CRYSTAL-STRUCTURE OF HYDRIDE AND DEUTERIDE OF UNIAL

The hydrogen and deuterium absorption behaviour of UNiAl was examined at hydrogen isotope pressures below 10(5) Pa and temperatures up to 97 3 K. Under these conditions UNiAl absorbed hydrogen and deuterium up t o UNiAlH0.8, and UNiAlD0.7 respectively. Neutron powder diffraction of UNiAlD0.7 revealed that each D atom occupies a 3U-2Ni site formed by three U atoms and two Ni atoms. It is at the centre of a triangle of U atoms which two SU-1Ni-type tetrahedral sites share. This occupation slightly changed the arrangement of metal atoms to enlarge the 3U tria ngle. Changes in X-ray diffraction pattern suggested the further movem ent of metallic atoms to form UNiAlH27.