Preferred hydrogen sites and hydride stoichiometry for A6B23 compounds

Abstract

The variety of interstices available for potential occupation by hydrogen in A6B23 intermetallic compounds makes them important tests of a geometric model recently developed for rationalizing observed hydride stoichiometries and preferred hydrogen sites. The model was applied to Y6Mn23D18 and Th6Mn23D16, for which structure determinations can be found in the literature. Both compounds have cubic structures of space groupFmSm, and they have 116 metal atoms per unit cell. Nine different kinds of sites (708 sites per unit cell) have been considered for potential occupancy. Hole radii and inter-site distances were calculated for all these types. Concepts of minimum hole radius (0.040 nm) and minimum H-H distance (0.210 nm) were used to predict hydrogen-site occupancy. The model correctly predicts that, in addition to a and f3 sites, j1 sites (Wyckoff notation) should be involved in both of these compounds at these H(D) concentrations. The location actually observed for the deuterium atom is in the hexahedral i site that has its centre in the trigonal face shared by two j1 sites. For Y6Mn23D23, the model correctly predicts the partial occupation of h sites, which are coordinated by two yttrium and three manganese atoms. Other predictions have been made, including one regarding partial occupation of I sites when there are more than 25 hydrogens per formula unit. The latter predictions have not been tested experimentally. With this crystal structure, both compounds appear to have a maximum absorption capacity of 33 hydrogens per formula unit.