In situ observation on hydrogenation of Mg-Ni films using environmental transmission electron microscope with aberration correction

In situ transmission electron microscopy (TEM) was performed to observe the hydrogenation of Mg-Ni films in a hydrogen atmosphere of 80-100 Pa. An aberration-corrected environmental TEM with a differential pumping system allows us to reveal the Angstrom-scale structure of the films in the initial stage of hydrogenation: first, nucleation and growth of Mg₂NiH₄ crystals with a lattice spacing of 0.22 nm in an Mg-rich amorphous matrix of the film occurs within 20 s after the start of the high-resolution observation, then crystallization of MgH₂ with a smaller spacing of 0.15 nm happens after approximately 1 min. Our in situ TEM method is also applicable to the analysis of other hydrogen-related materials.