Iron(III) selenide, or iron(III) triselenide, is an inorganic compound with the formula Fe_2Se_3. It is a dark brown solid that decomposes in water to give hydrated iron(III) selenide and hydrogen selenide. It is a member of a series of compounds that contain iron and selenium in various oxidation states. The properties of these compounds depend strongly on the ratio of Fe:Se. For example, at Fe:Se = 1:1, iron(III)selenite (Fe_2SeO_4·H_2O), iron(II)selenate (Fe SeO_4 2-), and elemental selenium can coexist in equilibrium. In contrast, at Fe:Se = 3:1, only the high temperature phase α-Fe_2Se_3 exists; this material is antiferromagnetic with Tc=15 K.
Iron(III) selenides have attracted interest as potential thermoelectric materials. They also show promise for applications in solar energy conversion and catalysis.