2. Quantum Chemistry in Perovskite Fluoride and Hydride: Nanoscale Hydride Ion Conduction

Hydride ion, which implies negatively charged hydrogen, has recently attracted much scientific attention from the viewpoint of nanoscale fast ion transport. Molecular orbital calculations based on density functional theory were performed for perovskite fluoride and hydride, in order to investigate the hydride ion-conducting mechanism. In hydride ion-doped KMgF₃ perovskite, it was found that hydride ion conduction occurs, combined with “competitive fluctuation”, which implies that fluorine anion also migrates around local minimum during hydride ion conduction. The activation energy for hydride ion conduction was estimated to be 0.61–0.85 eV. It was also found that hydride ion conduction occurs in KMgH₃ perovskite. The activation energy for hydride ion conduction was estimated to be 0.40–0.61 eV. From the viewpoint of structural stability at high temperature, it was concluded that hydride ion-doped KMgF₃ perovskite is more favourable than KMgH₃ perovskite. In comparison with proton-conducting perovskites, it was concluded that hydride ion conducting perovskites can be utilized as fast ion conductor. Finally, we discuss hydride ion safety and outlook.