The crystal structures of Rb2MgH4 and Rb3MgH5 by neutron powder diffraction

The title compounds and corresponding deuterides were prepared by sintering stoichiometric mixtures of RbH and MgH2 under a hydrogen (deuterium) pressure of 80 bar at 650 K. Neutron powder diffraction on the deuterides indicate that Rb2MgH4 adopts the beta-K2SO4 structure type (space group Pnma, Z=4), while Rb3MgH5 crystallises with the Cs3CoCl5 structure type (space group I4/mcm, Z=4). In both compounds magnesium is tetrahedrally co-ordinated by hydrogen with ME-D bond distances in the range 1.79-1.87 Angstrom and D-Mg-D bond angles in the range 104.7-114.9 degrees. The cell dimensions and metal hydrogen tetrahedra are significantly bigger than those in the isostructural zinc analogues, thus underlining the difficulty of defining a consistent set of ionic radii (or partial atomic volumes) for metal hydride structures. (C) 1998 Elsevier Science S.A.