Title: Electronic structure of the magnetic halide double perovskites Cs2(Ag,Na)FeCl6 from first-principles

Abstract: A family of magnetic halide double perovskites (HDPs) have recently attracted attention due to their potential to broaden application areas of halide double perovskites into e.g. spintronics. Up to date the theoretical modelling of these systems have relied on primitive approximations to the density functional theory (DFT). In this paper, we study structural, electronic and magnetic properties of the Fe$^{3+}$-containing HDPs Cs$_2$AgFeCl$_6$ and Cs$_2$NaFeCl$_6$ using a combination of more advanced DFT-based methods, including DFT+U, hybrid-DFT and treatments of various magnetic states. We examine the effect of varying the effective Hubbard parameter, U$_{eff}$, in DFT+U and the mixing-parameter, $\alpha$, in hybrid DFT on the electronic structure and structural properties. Our results reveal a set of localized Fe(d) states that are highly sensitive to these parameters. Cs$_2$AgFeCl$_6$ and Cs$_2$NaFeCl$_6$ are both antiferromagnets with Ne\'el temperatures well below room temperature and are thus in their paramagnetic (PM) state at the external conditions relevant to most applications. Therefore, we have examined the effect of disordered magnetism on the electronic structure of these systems and find that while Cs$_2$NaFeCl$_6$ is largely unaffected, Cs$_2$AgFeCl$_6$ shows significant renormalization of its electronic band structure.