Analysis of phase transformation in Mg2NiH4 via in situ synchrotron X-ray measurements

Mg-based alloys such as Mg2NiH4 are still attracting attention as high-capacity hydrogen storage materials. Mg2NiH4 undergoes the phase transformation between low-temperature (LT) and high temperature (HT) phases near the hydrogen desorption temperature. Herein, the crystallographic changes of Mg2NiH4 were investigated to clarify the behavior of the phase transformation by in situ synchrotron radiation X-ray measurements. The samples with three thermal histories (slow cooling, quenching from the HT phase, and hydrogenation below the transformation temperature) were prepared. These thermal histories affected the ratio of the LT phase without microtwinning (LT1) and with microtwinning (LT2). As the LT2/LT1 ratio increases, the phase transformation temperature decreases, and the phase transformation accelerates. X-ray diffraction peak analyses showed that the crystallite size and the lattice strain depend on the thermal history of the sample. X-ray absorption spectra indicated an increase in the short-range disorder of Mg2NiH4 when hydrogenated below the transformation temperature, which causes anomalous phase transformation and modulates the transformation temperature.(c) 2022 Elsevier B.V. All rights reserved.