Configuration\nand Dynamics of Hydride Ions in the\nNitride-Hydride Catalyst Ca<sub>3</sub>CrN<sub>3</sub>H

We report results on the configuration and vibrational\ndynamics\nof hydride ions (H<sup>–</sup>) in the novel mixed-anion, nitride-hydride,\ncatalyst Ca<sub>3</sub>CrN<sub>3</sub>H by means of variable temperature\ninelastic neutron scattering experiments, harmonic phonon calculations,\nand machine-learning molecular dynamics calculations. The combined\nanalyses of experimental and theoretical data show that the vibrational\ndynamics of H<sup>–</sup> are manifested as a broad, asymmetric\nvibrational band between 80 and 130 meV. These vibrational dynamics\nare generally anharmonic in nature and polarized along the crystallographic <i>c</i> axis, and their dispersive character reveals significant\ninteractions between neighboring H<sup>–</sup> in the material.\nWe find that most H<sup>–</sup> are surrounded by two other\nH<sup>–</sup> and that the H<sup>–</sup> sites of the\nstudied sample have an occupancy of at least 95%. We argue that this\nhigh H<sup>–</sup> occupancy may be related to the material’s\nhigh efficiency as a catalyst for ammonia synthesis.