Stability of iron selenide nanophases prepared by mechanosynthesis

We have recently reported a method for obtaining about 30% of tetragonal β-FeSe in coexistence with hexagonal δ-FeSe and α-Fe phases by ball milling under an inert atmosphere, starting from FexSe powders with x = 1.00, 1.25 or 1.50, in which the use of Fe1.25Se or Fe1.50Se leads to more than 93% of β-FeSe after annealing at 400 °C for 1 h under vacuum [K. F. Ulbrich and C. E. M. Campos, RSC Adv. 8(15), 8190–8198 (2018)]. In this paper, we describe the phase stability of the samples and reveal a remarkable δ→β phase transition with storage time. The β-FeSe phase content increased exponentially during the first 30 days of storage, reaching about 70% in Fe1.5Se. X-ray powder diffraction and Rietveld analysis were used to evaluate changes in crystal structure and microstructure. Aging had only a mild effect on the magnetization curves of the samples, suggesting that the magnetism of mechanically alloyed iron selenide is governed mainly by the content of α-Fe.