Rigidity-Driven Structural Isomers in the NaCl-Ga₂S₃ System: Implications for Energy Storage

Alternative energy sources require the search for innovative materials with promising functionalities. Systems with unusual chemical properties represent an insufficiently explored domain, concealing unexpected features. Using diffraction and Raman spectroscopy over a wide temperature range, supported by first-principles simulations, a rare phenomenon is unveiled: phase-dependent chemical interactions between binary components in the NaCl-Ga₂S₃ system. In this unique occurrence, previously intact binary crystalline species transform upon melting into mixed liquid structural isomers, forming bonds with new partners. The chemical combinatorics appears to be fully reversible for stable crystals and liquids. Despite this, rapidly frozen glasses out of thermodynamic equilibrium remain in a metastable isomeric state, offering remarkable properties, particularly a high room-temperature Na⁺ conductivity, comparable to the best sodium halide superionic conductors and therefore encouraging for sodium solid-state batteries and energy applications. A rigidity paradigm is responsible for the observed phenomenon, as the extremely constrained Ga₂S₃ crystal lattice does not survive viscous flow, breaking up at a short-range level. The removal of rigidity constraints and dense packing leads to a significant increase in empty space, which is the origin of high sodium diffusivity. Broadly, the rigidity-driven structural isomerism opens up an inspiring path to the discovery of atypical materials.