Temperature Dependent Local Atomic Structure and Vibrational Dynamics of Barium Hydride and Calcium Hydride

Abstract

Solid-state ionic conductors that exhibit pure ionic transport of hydride anions are rare. In this study, we investigate two alkaline earth metal hydrides, barium hydride and calcium hydride, using neutron scattering techniques to understand how the local atomic environment plays a role in the diffusion of hydride ions. At high temperatures, barium hydride exhibits exceptional transport properties with ionic conductivities that are higher than those of many of the typical proton and oxide ion conductors in use today. Total neutron scattering and pair distribution function analysis reveal how a structural phase transition converts barium hydride from a modest ionic conductor into a fast ionic conductor through the introduction of disorder, deuterium site splitting, and dynamic structural fluctuations. Furthermore, neutron vibrational spectroscopy is employed to probe changes in the temperature evolution of the lattice dynamics and local energy landscape. These results improve our fundamental knowledge of the interplay between structure and dynamics governing a rare conduction process of hydride ions in solid-state materials.

A Heterogeneous Oxide Enables Reversible Calcium Electrodeposition for a Calcium Battery

“…29 The Ca(BH 4 ) 2 -tetrahydrofuran (THF) electrolyte is another well-studied system because of its best-in- 30 Computed migration barriers suggest that Ca-deficient CaH 2 is Ca 2+ -transmissive, while CaO and CaCO 3 are Ca 2+ -blocking at room temperature. 20 However, theory 31 and experiments 32,33 both indicate large energies for cation vacancy formation, making CaH 2 an unlikely cation conductor. These results illustrate that there is not yet a consensus on the chemical phase(s) and structural features that should be present in a practical Ca SEI, underscoring a critical need to clarify and spatially map the chemistry and structure of SEIs derived from best-in-class Ca electrolytes.…”

Uncovering the hydride ion diffusion pathway in barium hydride via neutron spectroscopy

“…1. The elastic scan reveals three distinct areas: (i) low T region (from 300 K up to about 670 K), (ii) intermediate T region (from about 670 K up to about 775 K) (iii) high T region (T > 775 K) 9 . In the low T region, the elastic intensity decrease is about 50%, which can be correlated to the temperature dependency of the vibrational density of states (VDOS), discussed later in this work and in the SI.…”

“…Barium hydride has been reported to have significant hydrogen sub-stoichiometries, particularly BaD x with x ~ 1.8, where the large concentration of vacancies leads to a vacancymediated transport mechanism 1,2,10 . A structural phase transition occurs around 773 K from the orthorhombic phase to a high symmetry hexagonal (Ni 2 In-type) phase with the space group P6 3 /mmc (194) 1,6,9 . The hexagonal unit cell has lattice parameters of a = 4.4571(2) Å and c = 6.7230(4) Å at T = 883 K 1 .…”

“…high-symmetry 2d site and instead resides on the 4f site 1,9 . Upon the phase transition, the ionic conductivity increases by more than an order of magnitude from 5 .…”

Strategies and Considerations for Least-Squares Analysis of Total Scattering Data

“…Discrete modeling of local coordination environments can provide answers beyond crystallography, but many times a material’s functionality is a combination of interactions across different length scales. A targeted approach to understanding multiscale interactions at play in a material is the box-car method. ,,− In this method, fits are performed by taking a set increment of r range, for example, 5 Å, and fitting the data against a candidate model at various length scales with this increment (i.e., 0–5, 5–10, 10–15 Å, etc.). A comparison of the derived parameters from each box-car, such as the R w or U iso , can indicate the interaction lengths where the model deviates from the data.…”