A Neutron Diffraction Study of V2H

Hydrogen arrangements in two polymorphic forms, β 1 and β 2 , of V 2 H have been studied by powder neutron diffraction up to 200°C. The hydrogen atoms in β 1 -V 2 H are fully ordered in the specific o z -sites of the body-centered pseudo-tetragonal vanadium lattice ( c 0 / a 0 ≃1.1), forming a monoclinic superstructure ( A ≃ C ≃( a 0 2 + c 0 2 ) 1/2 , B = a 0 , β≃2tan -1 c 0 / a 0 ) isomorphic with β-V 2 D. The structure of β 2 -V 2 H, which exists between 175° and 220°C, is partially ordered; the hydrogen atoms are distributed among all the o z -sites with a probability of 1/2. The hydrogen distribution in the VH superstructure is proposed to be the full occupation of these sites.