Metal aluminum amides for hydrogen storage – Crystal structure studies

• Deuterated metal aluminum amides M[Al(ND2)4]x were synthesized.
• Crystal structure of the compounds was studied by neutron and X-ray diffraction.
• Structure determination was successful for β-KAl(ND2)4 and Ca[Al(ND2)4]2.
• The structural features of the compounds have been compared with the alanates.

Crystal structure of metal aluminum amides has been investigated by synchrotron radiation powder X-ray diffraction (SR-PXD) and powder neutron diffraction (PND). Alkali and alkaline-earth metal aluminum amides M[Al(NH2)4]x desorb ammonia during thermal decomposition and have recently been studied as possible hydrogen storage system by making composites with metal hydrides. In this work, deuterated metal aluminum amides M[Al(ND2)4]x with M = Li, Na, K, Mg and Ca were synthesized by ball-milling of M (or MDx) + Al (or AlD3) under liquid ND3. The crystal structures of MAl(ND2)4 phases with M = Li, Na, K (α-phase) have been refined by simultaneous Rietveld refinement using PND and SR-PXD data. Structure determination was successful for β-KAl(ND2)4 and Ca[Al(ND2)4]2. The structures are composed of [Al(ND2)4]− tetrahedra and M cations, and the unit cells are: M = Li; monoclinic space group P21/n, a = 9.5075(3) Å, b = 7.3610(2) Å, c = 7.4076(2) Å, β = 90.165(3)°, Na; monoclinic space group P21/c, a = 7.3317(9) Å, b = 6.0447(8) Å, c = 13.151(2) Å, β = 94.110(9)°, K (α-phase); orthorhombic space group C2221, a = 10.17258(7) Å, b = 5.78762(5) Å, c = 9.98651(7) Å, K (β-phase); orthorhombic space group Pnma, a = 11.4183(4) Å, b = 8.8588(2) Å, c = 6.1696(2) Å, and Ca; monoclinic space group Pc, a = 6.4321(2) Å, b = 6.4377(2) Å, c = 12.2939(3) Å, β = 90.612(2)°. The structural features of metal aluminum amides have been compared with the corresponding alanates.