Emergence of Glass-like Thermal Transport through Anion-Engineering in Complex Silver Gallium Germanium Chalcogenides

Designing crystalline materials with ultralow thermal conductivity remains an important challenge for optimizing thermoelectric energy conversion and thermal barrier coatings. Crystalline solids exhibiting glass-like thermal transport are of special interest because they approach the minimum thermal conductivity achievable in an ordered structure. Here, we investigate the ultralow thermal conductivity of crystalline AgGaGeS8-8xSe8x and Ag2-xGa2-xGe2+xS8-8xSe8x (x = 0.00, 0.25, 0.50, 0.75, 1.00). In these systems, selenium incorporation strongly suppresses crystalline thermal transport, leading to glass-like properties while preserving structural crystallinity. Across all compositions, the ultralow thermal conductivity is associated with a disordered silver substructure characterized by weak Ag–Ch interactions and pronounced anharmonicity, where selenium substitution promotes softening of the silver dynamics. Heat-capacity measurements reveal boson peaks in the endmember compounds, which for AgGaGe3Se8 is significantly more intense and shifted to lower temperature. Together, these results suggest that selenium promotes low-frequency silver vibrations that drive the emergence of glass-like thermal transport. Furthermore, selenium exhibits a pronounced preference for the chalcogen site most strongly interacting with silver, demonstrating that local anion chemistry, rather than average composition, governs the thermal transport response. This selective occupancy provides a structural basis for the emergence of glass-like thermal conductivity at remarkably low selenium contents.