Direct synthesis of BaAlH5 and Ba2AlH7 from BaH2 and Al system and their hydriding/dehydriding characteristics

BaAlH5 and Ba2AlH7 were successfully synthesized through reactive ball milling commercially available BaH2 and Al powders for the first time. The effects of the mole ratios of BaH2/Al on the phase formation and structural properties were investigated by means of X-ray diffractions and a subsequent Rietveld refinement. For different BaH2/Al mole ratios (0.5, 1.0 and 2.0) the main phases were BaAlH5, Ba2AlH7 and BaH2, respectively. Ba2AlH7 can also be prepared by ball milling BaH2/Al (1:1) under At and a subsequent hydrogenation. The phase transition and hydriding/dehydriding characteristics of the ball milled BaH2/Al (1:1) system under At were investigated using in-situ differential scanning calorimeter (DSC) under H-2 atmosphere and pressure-composition isotherms (PCI). The mechano-chemistry induced phase transformation of the BaH2/Al (1:1) mixture to a BaH2/BaAl4 (3:1) mixture during ball milling under At was first observed. The hydrogenation enthalpy of BaH2/BaAl4 (3:1) to form Ba2AlH7 was determined to be -20.0 kJ mol(-1) from a van't Hoff plot. The phase transitions during ball milling under At and PCT measurements were proposed as follows: 28BaH(2) + 28Al ->(Ball milling)(Ar)21BaH(2) + 7BaAl(4) reversible arrow(+H2)(-H2) 12Ba(2)AlH(7) + 4BaAl(4) (c) 2008 Elsevier B.V. All rights reserved.