Crystal Structure of Colloidally Prepared Metastable Ag₂Se Nanocrystals

Structural polymorphism is known for many bulk materials; however, on the nanoscale metastable polymorphs tend to form more readily than in the bulk, and with more structural variety. One such metastable polymorph observed for colloidal Ag₂Se nanocrystals has traditionally been referred to as the "tetragonal" phase. While there are reports on the chemistry and properties of this metastable polymorph, its crystal structure, and therefore electronic structure, has yet to be determined. We report that an anti-PbCl₂-like structure type (space group P2₁/n) more accurately describes the powder X-ray diffraction and X-ray total scattering patterns of colloidal Ag₂Se nanocrystals prepared by several different methods. Density functional theory (DFT) calculations indicate that this anti-PbCl₂-like Ag₂Se polymorph is a dynamically stable, narrow-band-gap semiconductor. The anti-PbCl₂-like structure of Ag₂Se is a low-lying metastable polymorph at 5-25 meV/atom above the ground state, depending on the exchange-correlation functional used.