Kinetically Stabilized Cation Arrangement in Li3YCl6 Superionic Conductor during Solid‐State Reaction

Abstract 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 ‐Li 3 YCl 6 , is discovered through in situ X‐ray diffraction after heating a mixture of LiCl and YCl 3 powders. While Cl − arrangement is represented as a hexagonal close packed structure in both metastable β ‐Li 3 YCl 6 synthesized below 600 K and stable α ‐Li 3 YCl 6 above 600 K, the arrangement of Li + and Y 3+ in β ‐Li 3 YCl 6 determined by neutron diffraction brought about the cell with a 1/√3 a ‐axis and a similar c ‐axis of stable α ‐Li 3 YCl 6 . Higher Li + ion conductivity and lower activation energy for Li + transport are observed in comparison with α ‐Li 3 YCl 6 . The computationally calculated low migration barrier of Li + supports the low activation energy for Li + conduction, and the calculated high migration barrier of Y 3+ kinetically stabilizes this metastable phase by impeding phase transformation to α ‐Li 3 YCl 6 . 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.