THE CRYSTAL-STRUCTURE OF NB3AUHX

The Al5 alloy Nb3Au and its hydrides Nb3AuHx with x=2.9 and x=4.2 prep ared at high hydrogen pressures were studied by neutron and X-ray diff raction. Their crystal structures were refined using the Rietveld meth od. Neutron diffraction showed that in both hydrides the hydrogen atom s occupy approximately 2.6 interstitial d sites of the 3 available per formula unit in the Al5 metal lattice. The remaining hydrogen fills i sites. The X-ray examination revealed that the metal lattice of the N b3Au alloy and its hydrides was partly disordered, with 8+/-1% of the regular Au positions randomly occupied by Nb. This atomic disorder is shown to be the cause of the observed maximum occupancy of d sites in the hydrides. (C) 1998 Elsevier Science S.A.