Investigations on the formation and decomposition behaviors of BaAlH ₅ and Ba₂AlH₇

The formation and decomposition behaviors of BaAlH₅ and Ba ₂AlH₇ 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 °C under 5 MPa H₂. When the hydrogenation temperature increased to 250 °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 °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 °C, 280 and 350 °C were BaAlH₅, a mixture of Ba₂AlH₇, BaH ₂ and BaAl₄, a mixture of BaH₂ and BaAl ₄, respectively. Independent decomposition experiments showed that BaAlH₅ partly desorbed H₂ to form a mixture of BaAlH ₅, Ba₂AlH₇, BaH₂, and BaAl ₄ at 280 °C, whereas both BaAlH₅ and Ba ₂AlH₇ completely decomposed into BaH₂ and BaAl₄ at 350 °C.