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

A new low-temperature solvothermal synthesis of superconducting β-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 β-FeSe, whereas syntheses performed in ultra-dry and oxygen-free conditions produced superconducting β-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 β-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 creat...