Hydride ion (H⁻) transport behavior in barium hydride under high pressure

Abstract

Hydride ions (H-) have an appropriate size for fast transport, which makes the conduction of H- attractive. In this work, the H- transport properties of BaH2 have been investigated under pressure using in situ impedance spectroscopy measurements up to 11.2 GPa and density functional theoretical calculations. The H- transport properties, including ionic migration resistance, relaxation frequency, and relative permittivity, change significantly with pressure around 2.3 GPa, which can be attributed to the structural phase transition of BaH2. The ionic migration barrier energy of the P63/mmc phase decreases with pressure, which is responsible for the increased ionic conductivity. A huge dielectric constant at low frequencies is observed, which is related to the polarization of the H- dipoles. The current study establishes general guidelines for developing high-energy storage and conversion devices based on hydride ion transportation.

Neutron diffraction study of the α- to β-phase transition in BaD2 under high pressure

“…If the ionic migration path is assumed to be parallel to the (002)-plane, D2 migration mediated by D1 vacancies, then this localisation should have little effect on the conductivity. DFT modelling shows that increased pressure reduces the hopping barrier energy for D2 along a shorter migration path perpendicular to the (002)-plane, resulting in an increase in ionic conductivity (using the 2d-model), confirmed with high-pressure impedance spectroscopy measurements [3]. The onset of this increase in conductivity appears approximately 2 GPa higher in pressure than the reported change in symmetry, and plateaus around 6 GPa, showing a much less pronounced increase beyond this.…”

“…This relaxed arrangement of Ba in the -phase is accommodated by the compression of the a-axis and the delocalisation of the D1 ions, splitting with a partial occupancy on the 4f Wyckoff sites, see Figure 1(ii), also see SI. This results in highly mobile H − , causing a sharp increase in the electrical conductivity of the sample as reported previously both at high temperature [2] and under applied pressure at room temperature [3]. It is also of interest to reconsider the effects of isotope enrichment in this sample, as it is to be expected that H ions are more electronically mobile than D ions, following classical theory (∝ 1∕ √ ) as is seen elsewhere [22].…”

“…More recently Zhang et al [3] investigated the ionic transport behaviour under applied pressure, confirming that the high-temperature and high-pressure phases show the same increase in H − conductivity. However, there is little structural information available at high pressures including accurate hydrogen positions, key to understanding the high-ionic conductivity in the material.…”

“…Heavy alkali-earth hydrides, such as BaH 2 , have received little attention for this due to their low gravimetric density, and high decomposition temperature. However, recently BaH 2 was found to show fast H − ionic conductivity at elevated temperatures, with a large increase in the overall conductivity, which may lead to interesting applications in electrochemical energy storage and conversion systems [2], which has led to a renewed interest around heavy hydrides [3,4].…”

Synthesis of molecular metallic barium superhydride: pseudocubic BaH12

“…The calculated maximum T C is only about 30-38 K 19,20 for predicted P4/mmm-BaH 6 stable at 100 GPa, which has a hydrogen sublattice consisting of H 2 molecules and Hanions 19 . Lower barium hydride, BaH 2 , well-known for its extraordinarily anionic (H -) conductivity 21 , exists in Pnma modification below 2.3 GPa, whereas above 2.3 GPa it undergoes a transition to hexagonal Ni 2 In-type P6 3 /mmc phase 22 . At pressures above 41 GPa, BaH 2 transforms into P6/mmm modification, which metallizes at over 44 GPa, but its superconducting T C is close to zero 23 .…”