A Solution for Solution-Produced β-FeSe: Elucidating and Overcoming Factors that Prevent Superconductivity

A new low-temperature solvothermal synthesis of superconducting beta-FeSe has been developed using elemental iron and selenium as starting materials. We have shown that syntheses performed in aerobic conditions resulted in the formation of nonsuperconducting antiferromagnetic beta-FeSe, whereas syntheses performed in ultra-dry and oxygen-free conditions produced superconducting beta-FeSe. Detailed characterization of both types of samples with magnetometry, resistivity, Mossbauer spectroscopy, synchrotron X-ray and neutron powder diffraction, and pair-distribution function analysis uncovered factors that trigger the loss of superconductivity in beta-FeSe. Vacancies in the iron sublattice and the incorporation of disordered oxygen-containing species are typical for nonsuperconducting antiferromagnetic samples, whereas a pristine structure is required to preserve superconductivity. Exposure to ambient atmosphere resulted in the conversion of superconducting samples to antiferromagnetic ones. This synthetic method creates new possibilities for soft chemistry approaches to the synthesis and modification of iron-based superconductors.