial pressure of the gaseous dioxide is relatively insensitive to the composition of the solid phase whereas the partial pressures of gaseous monoxide and oxygen (both atomic and molecular) show a marked dependence and become predominant near the lower and upper phase boundaries, respectively. Known thermodynamic data for the solid phase are combined with the vapor pressure data to yield equations for the standard free energies of formation of gaseous dioxide and monoxide: A(7f0[(Pu02(g)] = -113,100 + 4.35T and &G°[ (PuO(g) ] = -29,000 -12.1 T. The congruently vaporizing composition is calculated to vary from O/Pu = 1.92 at 1600°K to 1.84 at 2400°K. ently vaporizing composition (minimum vapor pressure) was shown to exist for the Pu02_z phase although(1) Based on work performed under the auspices of the U. S. Atomic Energy Commission.(2) T. E. Phipps, G. W. Sears, and O. O. Simpson, J, Chem. Phys 18, 724 (1950).

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