Comparative ab initio study of the structural, electronic, magnetic, and dynamical properties of LiOsO3 and NaOsO3

Despite similar chemical compositions, LiOsO3 and NaOsO3 exhibit remarkably distinct structural, electronic, magnetic, and spectroscopic properties. At low temperature, LiOsO3 is a polar bad metal with a rhombohedral R3c structure without the presence of long-range magnetic order, whereas NaOsO3 is a G-type antiferromagnetic insulator with an orthorhombic Pnma structure. By means of comparative first-principles DFT+U calculations with the inclusion of spin-orbit coupling, we (i) identify the origin of the different structural (R3c vs Pnma) properties using a symmetry-adapted soft-mode analysis, (ii) provide evidence that all considered exchange-correlation functionals (local-density approximation, the Perdew-Burke-Ernzerhof functional as well as its improved version for solids, the strongly constrained appropriately normed functional, and the hybrid functional HSE06) and the spin disordered polymorphous descriptions are unsatisfactory to accurately describe the electronic and magnetic properties of both systems simultaneously, and (iii) clarify that the distinct electronic (metallic vs insulating) properties originate mainly from a cooperative steric and magnetic effect. Finally, we find that although at ambient pressure LiOsO3 with a Pnma symmetry and NaOsO3 with a R3¯c symmetry are energetically unfavorable, they do not show soft phonons and therefore are dynamically stable. A pressure-induced structural phase transition from R3c to Pnma for LiOsO3 is predicted, whereas for NaOsO3 no symmetry change is discerned in the considered pressure range.