Silver(I) selenide

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Silver(I) selenide

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Template:Longitem	Ag2Se
Molar mass	294.7 g/mol
Density	8.216 g/cm3, solid
Melting point	Template:Chembox CalcTemperatures
Band gap	0.15 eV
Template:Longitem	orthorhombic, oP12
Template:Longitem	P212121, No. 19

Silver selenide (Ag₂Se) is the reaction product formed when selenium toning analog silver gelatine photo papers in photographic print toning. The selenium toner contains sodium selenite (Na₂SeO₃) as one of its active ingredients, which is the source of the selenide (Se²⁻) 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-Ag₂Se. This orthorhombic phase, stable at room temperature, is a narrow-gap semiconductor, with space group P2₁2₁2₁. 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-Ag₂Se., 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 P2₁/n is known to form for colloidal Ag₂Se nanocrystals. The crystal structure of this polymorph is highly related to the acanthite phase of silver sulfide. For Ag₂Se, 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 Ag₂Se 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 Ag₂Se. This terminology, while not technically correct, is prevalent in the literature pertaining to this metastable phase of Ag₂Se.

References

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