Iron(II) selenide refers to a number of

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

s of ferrous iron and

selenide A selenide is a chemical compound containing a selenium anion with oxidation number of −2 (Se2−), much as sulfur does in a sulfide. The chemistry of the selenides and sulfides is similar. Similar to sulfide, in aqueous solution, the selenide ion ...

(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 becomes a rectangular prism with a squar ...

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'' has ...

NiAs Nias ( id, Pulau Nias, Nias language: ''Tanö Niha'') (sometimes called Little Sumatra in English) 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 ...

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 refers to the relationship between the quantities of reactants and products before, during, and following chemical reactions. Stoichiometry is founded on the law of conservation of mass where the total mass of the reactants equ ...

, and FeSe₂, which occurs as the

marcasite The mineral marcasite, sometimes called “white iron pyrite”, is iron sulfide (FeS2) with orthorhombic crystal structure. It is physically and crystallographically distinct from pyrite, which is iron sulfide with cubic crystal structure. Bot ...

-structure natural mineral

ferroselite Orthorhombic ferroselite and its isometric polymorph dzharkenite are iron selenides of general formula FeSe2 precipitated under reducing conditions in anoxic environments. They are a source of selenium in the Rocky Mountains where selenium occurr ...

, 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 giv ...

-structure mineral dzharkenite. It is used in electrical

semiconductor A semiconductor is a material which has an electrical conductivity value falling between that of a conductor, such as copper, and an insulator, such as glass. Its resistivity falls as its temperature rises; metals behave in the opposite way. ...

Superconductivity

''β''-FeSe is the simplest

iron-based superconductor Iron-based superconductors (FeSC) are iron-containing chemical compounds whose superconducting properties were discovered in 2006. In 2008, led by recently discovered iron pnictide compounds (originally known as oxypnictides), they were in the firs ...

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 Intercalation may refer to: * Intercalation (chemistry), insertion of a molecule (or ion) into layered solids such as graphite *Intercalation (timekeeping), insertion of a leap day, week or month into some calendar years to make the calendar foll ...

, or after

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

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 Superconductivity is a set of physical properties observed in certain materials where electrical resistance vanishes and magnetic flux fields are expelled from the material. Any material exhibiting these properties is a superconductor. Unlike ...

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

Iron(II) compounds Selenides Semiconductor materials Nickel arsenide structure type Superconductors {{Inorganic-compound-stub