Silver(I) selenide

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.

Silver(I) selenide

Silver(I) selenide
Names
IUPAC name Silver(I) selenide
Other names Naumannite Argentous selenide
Identifiers
CAS Number	1302-09-6 Y
3D model (JSmol)	Interactive image
ECHA InfoCard	100.013.727
PubChem CID	6914520
UNII	QZ0B90W7EQ Y
CompTox Dashboard (EPA)	DTXSID2061644
InChI InChI=1S/2Ag.Se Key: GNWCVDGUVZRYLC-UHFFFAOYSA-N
SMILES [Se].[Ag].[Ag]
Properties
Chemical formula	Ag2Se
Molar mass	294.7 g/mol
Density	8.216 g/cm3, solid
Melting point	896.85 °C (1,646.33 °F; 1,170.00 K)
Solubility in water	insoluble
Band gap	0.15 eV [1]
Structure
Crystal structure	orthorhombic, oP12
Space group	P212121, No. 19
Thermochemistry
Enthalpy of fusion (ΔfH⦵fus)	56.9 J/g
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). N verify (what is YN ?) Infobox references

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.^[2][3]

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.^[4]

There is also a high temperature cubic phase, α-Ag₂Se.,^[5] 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.^[6] 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.^[7] 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,^[6] which is supported by experimental evidence as well.^[8] 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.^[9] This terminology, while not technically correct, is prevalent in the literature pertaining to this metastable phase of Ag₂Se.