Influence of Halide Substitution on Local and Average Structure, Lattice Dynamics, and Transport Properties in Cu6PS5X Argyrodites

Halide-substituted argyrodite materials have attracted increasing attention for energy applications since compositional tuning provides an effective strategy to modulate their structure and transport characteristics. While Li⁺-based halide argyrodites have been extensively studied, a unified composition-resolved understanding of Cu⁺-based halide argyrodites that integrates phase evolution, local structure, lattice dynamics, and electronic and ionic transport remain limited. In this work, we investigate Cu₆PS₅X (X = Cl, Br, I, Cl_0.5Br_0.5, Cl_0.5I_0.5, and Br_0.5I_0.5) within a combined experimental and computational framework. All compositions adopt an average cubic  structure at room temperature, while local structural analysis reveals deviations from cubic symmetry consistent with a monoclinic  model involving PS₄³⁻ tetrahedral tilting. ³¹P MAS NMR spectroscopy corroborates this local symmetry breaking through multiple distinct phosphorus environments arising from relative tetrahedral orientation rather than S²⁻ / X⁻ site disorder. Halide substitution modifies the Cu⁺ conductivity through changes in the activation energy and the Arrhenius pre-exponential factor, following the Meyer-Neldel behavior, with additional contributions from variations in jump distances and migration pathways. Direction-projected phonon density of states analysis identifies low-frequency Cu⁺ vibrational components along the crystallographic migration pathways. Analysis of the Meyer-Neldel slope further suggests phonon assisted ion hopping involving multi-phonon excitation of low-frequency Cu⁺ vibrational modes. Together, these findings offer insight into structure-property relationships in Cu₆PS₅X and suggest that, alongside the migration energy landscape, the vibrational energy scale, thermal population, and directionality of mobile ion modes should be considered when interpreting ion transport, thereby providing a vibrational perspective for the design of solid-state ion conductors.