Single-crystal X-ray diffraction study on the hydrogen ordering in Ta₂H

A crystal of Ta₂H with a single ordered domain has been prepared by the reaction of hydrogen gas with a tantalum single-crystal. Precession and Weissenberg photographs show the superlattice reflections which are indexed in terms of an orthorhombic unit cell with lattice parameters A = a[2(1 +cos γ)]^{1/ 2}, B = c and C = a[2(1−cos γ)]^1/2, where a(= b) = 3.359, c = 3.398 Å and γ = 90.3° are the parameters of the host monoclinic tantalum lattice. Quantitative intensity measurements indicate that these reflections arise mostly from the systematic displacement of metal atoms caused by the ordered hydrogen arrangement. The amount of displacement has been determined as 0.086 Å, which is a little larger than a previous neutron diffraction result (0.057 Å) on Ta₂D.