Structure Analysis of H by Channeling <sup>15</sup>N-NRA and Discovery of H-induced Jahn-Teller Effects

Hydrogen readily permeates various materials and modulates their physical properties. Identification of the hydrogen lattice location in crystals is key to understanding and controlling the hydrogen-induced phenomena. Combining nuclear reaction analysis (NRA) with the ion channeling technique, we experimentally determined the locations of H and D in epitaxial nanofilms of titanium hydrides. We found that 11 at.% of H are located at the octahedral site with the remaining H atoms in the tetrahedral site. Density functional theory calculations revealed that the partial octahedral site occupation is attributed to the Jahn-Teller effect induced by hydrogen. In contrast, D was found to solely occupy the tetrahedral site owing to the mass effect on the zero-point vibrational energy. This study demonstrates the potential to fabricate hydrides with arbitrary site occupancy by tuning the isotope ratio, which potentially allows for control over their electronic properties and leads to the discovery of novel phenomena.