An electrical conductivity and X-ray study of a high-temperature transition in U 4O 9

Abstract

The high temperature transition in U 4O 9 has been studied by electrical conductivity measurements and X-ray diffraction. From the electrical conductivity measurements, a similar variation of log σ T with reciprocal temperature to that in the transition range near room temperature is observed in the temperature range from about 300 to 800°C. Like the low temperature transition, a small lattice contraction is also observed in that temperature range by means of X-ray diffractometry, and the transition temperature increases from 530 to 620°C with increasing O/U ratio. After the transition the intensity of 4a 0 superlattice reflections increases, but that of 8a 0 superlattice reflections disappears. The mechanism of this high temperature transition is considered to be a second-order transition of the order—disorder type based on the configurational change of U 4+ and U 5+ with the shift of some portions of the lattice oxygen atoms from the lattice sites to the interstitial positions. The phase diagram of U 4O 9 is presented on the basis of the electrical conductivity and X-ray data.