Anion ordering enables fast H ⁻ conduction at low temperatures

Abstract

The introduction of chemical disorder by substitutional chemistry into ionic conductors is the most commonly used strategy to stabilize high-symmetric phases while maintaining ionic conductivity at lower temperatures. In recent years, hydride materials have received much attention owing to their potential for new energy applications, but there remains room for development in ionic conductivity below 300°C. Here, we show that layered anion-ordered Ba2−δH3−2δX (X = Cl, Br, and I) exhibit a remarkable conductivity, reaching 1 mS cm−1 at 200°C, with low activation barriers allowing H− conduction even at room temperature. In contrast to structurally related BaH2 (i.e., Ba2H4), the layered anion order in Ba2−δH3−2δX, along with Schottky defects, likely suppresses a structural transition, rather than the traditional chemical disorder, while retaining a highly symmetric hexagonal lattice. This discovery could open a new direction in electrochemical use of hydrogen in synthetic processes and energy devices.

Trihalide Mixing by Size-Flexible H– Ions in Layered Ba2H3 (Cl, Br, I)

“…Shown in Figure 2b are SXRD patterns of all of the Ba 2−δ H 3−2δ Cl x Br 1−x−y I y compounds (BHX1 to BHX5), with v 2 = 0.72−2.52% (Figure 2a and Table 1), plotted together with BHX0. 12 As expected, the BHX1−4 samples have nearly the same in-plane constant of a ∼ 4.46 Å (Figure 2c), in rough agreement with that of BHX0 (a = 4.46507(3) Å). 9 The c parameter of ∼7.63 Å is slightly longer (vsc = 7.61687(8) Å in BHX0 ).…”

“…Bond valence sum (BVS) calculations for BHX2 were conducted using the refined atomic positions and tabulated parameters. 39,40 The BVS values for Cl − (−0.79) and Br − (−1.32) are more or less acceptable, considering similar deviations in the end members 12 (−1.14 The variance v 2 is defined as…”

“…The BVS calculations gave reasonable values for all of the Figure 6a displays the anion-centered octahedra of BHX4 based on the triphasic model. The octahedral volumes for halide-and hydride-centered octahedra, V X and V H , are compared with the end members, 12 as shown in Table 2. Note that the volume of the XBa 6 octahedron varies with the z coordinate of the Ba cation at the 2d site.…”

“…For comparison, the octahedral volumes in the end members Ba 2−δ H 3−2δ X (X = Cl, Br, or I) obtained by Rietveld refinements are also shown. 12 Copyright©2021, CC BY-NC 4.0. conductivity without any obvious v 2 dependences (e.g., for BHX0−4, 2.0−0.96 × 10 −5 S cm −1 at 100 °C). Considering that the strategy of introducing chemical disorder by substitution is generally taken to improve ionic conductivity, 55−57 the present result that the conductivity does not change much seems surprising.…”

“…We have recently reported fast H − ion conductivity at low temperatures in Ba 2−δ H 3−2δ X (X = Cl, Br, and I) with alternate stacking of face-sharing HBa 6 and XBa 6 octahedral layers (Figure 1). 12 Prior to this report, the structurally related hexagonal BaH 2 was known to exhibit the highest H − ion conductivity at high temperatures. 13 This feature was shown to be lost below 450 °C due to a structural transition to a distorted orthorhombic phase.…”