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 ...

. 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; it has Symbol (chemistry), symbol Ag () and atomic number 47. A soft, whitish-gray, lustrous transition metal, it exhibits the highest electrical conductivity, thermal conductivity, and reflectivity of any metal. ...

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 Lattice (group), lattices result from stretching a cubic crystal system, cubic lattice along two of its orthogonal pairs by two different factors, res ...

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

Imm, No. 229, Pearson symbol cI20. The phase transition increases ionic conductivity by 10,000 times to about 2 S/cm. A third

metastable In chemistry and physics, metastability is an intermediate energetic state within a dynamical system other than the system's state of least energy. A ball resting in a hollow on a slope is a simple example of metastability. If the ball is onl ...

phase with a

monoclinic In crystallography, the monoclinic crystal system is one of the seven crystal systems. A crystal system is described by three Vector (geometric), vectors. In the monoclinic system, the crystal is described by vectors of unequal lengths, as in t ...

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 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 ob ...

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

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 (geometry), cube becomes a rectangular Pri ...

" 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