Hydride ion diffusion along grain boundaries in titanium nitride

Density functional theory (DFT) simulations are utilized to study the absorption and diffusion of hydride ions along four types of titanium nitride grain boundaries: open and compact structures of Σ5 (210)[001] and Σ5 (310)[001]. The absorption energy into bulk vacancies ranges from approx. −0.7 eV in TiN0.7 to −0.38 eV for a single vacancy in TiN, and shows a minor dependence on the hydrogen content in the structure. The absorption energy for vacancy sites at the grain boundaries ranges from −0.44 eV to −0.66 eV. Hydrogen absorption into interstitial sites at the grain boundaries is less favourable, in part due to a positive and fairly large zero-point energy contribution, with absorption enthalpies between −0.09 eV and 0.06 eV. The interstitial diffusion barriers are 1.34 eV and 0.83 eV in the open Σ5 (310) and Σ5 (210) grain boundaries, respectively. In the compact grain boundaries, the migrating hydrogen specie changed oxidation state from a hydride ion (H−0.5) to a proton (H0.3+) at the transition state as it formed bonds to nitrogen along the diffusion path. The diffusion barriers are larger than in the corresponding open structures: 1.75 eV and 1.02 eV in the compact Σ5 (310) and Σ5 (210) structures, respectively. The considered grain boundaries and diffusion pathways cannot explain the fast interfacial hydride ion transport reported for hydrogen separation membranes based on TiNx thin films with columnar microstructure.