Kinetically Stabilized Cation Arrangement in Li₃ YCl₆ Superionic Conductor during Solid-State Reaction

The main approach for exploring metastable materials is via trial-and-error synthesis, and there is limited understanding of how metastable materials are kinetically stabilized. In this study, a metastable phase superionic conductor, β-Li₃ YCl₆ , is discovered through in situ X-ray diffraction after heating a mixture of LiCl and YCl₃ powders. While Cl^- arrangement is represented as a hexagonal close packed structure in both metastable β-Li₃ YCl₆ synthesized below 600 K and stable α-Li₃ YCl₆ above 600 K, the arrangement of Li⁺ and Y³⁺ in β-Li₃ YCl₆ determined by neutron diffraction brought about the cell with a 1/√3 a-axis and a similar c-axis of stable α-Li₃ YCl₆ . Higher Li⁺ ion conductivity and lower activation energy for Li⁺ transport are observed in comparison with α-Li₃ YCl₆ . The computationally calculated low migration barrier of Li⁺ supports the low activation energy for Li⁺ conduction, and the calculated high migration barrier of Y³⁺ kinetically stabilizes this metastable phase by impeding phase transformation to α-Li₃ YCl₆ . This work shows that the combination of in situ observation of solid-state reactions and computation of the migration energy can facilitate the comprehension of the solid-state reactions allowing kinetic stabilization of metastable materials, and can enable the discovery of new metastable materials in a short time.