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

According to the Hume-Rothery rule, anions with a large difference in ionic radii, for example, chloride and iodide anions, cannot form solid solutions. In this study, we demonstrate that Ba2H3X (X = Cl, Br, or I) with the anti-Li3LaSb2-type structure, recently discovered to exhibit excellent hydride conductivity at low temperatures, can afford a trihalide solid solution containing a substantial amount of each halogen (e.g., Ba2H3Cl0.40Br0.35I0.25). The systematic study using Rietveld analysis and pair distribution function (PDF) analysis of synchrotron X-ray diffraction data revealed a significant deviation of the local structure from the average structural model for the solid solutions with fixed average radius but different size variances of X– anions. Analysis based on a triphasic model indicates that size-flexible H– anions play an important role in the formation of trihalide solid solutions, i.e., depending on the type of X, the HBa6 octahedron locally changes its size to accommodate the neighboring XBa6 octahedron. The chemical disorder in the hydride layer induced by the halide mixing reduces the activation barrier of the H– conduction, which is important for lowering operating temperatures. This study opens up the possibility to significantly expand the compositional space of anionic solid solutions by including hydride anion toward exploring novel functions.