Metastable Ag–Ln–S Nanocrystals with Ordered Cations and Vacancies Form through Structure-Templating Ag2S Intermediate
来源：ACS Publications
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.
Here, we report the isolation of a metastable pseudohexagonal Ag–Ln–S (Ln = Pr, Nd, Sm, Gd) phase as colloidal nanocrystals. Our synthesis strategy relies on the decomposition of metal-dithiocarbamate (DTC) complexes in solution, the use of which circumvents some noted synthetic challenges by preforming the metal–sulfur bonds in an organometallic precursor. We identify a seed-mediated mechanism for product formation, which progresses through a crystalline α-Ag2S intermediate. Based on structural similarities to the isolated intermediate and evidence from diffraction, elemental composition, and microscopy, we propose the formation of an anti-La2O3 structure that does not appear on the bulk phase diagram in this composition space. This structure features a hexagonally close-packed anion sublattice, octahedrally coordinated lanthanide cations, and tetrahedrally coordinated silver cations. We find that the as-synthesized Ag–Ln–S nanocrystals have band gaps in the visible region and exhibit paramagnetic behavior at low temperatures.