Fe-Vacancy-Ordered Fe4Se5: The Insulating Parent Phase of FeSe Superconductor

We carried out a detailed study to investigate the existence of an insulating parent phase for FeSe superconductors. The insulating Fe4Se5 with √5 × √5 Fe-vacancy order shows a 3D-Mott variable-range-hopping behavior with a Verwey-like electronic correlation at around 45 K. The application of the RTA process at 450°C results in the destruction of Fe-vacancy order and induces more electron carriers by increasing the Fe3+ valence state. Superconductivity emerges with Tc ∼ 8 K without changing the chemical stoichiometry of the sample after the RTA process, resulting in the addition of extra carriers in favor of superconductivity.