Investigations on the Formation and Decomposition Behaviors of BaAlH5 and Ba2AlH7

The formation and decomposition behaviors of BaAlH5 and Ba2AlH7 were investigated by means of X-ray diffraction and pressure-composition isotherms (PCT). The phase transitions, phase compositions, and lattice parameters were determined by Rietveld refinement. Significantly pure BaAlH5 with a purity of 99.1% was obtained through hydriding of Ba4Al5 alloy at 100 degrees C under 5 MPa H-2. When the hydrogenation temperature increased to 250 degrees C, the main phase became Ba2AlH7 with some impurity phases of BaAl4 and BaH2. PCT measurements of Ba4Al5 showed that the plateau pressure at 150 degrees C was about 0.5 MPa, and the maximum absorption H2 content was 2.15wt %. The final products obtained after PCT measurements of Ba4Al5 at 150 degrees C, 280 and 350 degrees C were BaAlH5, a mixture of Ba2AlH7, BaH2 and BaAl4, a mixture of BaH2 and BaAl4, respectively. Independent decomposition experiments showed that BaAlH5 partly desorbed H-2 to form a mixture of BaAlH5, Ba2AlH7, BaH2, and BaAl4 at 280 degrees C, whereas both BaAlH5 and Ba2AlH7 completely decomposed into BaH2 and BaAl4 at 350 degrees C.