Metastable Ag-Ln-S Nanocrystals with Ordered Cations and Vacancies Form through Structure-Templating Ag₂S Intermediate

Rare-earth chalcogenides have unusual crystal chemistry and properties, and have been difficult to access as nanocrystals, representing an opportunity for new materials discovery. Here, we report the colloidal synthesis of ternary Ag-Ln-S nanocrystals (Ln = Pr, Nd, Sm, and Gd) with a metastable crystal structure. Structural analysis suggests that the products adopt a trigonal anti-La₂O₃ structure that does not have a bulk congener in this phase space. The anti-La₂O₃ structure adopted by the Ag-Ln-S nanocrystals features a hexagonal close-packed array of sulfur anions and ordered cation positions: silver in tetrahedral coordination and octahedrally coordinated lanthanide cations, with a nonzero vacancy concentration on the lanthanide site. Product formation proceeds through a crystalline α-Ag₂S intermediate. The anti-La₂O₃ product structure mirrors several key crystallographic features with the intermediate, suggesting that α-Ag₂S serves as a structural template that directs the formation of the metastable product through a seed-mediated mechanism. All products have band gaps in the visible range and are shown to be weakly paramagnetic at low temperature.