Comparative ab initio study of the structural, electronic, magnetic, and dynamical properties of LiOsO${}_3$ and NaOsO${}_3$

Abstract

Despite similar chemical compositions, LiOsO${}_3$ and NaOsO${}_3$ exhibit remarkably distinct structural, electronic, magnetic, and spectroscopic properties. At low temperature, LiOsO${}_3$ is a polar bad metal with a rhombohedral $R3c$ structure without the presence of long-range magnetic order, whereas NaOsO${}_3$ is a $G$-type antiferromagnetic insulator with an orthorhombic $Pnma$ structure. By means of comparative first-principles DFT+$U$ calculations with the inclusion of the 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 (LDA, PBE, PBEsol, SCAN, and 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 originates mainly from a cooperative steric and magnetic effect. Finally, we find that although at ambient pressure LiOsO${}_3$ with a $Pnma$ symmetry and NaOsO${}_3$ with a $R\bar{3}c$ 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 LiOsO${}_3$ is predicted, whereas for NaOsO${}_3$ no symmetry change is discerned in the considered pressure range.

9 pages, 9 figures