Statistco-Thermodynamical Study of the Equilibrium between Hydrogen Gas and the Plutonium Dihydride Phase

The hydrogen pressure-composition isotherms for the plutonium-hydrogen system have been studied experimentally by Mulford and Sturdy.It has been considered that the stoichiometric plutonium dihydride phase has the fluorite structure, in which plutonium atoms occupy the face centred cubic lattice sites and hydrogen atoms are located in the tetrahedral sites of the plutonium lattice, and the plutonium dihydride phase is a non-stoichiometric berthollide compound which can exist in the range of composition from 1.8 to 2.75 hydrogen atoms per plutonium atom. But, it is expected that hydrogen atoms can occupy the tetrahedral and octahedral sites in the face centred cubic lattice of plutonium atoms.Based on the assumption described above, the equilibrium between the non-stoichiometric plutonium dihydride phase and hydrogen gas was investigated statistico-thermodynamically. The energies of the mutual interaction between hydrogen atoms in tetrahedral or octahedral sites and plutonium atoms surrounding them and the potential energies of pairs of hydrogen atoms were calculated, respectively. The ratio of hydrogen atoms in tetrahedral and octahedral sites was estimated as a function of temperature, hydrogen concentration and the difference between the energies of hydrogen atoms in both sites.From the results of the theoretical study, it was concluded that hydrogen atoms were located preferentially in tetrahedral sites and the number of hydrogen atoms in octahedral sites increased with increasing temperature and hydrogen concentration.It was found that the calculated hydrogen equilibrium pressure for the non-stoichiometric plutonium dihydride phase agreed well with the experimental results of Mulford and Sturdy.