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

Abstract

Several K2NiF4-type oxyhydrides (La2-x-ySrx+yLiH1-x-yO3-y) have in recent years been shown to exhibit significant transport of hydride ions. The migration mechanism of hydride conduction is, however, still not understood. In the following contribution, we explore formation and stability of defects, and the hydride ion migration mechanisms in three fast hydride ion conducting K2NiF4-type oxyhydrides (La2LiHO3, LaSrLiH2O2 to Sr2LiH3O) based on first principles calculations. Our results demonstrate that O_H^/ and v_H^• are the dominant defects, coexistent with minor amounts of v_O^(••). Both v_H^• and v_O^(••) exhibits strong site preference at axial sites of 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 oxyhydrides’ composition. 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, O_H^/ and v_H^• 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 give the advantage to enhance the hydride ion conductivity, for instance by introducing acceptor dopants on cations sites to create more vacant sites.

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Publication details

The article was received on 23 Sep 2017, accepted on 18 Nov 2017 and first published on 18 Nov 2017


Article type: Paper
DOI: 10.1039/C7TA08435H
Citation: J. Mater. Chem. A, 2017, Accepted Manuscript
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    Defects formation and their effects on hydride ion transport properties in a series of K2NiF4-type oxyhydrides

    X. Liu, T. S. Bjorheim and R. Haugsrud, J. Mater. Chem. A, 2017, Accepted Manuscript , DOI: 10.1039/C7TA08435H

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