Abstract
The debate of whether or not phase separation occurs in the potassium doped iron selenide KxFe2-ySe2, with superconductivity appearing with the stoichiometric I4/mmm crystal phase while the host, non-superconducting, lattice is non-stoichiometric and magnetic at the same time, with the I4/m symmetry, is still ongoing. The latter consists of Fe and K vacancies that go through a disordered (high temperature) to an ordered (low temperature) transition in which the partially filled Fe sites create a supercell for which several different types have been reported, the most common being the √𝟓×√𝟓×1 type. This conundrum arising from vacancy ordering not observed in any of the other Fe-based superconductors combined with the absence of hole pockets may signal a new mechanism to pairing. Herein, we show by focusing on the optimally doped composition, K0.8Fe2-ySe2, that the magnetic and non-
2
stoichiometric phase can be systematically controlled. Moreover, we find that the distribution of Fe vacancies can be controlled as well as this is strongly coupled to the size of the magnetic moment. The intrinsic crystal structure of the K0.8Fe2-ySe2 may not be phase separated after all.
