Microstructures and hydrogen absorption–desorption behavior of an A₂B₇-based La-Mg-Ni alloy

The microstructural changes during hydrogen absorption–desorption cycles of an A₂B₇-based La-Mg-Ni alloy with a nominal composition of La_1.5Mg_0.5Ni_7.0 were systematically investigated by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM). The ternary La-Mg-Ni alloy was mostly composed of 2H-A₂B₇ phase with minor inclusions of 3R-A₅B₁₉, 2H-A₅B₁₉ and 3R-AB₃ phases existing as parts of intergrowth structures with the major A₂B₇ phase. Most parts of the major 2H-A₂B₇ phase containing Mg exhibited an excellent crystal structure retention after the hydrogen absorption–desorption cycles at 80 °C. Two types of defected bands were found to develop after the first hydrogen absorption–desorption cycle. The first ones are amorphous bands developed inside the minor 3R-AB₃ phase, while the second ones develop as heterogeneously strained regions inside the major 2H-A₂B₇ phase. Both the defected bands are considered to be responsible for the irreversible hydrogen capacity of the A₂B₇-based La_1.5Mg_0.5Ni_7.0 alloy during the hydrogen absorption–desorption cycles at 80 °C.