Silver(I) selenide
# Silver(I) selenide

Silver selenide $(Ag_2Se)$ is the reaction product formed when selenium toning analog silver gelatine photo papers in photographic print toning. The selenium toner contains sodium selenite $(Na_2SeO_3)$ as one of its active ingredients, which is the source of the selenide $(Se^{2−})$ anion combining with the silver in the toning process.

It is found in nature as the mineral naumannite, a comparatively rare silver mineral which has nevertheless become recognized as important silver compound in some low-sulfur silver ores from mines in Nevada and Idaho.

## Structure

Silver selenide has two crystal phases on the bulk phase diagram. At lower temperatures, it has an orthorhombic structure, $β-Ag_2Se.$ This orthorhombic phase, stable at room temperature, is a narrow-gap semiconductor, with space group $P2_12_12_1.$ The exact size of the band gap has been given variously from 0.02 eV to 0.22 eV.

There is also a high temperature cubic phase, $α-Ag_2Se.,$ which it transforms into at temperatures above 130 °C. This high temperature phase has space group Im3m, No. 229, Pearson symbol cI20. The phase transition increases ionic conductivity by 10,000 times to about 2 S/cm.

A third metastable phase with a monoclinic structure and space group $P2_1/n$ is known to form for colloidal $Ag_2Se$ nanocrystals. The crystal structure of this polymorph is highly related to the acanthite phase of silver sulfide. For $Ag_2Se,$ this polymorph is increasingly unstable for larger crystallites, which explains its absence on the bulk phase diagram. It is thought that the influence of the surface energy plays a role stabilizing this phase on the nanoscale, thus allowing its experimental isolation. Electronic structure calculations for this polymorph of $Ag_2Se$ suggest that it is a narrow-gap semiconductor, which is supported by experimental evidence as well. Prior to 2021, the crystal structure of this polymorph was unknown, and this material was informally referred to as a "tetragonal" or "pseudo-tetragonal" polymorph of $Ag_2Se.$ This terminology, while not technically correct, is prevalent in the literature pertaining to this metastable phase of $Ag_2Se.$

