Orthorhombic Ba6Mg7H26: A new fluoride-related ternary alkaline earth hydride

Ba6Mg7H26 and its deuteride were synthesized by sintering 6:7 mixtures of BaH2 (BaD2) and Mg powder at 730 K and 100 bar hydrogen (deuterium) pressure for two weeks and characterized by X-ray and neutron powder diffraction on a multiphase sample. They crystallize with a new structure type in space group Immm (No. 71), cell parameters at 295 K, hydride: a=5.8258(3) Angstrom, b=11.9791(6) Angstrom, c=14.905(1) Angstrom, V=1040.2(1) Angstrom(3); deuteride: a=5.8005(6) Angstrom, b=11.925(1) Angstrom, c=14.836(2) Angstrom, V=1026.2(1) Angstrom(3); 2=2. The structure contains two barium and four magnesium sites having cuboctahedral (Ba) and octahedral (Mg) deuterium coordinations. The metal-deuterium distances are in the range 2.61-3.14 Angstrom (Ba-D) and 1.90-2.28 A (Mg-D). The shortest D-D distance is 2.62 Angstrom. The ternary fluoride Ba6Zn7F26 can be considered as a monoclinic distortion variant of Ba6Mg7D26.