Configuration and Dynamics of Hydride Ions in the Nitride-Hydride Catalyst Ca₃CrN₃H

We report results on the configuration and vibrational dynamics of hydride ions (H–) in the novel mixed-anion, nitride-hydride, catalyst Ca3CrN3H by means of variable temperature inelastic neutron scattering experiments, harmonic phonon calculations, and machine-learning molecular dynamics calculations. The combined analyses of experimental and theoretical data show that the vibrational dynamics of H– are manifested as a broad, asymmetric vibrational band between 80 and 130 meV. These vibrational dynamics are generally anharmonic in nature and polarized along the crystallographic c axis, and their dispersive character reveals significant interactions between neighboring H– in the material. We find that most H– are surrounded by two other H– and that the H– sites of the studied sample have an occupancy of at least 95%. We argue that this high H– occupancy may be related to the material’s high efficiency as a catalyst for ammonia synthesis.