Metal aluminum amides for hydrogen storage - Crystal structure studies

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(NH2)(4)](x) with M = Li, Na, K, Mg and Ca were synthesized by ball-milling of M (or MD)(x) + Al (or AlD3) under liquid ND3. The crystal structures of MAl(ND2)(4) phases with M = Li, Na, K (alpha-phase) have been refined by simultaneous Rietveld refinement using PND and SR-PXD data. Structure determination was successful for beta-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 P2(1)/n, a = 9.5075(3) angstrom, B = 7.3610(2) angstrom, c = 7.4076(2) angstrom, beta = 90.165(3)degrees, Na; monoclinic space group P2(1)/c, a = 7.3317(9) angstrom, B = 6.0447(8) angstrom, beta = 13.151(2) angstrom, beta = 94.110(9)degrees, K (alpha-phase); orthorhombic space group C222(1), a = 10.17258(7) angstrom, b = 5.78762(5) angstrom, c = 9.98651(7) degrees, K (beta-phase); orthorhombic space group Pnma, a = 11.4183(4) angstrom, b = 8.8588(2) angstrom, c = 6.1696(2) angstrom, and Ca; monoclinic space group Pc, a = 6.4321(2) angstrom, b = 6.4377(2) angstrom, c = 12.2939(3) angstrom, = 90.612(2)degrees. The structural features of metal aluminum amides have been compared with the corresponding alanates. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.