Iron(II) selenide  Iron(II) selenide refers to a number of inorganic compounds of ferrous iron and selenide (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 PbO-structure (P4/nmm) β-Fe1−"x"Se and α-Fe7Se8. The high temperature phase is the hexagonal, NiAs structure (P63/mmc) δ-Fe1−"x"Se. Iron(II) selenide occurs naturally as the NiAs-structure mineral achavalite. More selenium rich iron selenide phases are the γ phases (γ and γˈ), assigned the Fe3Se4 stoichiometry, and FeSe2, which occurs as the marcasite-structure natural mineral ferroselite, or the rare pyrite-structure mineral dzharkenite. It is used in electrical semiconductors.  "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 and by means of intercalation. 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. It has been suggested that alternating layers of FeSe and CoSe (cobalt selenide) might boost "T"c even further due to proximity effects. Cobalt has been used in other pnictogen compounds as a substitute for iron and found to work as well.