Iron(II) selenide

Iron selenide refers to a number of inorganic compounds of ferrous iron and selenide. 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 β-Fe_1−xSe and α-Fe₇Se₈. The high temperature phase is the hexagonal, NiAs structure δ-Fe_1−xSe. Iron selenide occurs naturally as the NiAs-structure mineral achavalite.
More selenium rich iron selenide phases are the γ phases, 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.

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 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.