  Tetragonal iron selenide, FeSe, the layered parent compound of the recently
discovered superconducting arsenide family, has previously been shown to be non
magnetic and superconducting with a critical temperature near 8 K. There has,
however, been a lack of consensus as to whether selenium vacancies present due
to large deviations from ideal stoichiometry are required to give rise to the
superconductivity. Here we describe the results of experiments that demonstrate
simply that superconducting iron selenide can only be synthesized as a pure
material when near stoichiometric (i.e. FeSe). Significant selenium deficiency
or excess gives rise to secondary magnetic phases, and a suppression of the
superconductivity.

Superconductivity