Thermodynamics of point defect disorder and off-stoichiometry in ThO2 and Th1-yUyO2

Abstract

A thermodynamic model for point defect disorder was developed to investigate the off-stoichiometry of ThO2 and Th1-yUyO2. The model shows that ThO2 mainly exists as a hypo-stoichiometric oxide from 1200 K to 2900 K for oxygen partial pressures ranging from 10−50 to 105 atm. The addition of U expands the thermodynamic window over which Th1-yUyO2 is hyper-stoichiometric, i.e., Th1-yUyO2+x, illustrating the impact of U addition with its 5f electrons on defect disorder in the mixed oxide. For instance, at 1473 K and 10−5 atm, Th0.95U0.05O2+x was found to accommodate an oxygen off-stoichiometry of x = 0.002 whereas Th0.8U0.2O2+x accommodates an off-stoichiometry of x = 0.03. The extent of hyper-stoichiometry of the mixed oxide was found to increase with reducing temperature and with increasing U content in the oxide up to a certain U content in the mixed oxide after which a plateau is reached in terms of the extent of hyper-stoichiometry achievable. With the inclusion of vibrational entropy of point defects in the defect disorder model, oxygen vacancies were found to dominate in the hypo-stoichiometric regime of Th1-yUyO2. As for the hyper-stoichiometric regime, cation vacancies were found to dominate at low temperatures while anion interstitials dominate at high temperatures.