Unraveling Metal-To-Metal Hydride Phase Transformation using In-Situ S/TEM Techniques

In our relentless pursuit of solutions to advance the decarbonization of our society and economy, hydrogen stands out for its exceptional qualities as a zero-emission fuel, energy storage medium, and chemical feedstock. Nevertheless, the challenge of compact hydrogen storage remains a daunting one in both science and technology. Safe storage of hydrogen, particularly in solid forms such as metal hydrides, offers numerous compelling benefits.<br/>However, further improving its storage properties requires a comprehensive understanding of nucleation and growth of metal-to-metal hydride phase transformation at the atomic scale. In this context, real-time visualization of the various steps of the transformation process is essential for a precise and quantitative understanding. For instance, for interpreting the hydrogen sorption property of materials, it is crucial to reveal the effect of stress/strain, the role of defects, and intermediate phase evolution during metal-to-metal hydride phase transformation.<br/>In this study, we use MgTi thin films as a model system to study metal-to-metal hydride phase transformation using in-situ Scanning/Transmission Electron microscopy (S/TEM). The phase transition from hexagonal to face-centered cubic in Mg to MgTiH_X is tracked through crystal structure changes observed by electron diffraction (ED) and in bulk plasmon resonance detected by electron energy loss spectroscopy (EELS). We also apply in-situ 4D STEM to investigate local structural displacements and strain developed during the process. Moreover, integrated differential phase contrast (iDPC) imaging aids in pinpointing hydrogen atom positions within the lattice. By combining these methods, we gain insights into the hydrogenation process and its effects on hydrogen storage properties.