1 Istituto Italiano di Tecnologia, Via Morego 30, 16163 Genova, Italy.
2 Department of Chemistry, University of Toronto, Toronto, Ontario M5S 3H6, Canada.
3 Department of Chemistry and Biochemistry and Centre for NanoScience Research, Concordia University, 7141 Sherbrooke Street West, Montréal, Quebec H4B 1R6, Canada.
4 Momentum Transfer GmbH, Luruper Hauptstraße 1, 22547 Hamburg, Germany.
5 University of Toronto, Department of Materials Science and Engineering, Toronto, Ontario M5S3E4, Canada.

We resolve a phase identification controversy in the Ag-Sn-S material system by unraveling the polymorphic structure of nanocrystals within the argyrodite material family. Argyrodites are a class of superionic materials used in their bulk form for applications in solid-state batteries and thermoelectrics, where their advantageous properties relate to their polymorphism. However, despite their well-studied bulk applications, the limited exploration at the nanoscale has left considerable potential for the discovery of emerging properties due to size effects. Further, phase identification presents a prominent challenge to the study of polymorphs in superionic conductors and related materials. In this work, we synthesize canfieldite-like (Ag₈SnS₆) nanocrystals to understand their formation and structural behavior at the nanoscale. We observe the emergence of emissive, metastable, cluster-like species. Then, high-resolution transmission electron microscopy reveals indistinguishable polymorphs of canfieldite due to identical heavy-atom frameworks. However, using synchrotron X-ray total scattering for pair distribution function analysis, we uncover structural distortions, showing a pseudo-orthorhombic configuration that likely gives rise to the red emission. Further, we investigate the optical properties and structure of Ag₈SnS₆ nanocrystals upon the addition of Zn²⁺, the cation of interest in the canfieldite vs pirquitasite (Ag₂ZnSnS₄) phase identification controversy. We show that Zn²⁺ is incorporated in the canfieldite-like structure through the replacement of Ag⁺, boosting the emission. Our results solve a standing phase identification challenge and uncover fundamental insights for the synthesis and structure of canfieldite nanocrystals, laying the ground for the exploration of other argyrodite materials with emerging properties at the nanoscale.