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Description
Ternary lithium lanthanide chlorides, Li₃RCl₆ (R = rare earth), have recently emerged as promising solid electrolytes. Historically, compounds of the Li₃MCl₆ family (M = Sc, In, Ho, Er, Y, Yb, etc.) were regarded as poor ionic conductors, typically showing conductivities several orders of magnitude below relevant values. Recent advances in mechanochemical synthesis have changed this view, as as-milled powders now reach conductivities up to 10⁻³ S cm⁻¹, exemplified by Li₃HoCl₆. These findings have renewed interest in the structural origins of ionic transport in this class of halide conductors. At ambient conditions, the crystal structure follows a systematic size dependence: heavier rare earths such as Y and Tb–Tm stabilize a trigonal close-packed chloride lattice (P-3m1), whereas smaller cations like Yb and Lu adopt an orthorhombic Pnma arrangement. Yet most structural studies have focused on annealed samples, which exhibit lower conductivities. Our measurements confirm that annealing reduces ionic conductivity by about one order of magnitude compared with as-milled powders, underscoring the critical role of microstructural disorder in fast lithium transport. In situ X-ray diffraction shows that annealing drives complex temperature-dependent transformations across the series. Li₃TmCl₆ undergoes a direct trigonal-to-orthorhombic transition between 500 and 550 K. Li₃TbCl₆ follows a sequential pathway, stabilizing in the trigonal phase below 450 K, converting to a triclinic phase from 450 to 600 K, and then transforming into the orthorhombic Pnma structure up to 670 K, before reverting to the trigonal P-3m1 lattice at higher temperature. Li₃DyCl₆ displays an orthorhombic window between 500 and 630 K, with the trigonal modification favored outside this range. In contrast, Li₃HoCl₆ remains predominantly trigonal, with only a brief orthorhombic appearance near 500 K. Kinetic studies at fixed temperature reveal that substantial microstructural reorganization precedes the phase transformation, occurring with remarkably fast kinetics even at relatively low annealing temperatures. The conductivity loss upon annealing is therefore closely linked to structural and microstructural relaxation. By correlating conductivity with symmetry evolution and thermal history, this work highlights the central role of processing conditions in enabling or suppressing high ionic conductivity and offers insight into the design of high-performance solid electrolytes.
