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