Deuterium site occupation in the oxygen-stabilized η-carbides Zr3V3ODx. I. Preparation and neutron powder diffraction

The structures of the single-phase, well-crystallized family of hydrides Zr3V3ODx for x=0, 1.86, 2.85, and 4.93 have been refined by the Rietveld method from neutron powder diffraction data taken at 298 K. The alloy and the three deuterides studied all possess the η-carbide structure (cubic Fd3m; origin at 4̄3m). Deuterium is found to occupy four different tetrahedral interstices preferentially. Sites with the largest number of Zr near neighbors appear to fill first unless occupation is hindered by nearby oxygen or simultaneously occupied deuterium sites. The volume expansion per number of deuterium atoms is nearly linear, even though the lowest stable hydride concentration at 298 K is near Zr3V3OD1.5 and markedly nonlinear distortions occur within the unit cell as different sites fill.