High-pressure synthesis and crystal structures of new ternary caesium magnesium hydrides, CsMgH_3, Cs_4Mg_3H_(10) and Cs_2MgH_4

摘要

The title compounds were synthesized by heating stoichiometric mixtures of the binary hydrides (deuterides) to 820 K in a multi-anvil press at 30 kbar pressure. X-ray and neutron powder diffraction on the deuterides at ambient conditions indicates rhombohedral symmetry for CsMgH_3 (space group R-3m, a=6.2521(6), C=22.382(3) A, BaRuO_3 type structure), orthorhombic symmetry for Cs_4Mg_3H_(10) (Cmca, a=6.2569(9), b=14.734(2), C=13.791(2) A, Cs_4Mg_3F_(10) type structure), and tetragonal symmetry for Cs_2MgH_4 (14/mmm, a=4.3264(5), c=14.760(2) A, K_2NiF_4 type structure). The compounds contain octahedral (MgH_6)~(4-) anions and have considerably better volume efficiencies for hydrogen than those containing tetrahedral (MgH_4)~(2-) anions that are synthesized under hydrogen gas pressure.