Ba₂ScHO₃: H^– Conductive Layered Oxyhydride with H^– Site Selectivity

Abstract

Hydride (H–) conduction is a new frontier related to hydrogen transport in solids. Here, a new H– conductive oxyhydride Ba2ScHO3 was successfully synthesized using a high-pressure technique. Powder X-ray and neutron diffraction experiments investigated the fact that Ba2ScHO3 adopts a K2NiF4-type structure with H– ions preferentially occupying the apical sites, as supported by theoretical calculations. Electrochemical impedance spectra showed that Ba2ScHO3 exhibited H– conduction and a conductivity of 5.2 × 10–6 S cm–1 at 300 °C. This value is much higher than that of BaScO2H, which has an ideal perovskite structure, suggesting the advantage of layered structures for H– conduction. Tuning site selectivity of H– ions in layered oxyhydrides might be a promising strategy for designing fast H– conductors applicable for novel electrochemical devices.

Anion ordering enables fast H − conduction at low temperatures

“…In Fig. 3C, we compare the thermal evolution of total conductivities of Ba 2− H 3−2 X ( ~ 0.2) with those of the reported H − conductors (13)(14)(15)(16)(17)(18)(19)27). The conductivities in the range of 10 −3 to 10 −2 S cm −1 above 300°C are comparable to those of known representative materials such as BaH 2 (~5 × 10 −3 S cm −1 at 420°C) (13) and LaH 2.52 O 0.24 (2.6 × 10 −2 S cm −1 at 342°C) (18).…”

“…The dotted black line is an extrapolation of the total conductivity of the HT phase of BaH 2 to low temperatures. (C) Thermal evolution of the total conductivity of the hydride-halides Ba 2− H 3−2 X, together with those of other hydride conductors(13)(14)(15)(16)(17)27). Note that the bulk conductivities are shown for LaH 2.52 O 0.24 and NdHO(18,19).…”

First-Principles Calculations of Point Defect Formation and Anion Diffusion Mechanisms in the Oxyhydride Ba2ScHO3

“…These atmospheric conditions matched those applied during the measurements of the ionic conductivity of Ba2ScHO3 in our previous work. 14 Figure S4, while data regarding the local bonds near the interstitial defects are presented in Tables S8 to S11. Figure S4 demonstrates clear distortion of the anions around the interstitials.…”

“…Previous studies 14 determined the ionic conductivities of stoichiometric Ba2ScHO3 specimens without dopants, and in the present work we assumed that the ionic conductivity of undoped…”

“…[10][11][12][13] Recently, Ba2ScHO3 was reported as a new hydride ion conductor having the same K2NiF4-type structure but differing from La2-x-ySrx+yLiH1-x+yO3-y in that the hydride ions occupy only apical rather than equatorial sites. 14 As a consequence, hydride ions are believed to have the capacity to migrate through the rock-salt layers in this compound. However, the point defect species responsible for hydride ion conduction in Ba2ScHO3 and the associated conduction mechanisms have not yet been clarified.…”