Superconductivity in solid-state synthesized (Li,Fe)OHFeSe by tuning Fe vacancies in FeSe layer

The discovery of iron-based superconductors (FeSCs) has attracted worldwide attention in recent years due to their abundant properties. Recently, an air-stable high-${T}_{c}$ FeSe-based superconductor, (Li,Fe)OHFeSe, was discovered, and it is thought to be an ideal platform for investigating the underlying physics of superconductivity. In this paper, we report on the solid-state synthesis of (Li,Fe)OHFeSe at low temperature. As-synthesized (Li,Fe)OHFeSe is nonsuperconducting, which can be attributed to the Fe vacancies in the FeSe layer. After annealing at high pressure or a solid-state reaction with lithium, the Fe vacancies in the FeSe layer can be eliminated, and superconductivity at 40 K is induced. X-ray photoelectron spectroscopy analysis also proves the decrease of the Fe valence, which indicates that the Fe vacancies are gradually eliminated upon annealing at high pressure or the solid-state reaction with lithium. Our finding suggests that the solid-state synthesis of (Li,Fe)OHFeSe after annealing at high pressure or the solid-state reaction with lithium can effectively eliminate the Fe vacancies in the FeSe layer, eventually inducing superconductivity in (Li,Fe)OHFeSe.