Synthesis and structure of an orthorhombic low-pressure polymorph of caesium magnesium hydride, CsMgH3

CsMgH3 was synthesized by heating mixtures of the binary hydrides in an autoclave at 600 K and a hydrogen pressure of 150 bars. Synchrotron X-ray and neutron powder diffraction reveals a new structure having orthorhombic symmetry (space group Pmmn, a=9.9958(1), b=6.13271(6), c=8.57364(9) A for the hydride). In contrast to the trigonal high-pressure polymorph, magnesium centred [MgH6] octahedrons are condensed into triangular rather than linear [3MgH3]3- trimers, and these trimers are connected via four corners to two-dimensional slabs rather than via six corners to a three-dimensional network. The Mg-D distances in the deuteride range from 1.94 to 1.96 A at the periphery to 2.00-2.08 A at the centre of the trimers. The Mg displacements within the octahedrons suggest repulsive Mg2+-Mg2+ interactions