Non-equilibration of Mg
2Ni by nickel enrichment was investigated by using the bulk mechanical alloying (BMA). Although little or no solubility of nickel in Mg
2Ni was present in the phase diagram of Mg-Ni system, BMA enables us to make solid-state synthesis of Mg
2-xNi for 0<x<0.5 with success. The synthesized Mg
2-xNi has a common crystalline, nano-sized structure of Mg
2Ni together with a small amount of amorphous phase. Through precise analysis of the measured pressure-composition isotherms for these Mg
2-xNi samples, it was found that two hydride phases should exist for Mg
2NiH
4. This formation of low-temperature phase Mg
2NiH
4 is common to hydride formation from the nano-structured Mg
2Ni. With nickel enrichment, the maximum storage capacity decreased irrespectively of the holding temperature. This might be because the hydrogen-site energy distribution is broadened by the intrinsic strains constraining the nano-grain of Mg
2Ni. This is just corresponding to the hydrogen ordering especially in low temperature. Effect of non-equilibration on the hydrogen absorption can be seen in the monotonic reduction of the transformation temperature from high-temperature to low-temperature phases of Mg
2NiH
4. The amorphous phase became never negligible in Mg
1.5Ni. Its hydrogen behavior was a little different from those for Mg
2Ni and Mg
1.75Ni, especially in low temperature.
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