Halide-Driven Stabilization of a Hexagonal Polymorph with a Distinct and Disordered Li Substructure in Li3.25GeS3.25Cl0.75

Abstract Targeted halide substitution provides an effective route to access structural motifs in thio-LISICON systems. Here, partial substitution of S2– by Cl– in Li4GeS4 stabilizes a hexagonal polymorph near the composition Li3.25GeS3.25Cl0.75. Ab initio structure determination by joint refinement of time-of-flight neutron powder diffraction and X-ray diffraction data supports a P63mc average structure built on a hexagonal close-packed anion framework. The resulting framework exhibits a distinct Li-site topology with symmetry-inequivalent Li environments, including a shared tetrahedral framework site and additional tetrahedral and octahedral interstitial sites. Systematic investigation of the Li4–xGeS4–xClx series shows that the hexagonal phase is preferentially stabilized near the nominal x = 0.75 composition, for which no secondary crystalline phase was detected by PXRD. Structural analysis indicates that halide substitution produces a rearranged and partially disordered Li substructure. Bond-valence energy landscape calculations show that the computed Li-energy topology is highly sensitive to the representation of Li/Ge occupational disorder, with a Ge-free sensitivity model recovering the crystallographic Li1-related minimum and markedly altering the calculated percolation topology. Li3.25GeS3.25Cl0.75 exhibits a room-temperature ionic conductivity of 3.91 × 10–6 S cm–1. These results demonstrate that halide substitution can stabilize hexagonal polymorphs and reorganize the Li substructure in sulfide solid electrolytes, providing a platform for examining how structural disorder and Li-site topology influence ion transport.