The Stoichiometry of FeSe

TL;DR

This paper investigates the precise stoichiometry of FeSe, demonstrating that superconductivity occurs only in near-stoichiometric compositions and that deviations lead to secondary phases and suppression of superconductivity.

Contribution

It provides experimental evidence that pure superconducting FeSe requires near-stoichiometric composition, clarifying the role of selenium vacancies in its superconductivity.

Findings

Superconducting FeSe is only stable near ideal stoichiometry.

Deviations from stoichiometry introduce magnetic impurities.

Superconductivity is suppressed by significant selenium deficiency or excess.

Abstract

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.