An X-ray and neutron diffraction study on a phase transition in the U 4O 9 phase

Abstract

A neutron and X-ray diffraction study was made on the phase transition of U 4O 9 and U 4O 9− y which occurs at a region slightly above room temperature. On the basis of the neutron data of the fluorite type reflections, and the X-ray diffraction data of the superlattice lines, the behaviour of oxygen atoms during the transition is discussed. In U 4O 9− y , the vacancies in the fluorite oxygen sites increase at the transition when it is approached from low-temperature side. Above the transition temperature some portions of the lattice oxygen atoms leave the lattice sites and enter into the interstitial positions. These interstitial oxygen atoms contribute to intensify the superlattice lines.