Determination of hydrogen ordering within the β-RH_2+x phase (R = Ho, Y) using electron diffraction techniques

Computer simulations of the electron diffraction patterns along the [10] zone axes of four ordered structures within the β-RH_2+x phase, with R = Ho or Y, and 0 ≤ x ≤ 0.25, have been performed to establish whether or not the hydrogen ordering could be detected using electron diffraction techniques. Ordered structures within other RH_2+x (R = Ce, Tb) systems have been characterized with neutron scattering experiments; however, for HoH(D)_2+x, neutron scattering failed to characterize the superstructure, possibly because of the low x concentration or lack of long-range order within the crystal. This paper aims to show that electron diffraction could overcome both of these problems. The structures considered were the stoichiometric face-centred cubic (f.c.c.) fluorite structure (x = 0), the D1 structure (x = 0.125), the D1_a structure (x = 0.2) and the D0₂₂ structure (x = 0.25). In the stoichiometric structure, with all hydrogen atoms located on the tetrahedral (t) sites, only the diffraction pattern from the f.c.c. metal lattice was seen; however, for the superstoichiometric structures, with the excess hydrogen atoms ordered on the octahedral (o) sites, extra reflections were visible. All the superstoichiometric structures showed extra reflections at the (001)_f.c.c. and (110)_f.c.c. type positions, with structure D1 also showing extra peaks at (½ ½ ½)_f.c.c.. These reflections are not seen in the simulations at similar hydrogen concentrations with the hydrogen atoms randomly occupying the o vacancies.