Determination of hydrogen ordering within the ss-RH2+x phase (R = Ho, Y) using electron diffraction techniques

Computer simulations of the electron diffraction patterns along the [(1) ov er bar 10] zone axes of four ordered structures within the beta-RH2+x phase , with R = Ho or Y, and 0 less than or equal to x less than or equal to 0.2 5, have been performed to establish whether or not the hydrogen ordering co uld be detected using electron diffraction techniques. Ordered structures w ithin other RH2+x (R = Ce, Tb) systems have been characterized with neutron scattering experiments; however, for HoH(D)(2+x), neutron scattering faile d to characterize the superstructure, possibly because of the low x concent ration or lack of long-range order within the crystal. This paper aims to s how that electron diffraction could overcome both of these problems. The st ructures considered were the stoichiometric face-centred cubic (f.c.c.) flu orite structure (x = 0), the D1 structure (x = 0.125), the D1(a) structure (x = 0.2) and the D0(22) structure (x = 0.25). In the stoichiometric struct ure, 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 t he superstoichiometric structures, with the excess hydrogen atoms ordered o n the octahedral (o) sites, extra reflections were visible. All the superst oichiometric 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 (1/2 1/2 1/2)(f.c.c.). These reflections are not seen in the simulations at similar hydrogen concentrations with the hydrogen atoms randomly occupying the o vacancies.