Giant charge fluctuations with Se height and Fe-vacancy formation inMxFe2−ySe2

Abstract

From first-principles calculations we show that there is an intimate relationship between the Se height and the spin configuration of the underlying Fe square in ${M}_{x}$Fe${}_{2\ensuremath{-}y}$Se${}_{2}$, where $M$ denotes an alkali metal. A displacement of the Se atom by an amount as small as 0.2 \AA{} is enough to change the amount of charge in the Fe plane by as much as 0.8 electrons per Fe. The height of the Se atom above the Fe square increases as the number of ferromagnetic Fe-Fe bonds increases, yielding an expansion of 2 \AA{} in the $c$ axis for the fully ferromagnetic spin configuration, which suggests the presence of a giant magnetoelastic coupling in this system. Interestingly, the strong coupling between the Se height and the spin configuration of the underlying Fe square is found to be rather local, such that both the energetics and the structural parameters of the $\sqrt{5}\ifmmode\times\else\texttimes\fi{}\sqrt{5}$ phase can be understood in terms of three different magnetic configurations of the ideal 122 structure. We show that the formation of Fe vacancies is energetically favorable and that their presence changes the ground state from double-stripe to block-antiferromagnetic ordering. The formation of K vacancies is found to be energetically unfavorable.