Influence of Halide Substitution on Local and Average Structure, Lattice Dynamics, and Transport Properties in Cu ₆ PS ₅ X 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 Cu 6 PS 5 X (X = Cl, Br, I, Cl 0.5 Br 0.5, Cl 0.5 I 0.5, and Br 0.5 I 0.5 ) within a combined experimental and computational framework. All compositions adopt an average cubic F4̅3m structure at room temperature, while local structural analysis reveals deviations from cubic symmetry consistent with a monoclinic Cc model involving PS 4 3– tetrahedral tilting. 31 P MAS NMR spectroscopy corroborates this local symmetry breaking through multiple distinct phosphorus environments arising from relative tetrahedral orientation rather than S 2– /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 multiphonon excitation of low-frequency Cu + vibrational modes. Together, these findings offer insight into structure–property relationships in Cu 6 PS 5 X 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.