Stability of iron selenide nanophases prepared by mechanosynthesis

We have recently reported a method for obtaining about 30% of tetragonal beta-FeSe in coexistence with hexagonal delta-FeSe and alpha-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 beta-FeSe after annealing at 400 degrees 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 delta ->beta phase transition with storage time. The beta-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 a-Fe. (C) 2019 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).