New Phase Transitions in Non-Stoichiometric U3O8-x at High Temperatures, (II)

Electrical conductivity and X-ray diffraction studies on non-stoichiometric U3O8-x phase were carried out simultaneously in the range 765°≦T≦995°C and 10−4≦Po2≦1 atm. The plot of logσ vs. logPo2 showed many refractions which corresponded with the phase transitions determined by thermogravimetry reported in the preceding paper. Based on the data of both electrical conductivity and thermogravimetry, the non-stoichiometric defect structures of various U3O8-x phases are interpreted as consisting of singly charged oxygen interstitials (Ol′) and doubly charged oxygen vacancies (Vo.)? Some of the X-ray diffraction lines were found to undergo splitting with decreasing oxygen partial pressure. These splits are qualitatively discussed in reference to the out-of-step structure model. The mechanism of electrical conduction in the high temperature hexagonal U3O8-x phases is surmised to be the hopping of small polarons.