Preparation and superconductivity of stoichiometric β-FeSe

The stoichiometric β-FeSe superconductor was prepared for the first time by mechanical alloying (MA) from elemental powders of iron and selenium (1 : 1) and subsequent annealing process. The formation of the stoichiometric β-FeSe is attributed to the MA resulting nanometric precursor containing high reaction activity for the following annealing process, and the MA mechanism here is a mechanic collision reaction (MCR). The crystalline phases in the MA precursor are tightly related to the milling time (t). Hexagonal γ-Fe₇Se₈, tetragonal β-FeSe, and residual Fe coexist when t≤5 h. A single δ-FeSe phase is observed when t=10 h, and the crystalline size further decreases to ~10 nm. In the nanocrystalline state, hexagonal δ-FeSe is the stable and preferred phase. With the increase of the annealing temperature, δ-FeSe rapidly transforms to the well-crystallized β-FeSe. At 400 °C, the pure stoichiometric β-FeSe phase is successfully obtained with size of ~60 nm and the highest superconductor transition temperature T_c^onset of about 8.9 K in the β-FeSe_1-x series.