LAUSR

Using density functional theory together with Monte Carlo simulations, we demonstrate that epitaxial strain, both compressive and tensile, attenuates the spin frustration of double perovskite ${\mathrm{Sr}}_{2}\mathrm{Fe}\mathrm{Os}{\mathrm{O}}_{6}$ to significantly enhance the critical temperature to 310 K, enabling room-temperature applications. We discover under tensile strain a tetragonal ($I4/m$)-to-monoclinic ($P{2}_{1}/n$) structural transition concomitant with an antiferromagnetic-to-ferrimagnetic transition. Furthermore, an indirect-to-direct band gap transition is observed with the valence and conduction states localized on different transition metal sublattices, opening a route to efficient electron-hole separation upon photoexcitation.