Formation of defects and their effects on hydride ion transport properties in a series of K2NiF4-type oxyhydrides

Abstract

Several K₂NiF₄-type oxyhydrides (La_2−x−ySr_x+yLiH_1−x+yO_3−y) have in recent years been shown to exhibit significant transport of hydride ions. The migration mechanism of hydride ion conduction is, however, still not understood. In the following contribution, we explore the formation and stability of defects and the hydride ion migration mechanisms in three fast hydride ion conducting K₂NiF₄-type oxyhydrides (La₂LiHO₃, LaSrLiH₂O₂ and Sr₂LiH₃O) based on first principles calculations. Our results demonstrate that and are the dominant defects, coexistent with minor amounts of . Both and exhibit strong site preference at the axial sites of the LiX₆ octahedron, showing that hydride ion transport is strongly anisotropic in the crystal lattice. Hydride ions migrate through a vacancy-mediated mechanism with calculated migration enthalpies varying from 0.21 to 0.45 eV depending on the composition of the oxyhydrides. By mapping all relevant minimum energy pathways for hydride and oxygen ions, we reveal that the transport of hydride ions in La₂LiHO₃ is inhibited by slow oxygen ion (vacancy) migration. For LaSrLiH₂O₂ and Sr₂LiH₃O, and defect associates inhibit the hydride ion conduction further due to the strong binding energies of the complexes. However, the structural and compositional flexibility of K₂NiF₄-type oxyhydrides gives advantages for enhancing the hydride ion conductivity, for instance by introducing acceptor dopants on cation sites to create more vacant sites.