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

Abstract

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.