Silver selenide (Ag₂Se) is the reaction product formed when selenium toning analog silver gelatine photo papers in

photographic print toning In photography, toning is a method of altering the color of black-and-white photographs. In analog photography, it is a chemical process carried out on metal salt-based prints, such as silver prints, iron-based prints (cyanotype or Van Dyke b ...

. 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 Silver is a chemical element with the symbol Ag (from the Latin ', derived from the Proto-Indo-European ''h₂erǵ'': "shiny" or "white") and atomic number 47. A soft, white, lustrous transition metal, it exhibits the highest electrical ...

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 In crystallography, the orthorhombic crystal system is one of the 7 crystal systems. Orthorhombic lattices result from stretching a cubic lattice along two of its orthogonal pairs by two different factors, resulting in a rectangular prism with ...

structure, β-Ag₂Se. This orthorhombic phase, stable at room temperature, is a

narrow-gap semiconductor Narrow-gap semiconductors are semiconducting materials with a band gap that is comparatively small compared to that of silicon, i.e. smaller than 1.11 eV at room temperature. They are used as infrared detectors or thermoelectrics. List of narrow- ...

, 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 In mathematics, physics and chemistry, a space group is the symmetry group of an object in space, usually in three dimensions. The elements of a space group (its symmetry operations) are the rigid transformations of an object that leave it uncha ...

Imm, No. 229,

Pearson symbol The Pearson symbol, or Pearson notation, is used in crystallography as a means of describing a crystal structure, and was originated by W. B. Pearson. The symbol is made up of two letters followed by a number. For example: * Diamond structure ...

cI20. The phase transition increases ionic conductivity by 10,000 times to about 2 S/cm. A third metastable phase with a

monoclinic In crystallography, the monoclinic crystal system is one of the seven crystal systems. A crystal system is described by three vectors. In the monoclinic system, the crystal is described by vectors of unequal lengths, as in the orthorhombic ...

structure and space group P2₁/n is known to form for colloidal AgSe nanocrystals. The crystal structure of this polymorph is highly related to the

acanthite Acanthite is a form of silver sulfide with the chemical formula Ag2S. It crystallizes in the monoclinic system and is the stable form of silver sulfide below . Argentite is the stable form above that temperature. As argentite cools below that te ...

phase of

silver sulfide Silver sulfide is an inorganic compound with the formula . A dense black solid, it is the only sulfide of silver. It is useful as a photosensitizer in photography. It constitutes the tarnish that forms over time on silverware and other silver o ...

. For AgSe, 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 In surface science, surface free energy (also interfacial free energy or surface energy) quantifies the disruption of intermolecular bonds that occurs when a surface is created. In solid-state physics, surfaces must be intrinsically less ener ...

plays a role stabilizing this phase on the nanoscale, thus allowing its experimental isolation. Electronic structure calculations for this polymorph of AgSe 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 In crystallography, the tetragonal crystal system is one of the 7 crystal systems. Tetragonal crystal lattices result from stretching a cubic lattice along one of its lattice vectors, so that the cube becomes a rectangular prism with a squar ...

" or "pseudo-tetragonal" polymorph of AgSe. This terminology, while not technically correct, is prevalent in the literature pertaining to this metastable phase of AgSe.

References

Silver compounds Selenides Photographic chemicals {{inorganic-compound-stub