New Insights into Hydride Bonding, Dynamics, and Migration in La₂LiHO₃ Oxyhydride

Abstract

Hydride anion-conducting oxyhydrides have recently emerged as a brand new class of ionic conductors. Here we shed a first light onto their local vibrations, bonding mechanisms, and anion migration properties using the powerful combination of high-resolution inelastic neutron scattering and a set of rigorously experimentally validated density functional theory calculations. By means of charge-density analysis we establish the bonding to be strongly anisotropic; ionic in the perovskite layer and covalent in the rock salt layer. Climbing nudged elastic band calculations allow us to predict the hydride migration paths, which crucially we are able to link to the observed exotic ionic-covalent hybrid bonding nature. In particular, hydride migration in the rock salt layer is seen to be greatly hindered by the presence of covalent bonding, forcing in-plane hydride migration in the perovskite layer to be the dominant transport mechanism. On the basis of this microscopic insight into the transport and bonding, we are able to propose future candidates for materials that are likely to show enhanced hydride conductivity.

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

“…The conductivity of La 2 LiHO 3 synthesized by the high‐pressure method is also plotted for comparison. The activation energy for ionic conduction of the prepared sample was estimated as 69.2 kJ/mol, which is not only in good agreement with the values of o ‐La 2 LiHO 3 and t ‐La 2 LiHO 3 synthesized under high pressure but also the lowest activation barrier (0.68 eV, 66 kJ/mol) for H − hopping between the adjacent axial anion sites in the octahedra formed by the Li and anions, calculated by Fjellvåg et al . The total ionic conductivity (sum of the bulk + grain‐boundary contributions) at 275°C was estimated to be 3.85 × 10 −6 S/cm.…”

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

“…The DINS validation of the ab initio LD calculations, performed within the harmonic approximation, is further corroborated by comparison to vibrational INS spectrum from the INS spectrometer TOSCA, as shown in figure 3 [8]. Namely, we observe three very distinct spectral peaks which correspond to independent hydride anion motions in each of the spatial dimensions (x-, yand z-direction indicated by H x , H y and H z respectively in figure 3), showing that the hydride anions are situated in a 3D-multivariate harmonic potential.…”

“…A recent in silico study by Liu et al it was established that the migration of hydride anions in La 2 LiHO 3 , LaSrLiH 2 O 2 and Sr 2 LiH 3 O took place within the perovskite layer with calculated migration energies of 0.21 to 0.45 eV [7]. Similarly, Fjellvåg et al connected this observation to the anisotropic ionic-covalent mixed bonding [8]. The covalent bonding in the LaO rock salt layer is effectively hindering the hydride anions to migrate into and within the rock salt layer, forcing the migration to take place in the ionic bonded perovskite layer.…”

Na3SO4H—The First Representative of the Material Class of Sulfate Hydrides

“…Mainly during the last decade, this concept has become important due to the discovery of new mixed anionic hydrides. For instance, fast conductivity in a pure hydride anion conductor, La 2− x − y Sr x + y LiH 1− x + y O 3− y , [6–10] electronic conductivity in oxide hydride perovskite‐type titanates, [11–13] and even superconductivity [14] were found in oxide hydrides. Tuning of the anion chemistry, for instance, allows for a tailoring of the bonding situations from more ionic to more covalent or varying the polarizabilities from rather hard to soft anions, tailoring crystal fields, or electronic properties [1] .…”