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-Fe_1-xSe and α-Fe₇Se₈. The high temperature phase is the hexagonal, NiAs structure (P6₃/mmc) δ-Fe_1-xSe. 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 feroselite, or the rare pyrite-structure mineral dzharkenite.

It is used in electrical semiconductors.

β-FeSe is the simplest iron-based superconductor but with the diverse properties. It starts to superconduct at 8 K at normal pressure but its critical temperature (T_c) is dramatically increasing to 38 K under pressure and by means of intercalation. The combination of both intercalation and pressure results in re-emerging superconductivity at 48 (see and references therein).

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 70K. 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.