Thermal and Structural Aspects of the Hydride-Conducting Oxyhydride La₂LiHO₃ Obtained via a Halide Flux Method

Abstract

Oxyhydrides, in which oxide and hydride anions share the same anionic lattice, are relatively rare compounds. LaLiHO belongs to this family. We report the synthesis of LaLiHO by means of an alkali halide flux method, which allows the production of larger quantities of material relative to the usually adopted synthesis routes. Powder X-ray and neutron diffraction studies show that LaLiHO adopts an n = 1 Ruddlesden-Popper (RP)-type structure with an orthorhombic distortion (Immm) due to hydride and oxide anion ordering. No sign of polymorphism is observed. LaLiHO is seen to decompose in an oxygen atmosphere at ∼450 °C into LaLiO. We show that the high mobility of hydride anions close to the decomposition temperature is likely the main factor in inducing the oxidation. The crystal structure of LaLiO is also determined and takes an n = 1 RP-type structure with an orthorhombic distortion (Fmmm). This newly reported large-scale synthesis approach, combined with the proven high thermal stability, is a key factor for potential practical applications of this oxyhydride in real devices.

Ambient pressure synthesis of La2LiHO3 as a solid electrolyte for a hydrogen electrochemical cell

“…The result of the Rietveld refinement of the NPD pattern is presented in Figure and the structural parameters that were obtained are summarized in Table with the corresponding reliability factors. The refined result was in good agreement with the reported structure with the space group of Immm . In addition to the NPD result, the Rietveld refinement of the synchrotron XRD pattern provides a good fitting with the model of the structure in which hydride and oxide ions are ordered in the axial sites of the perovskite layer (Figure and Table ).…”

“…The o ‐La 2 LiHO 3 pellet was synthesized using a conventional solid‐state reaction with 1.0 g of the La 2 LiHO 3 per unit volume in the synthesis container and 200 wt.% excess of LiH. The crystal structure was consistent with previously reported structures that were prepared by a different synthetic method . The ionic conductivity of o ‐La 2 LiHO 3 is approximately equal to the value of the sample obtained by the high‐pressure method.…”

“…The stability of the solid electrolyte is a key factor for developing an electrochemical device. The stability of La 2 LiHO 3 against ambient air and dry oxygen was studied in detail by Sjåstad, who found that this material decomposes into La 2 O 3 and La 2 LiO 3.5 above 300 and 450°C in ambient air, respectively. Although H − is unstable in an oxidizing atmosphere, the stability of oxyhydride La 2 LiHO 3 against water remained unknown, and thus we investigated the stability of La 2 LiHO 3 against water at room temperature.…”

“…The establishment of electrochemical devices with a gas/solid/gas configuration based on the H − conduction phenomenon would require the development of a synthetic method to produce gas‐tight pellets of H − conducting oxyhydrides. However, the few known synthetic methods to obtain these H − conducting oxyhydrides, for example, the high‐pressure method and LiCl flux method, limit the examination of the gas/solid/gas cell, which must be conducted with gas‐tight solid electrolyte pellets in the case of an electrolyte‐supported cell . In this respect, a synthetic method to obtain the sintered pellets of a H − conductive oxyhydride at large scale was reported for LaSrLiH 2 O 2 at ambient pressure, although the gas‐sealing property of the material was not examined .…”

A combined deep inelastic neutron scattering and ab initio lattice dynamics study of the hydride anion dynamics and bonding in La2LiHO3 oxyhydride

“…Several methods are in use to prepare La 2 LiHO 3 . Both solid-state techniques (high-pressure methods or under a H 2 -pressure) and various halide salt flux approaches that give phase pure compounds, including our recent discovery to utilize the flux below its melting temperature [1][2][3][4][5][6]. The compound normally takes an anion ordered orthorhombic Ruddlesden-Popper (RP) n=1 type structure, which is described as being built of n perovskite layers (n denoting number of perovskite layers) separated by one-half rock salt layer (figure 1).…”