Iron(II) selenide refers to a number of

inorganic compound An inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds⁠that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as ''inorganic chemistry''. Inorgan ...

s of ferrous iron and

selenide A selenide is a chemical compound containing a selenium with oxidation number of −2. Similar to sulfide, selenides occur both as inorganic compounds and as organic derivatives, which are called organoselenium compound. Inorganic selenides Th ...

(Se²⁻). The phase diagram of the system Fe–Se reveals the existence of several non-stoichiometric phases between ~49 at. % Se and ~53 at. % Fe, and temperatures up to ~450 °C. The low temperature stable phases are the

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 ...

PbO-structure (P4/nmm) β-Fe_1−''x''Se and α-Fe₇Se₈. The high temperature phase is the

hexagonal In geometry, a hexagon (from Greek , , meaning "six", and , , meaning "corner, angle") is a six-sided polygon. The total of the internal angles of any simple (non-self-intersecting) hexagon is 720°. Regular hexagon A regular hexagon is d ...

NiAs Nias (, Nias: ''Tanö Niha'') is an island located off the western coast of Sumatra, Indonesia. Nias is also the name of the archipelago () of which the island is the centre, but also includes the Batu Islands to the southeast and the small ...

structure (P6₃/mmc) δ-Fe_1−''x''Se. Iron(II) selenide occurs naturally as the

-structure mineral achavalite. More selenium rich iron selenide phases are the γ phases (γ and γˈ), assigned the Fe₃Se₄

stoichiometry Stoichiometry () is the relationships between the masses of reactants and Product (chemistry), products before, during, and following chemical reactions. Stoichiometry is based on the law of conservation of mass; the total mass of reactants must ...

, and FeSe₂, which occurs as the marcasite-structure natural mineral ferroselite, or the rare

pyrite The mineral pyrite ( ), or iron pyrite, also known as fool's gold, is an iron sulfide with the chemical formula Fe S2 (iron (II) disulfide). Pyrite is the most abundant sulfide mineral. Pyrite's metallic luster and pale brass-yellow hue ...

-structure mineral dzharkenite. It is used in electrical

semiconductor A semiconductor is a material with electrical conductivity between that of a conductor and an insulator. Its conductivity can be modified by adding impurities (" doping") to its crystal structure. When two regions with different doping level ...

Superconductivity

''β''-FeSe is the simplest iron-based superconductor but with diverse properties. It starts to superconduct at 8 K at normal pressure but its critical temperature (''T''_c) is dramatically increased to 38 K under pressure, by means of intercalation, or after

quenching In materials science, quenching is the rapid cooling of a workpiece in water, gas, oil, polymer, air, or other fluids to obtain certain material properties. A type of heat treating, quenching prevents undesired low-temperature processes, suc ...

at high pressures. The combination of both intercalation and pressure results in re-emerging superconductivity at 48 K. In 2013 it was reported that a single atomic layer of FeSe epitaxially grown on SrTiO₃ is superconductive with a then-record transition temperature for iron-based superconductors of 70 K. This discovery has attracted significant attention and in 2014 a superconducting transition temperature of over 100K was reported for this system.

References

{{Selenides Iron(II) compounds Selenides Semiconductor materials Nickel arsenide structure type Superconductors