Improving Hydride Conductivity in Layered Perovskites via Crystal Engineering

Abstract

Hydride ion conduction in layered perovskites is of great interest for sustainable-energy applications. In this report we study Ba2ScHO3, a recently synthesized oxyhydride with an unusual anion ordering, using a multifaceted density functional theory approach involving both transition state calculations and molecular dynamics simulations. Beyond simply identifying the key ion migration pathways, we perform detailed analysis of transition states and identify key interactions which drive trends in ionic mobility. Our key findings are that ionic mobility is, remarkably, independent of hydride-oxide disorder, the dominant migration pathway changes under pressure, and a reduction in A-site cation size accelerates hydride diffusion. Local structural flexibility along migration pathways is understood in terms of dimensionality and ionic size, and we thus identify crystal engineering principles for rational design of ion conductors. On the basis of our new insights into these materials, we predict that Sr2ScHO3 will show improved conductivity over existing analogues.

Energy landscapes of perfect and defective solids: from structure prediction to ion conduction

“…Oxide ion conductivity has been investigated much more thoroughly than hydride ion conductivity, a much more recently observed phenomenon that has so far been restricted to a small number of recent studies of metal hydrides and oxyhydrides. We have recently studied hydride transport in Ba 2 ScHO 3 , a recently synthesised oxyhydride with an unusual anion ordering, examining with DFT the minima in the energy landscape and the transition states between them and also carrying out ab initio molecular dynamics simulations [88]. Ionic mobility depends on the hydride-oxide disorder but activation energies to migration do not; understanding how the migration energies depend on local structural flexibility has allowed us to predict that Sr 2 ScHO 3 will have larger conductivity than the Ba analogue.…”

Sequential Pressure-Induced B1–B2 Transitions in the Anion-Ordered Oxyhydride Ba2YHO3

“…As a result, there may be an intermediate region in which half of the unit cell adopts CsCl ordering, while the other half retains rock salt ordering. The primary motivation for studying these materials is their ability to act as hydride anion conductors, and a previous computational study has shown that the principal anion conduction mechanism in Ba 2 ScHO 3 changes under pressure, from an interstitial-mediated pathway to one involving vacancies 19 via a "bottleneck" transition state that has been observed in other hydride conductors. 21,22 The identification of materials with new hydride ordering patterns is, therefore, an important step in the study of novel hydride-ion conductors.…”

Hydride Conductivity in Nitride Hydrides

“…33 Our calculated activation barrier for hydride diffusion is lower than the measured barrier in LaHO, 34 the most conductive oxyhydride known. Among hydride conductors studied to date, SLHN is relatively unique in that interstitial hydrogen (rather than the hydrogen vacancy) is the primary defect driving the hydride migration; another example is the oxyhydride Ba 2 ScHO 3 , 35,36 in which equal mobility for hydride vacancies and interstitials has been predicted. We also use our methodology to assess the energetics of oxygen incorporation, highlighting the need to protect the material from oxygen.…”