Impact of Na Concentration on the Phase Transition Behavior and H⁻ Conductivities in the Ba–Li–Na–H–O Oxyhydride System

Abstract

K2NiF4‐type Ba–Li oxyhydride (BLHO) transitions to a so‐called hydride superionic conductor, exhibiting a high and essentially temperature‐independent hydride ion (H−) conductivity over 0.01 S cm−1 through the disordering of H− vacancies above 300 °C. In this study, a Ba–Li–Na–H–O oxyhydride system synthesized in which lithium is partially substituted with sodium in BLHO and investigated the effects of Na content on the phase transition behavior and the conductivity. Structural refinements and differential scanning calorimetry experiments confirmed a lowering trend in the phase transition temperatures and decreasing enthalpy changes for the transition with increasing Na content. Substitution of not <40% of Li with Na lowered the degree of ordered vacancies at the H− sites at room temperature and improved conductivities by more than two orders of magnitude in the low‐temperature region (T < 300 °C) before the phase transition. These findings clearly show that introducing Na into the lattice effectively stabilizes the high‐conductive phase of BLHO.

Pressure-Induced Anion Order–Disorder Transition in Layered Perovskite Sr2LiHOCl2

“…Thus, in order to attain the long-range hydride diffusion, it is necessary to increase in-plane hydride occupancy along with the introduction of anion vacancies, for example, by aliovalent substitution of Sr 2+ with Na + . 11,53 If one allows nextnearest-neighbor (NNN) hydride hopping, ionic conduction is more possible in the anion-ordered α phase as seen in BaTiO 3−x H x . 62,63 The ordered α-Sr 2 LiHOCl 2 with trans-and cis-coordination have both NN and NNN hopping pathways in contrast to Ln 2 LiHO 3 (Ln = La, Pr, Nd) with a trans-LiH 2 O 2 coordination only.…”

“…However, the site-percolation threshold for the 2D square lattice is approximately 0.59, , indicating that β-Sr 2 LiHOCl 2 with a hydride ion concentration (occupancy) of 0.5 at the equatorial site cannot achieve a long-range diffusion of H – via NN sites only. Thus, in order to attain the long-range hydride diffusion, it is necessary to increase in-plane hydride occupancy along with the introduction of anion vacancies, for example, by aliovalent substitution of Sr 2+ with Na + . , If one allows next-nearest-neighbor (NNN) hydride hopping, ionic conduction is more possible in the anion-ordered α phase as seen in BaTiO 3– x H x . , The ordered α-Sr 2 LiHOCl 2 with trans - and cis -coordination have both NN and NNN hopping pathways in contrast to Ln 2 LiHO 3 ( Ln = La, Pr, Nd) with a trans -LiH 2 O 2 coordination only. , Comparing different ion diffusion pathways would be a fascinating future endeavor with bond valence mapping serving as a widely utilized technique for analyzing conduction pathways in ionic conductors. , However, the bond valence calculation is not well-suited for hydride ions due to their large size flexibility (and covalency), preventing obtaining reliable bond-valence parameters. ,, …”

“…K 2 NiF 4 -type hydride-containing mixed-anion compounds have recently been developed as hydride ion conductors, ,− and thus Sr 2 LiHOCl 2 may also be a potential candidate provided that anion deficiency is introduced at the equatorial site. La 2– x – y Sr x + y LiH 1– x + y O 3– y exhibits a hydride conductivity of ∼10 –5 S·cm –1 at 300 °C, but in addition to in-plane diffusion through a vacancy-mediated mechanism, interlayer diffusion via apical sites is also suggested, which complicates the understanding of ionic conduction .…”

A new family of anti-perovskite oxyhydrides with tetrahedral GaO4 polyanions

“…Moreover, H − conduction in oxyhydrides, which consist of electropositive cations including alkali, alkaline-earth, and rare-earth ions, has also attracted attention from the perspective of the electrochemical use of hydrogen. By tuning the carrier amount and conduction path in crystal frameworks, fast ionic conduction was achieved in several compounds, such as La 2−x−y Sr x+y LiH 1−x+y O 3−y , 10 Ba 2 MHO 3 (M = Sc, Y), 11,12 Ba 1.75 LiH 2.7 O 0.9 , [13][14][15] and LaH 3−x O x/2 . 16,17 Despite such a rapid growth of oxyhydrides, the use of main group elements ( p-block metals) has still been limited, possibly due to the strong reducing property of H − that easily induces p-block metal deposition.…”

“…Moreover, H − conduction in oxyhydrides, which consist of electropositive cations including alkali, alkaline-earth, and rare-earth ions, has also attracted attention from the perspective of the electrochemical use of hydrogen. By tuning the carrier amount and conduction path in crystal frameworks, fast ionic conduction was achieved in several compounds, such as La 2− x − y Sr x + y LiH 1− x + y O 3− y , 10 Ba 2 MHO 3 (M = Sc, Y), 11,12 Ba 1.75 LiH 2.7 O 0.9 , 13–15 and LaH 3− x O x /2 . 16,17…”