Theoretical prediction of room-temperature two-dimensional ferromagnetism in an yttrium iodide electride

Through a global structure search combined with density functional theory calculations, we predict a two-dimensional (2D) electride ferromagnet, yttrium iodide (YI). The ground-state YI structure has YI layers bound by relatively weak van der Waals interactions; the interlayer binding energy is only $--0.016\phantom{\rule{0.16em}{0ex}}\mathrm{eV}/{\AA{}}^{2}$. Each YI layer has strongly localized anionic electrons in interstitial spaces, which is a crucial characteristic of electrides. The hybridization of these interstitial $s$-band lone-pair electrons and the $d$-band electrons of Y gives rise to a Stoner-type instability, resulting in 2D ferromagnetic orders with magnetic moments of $1.28\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}$ per unit cell. Our Monte Carlo simulations further demonstrate that the Curie temperatures of bulk and monolayer YI are as high as 380 and 383 K, respectively. Phonon dispersion calculations and ab initio molecular dynamics simulations also show that the YI layers are dynamically and thermodynamically stable at room temperature. These findings suggest that monolayer YI is a promising candidate for a room-temperature 2D ferromagnet.