Structural transition and superconductivity in hydrothermally synthesized FeX (X = S, Se).

Iron selenide obtained by mild hydrothermal reaction is not superconducting and exhibits a triclinic crystal structure below 60 K unlike superconducting FeSe from conventional solid state synthesis which is orthorhombic. In contrast, tetragonal iron sulphide FeS from hydrothermal synthesis is superconducting but undergoes no structural change on cooling.