Direct observation of electron density reconstruction at the metal-insulator transition in NaOsO3

5d transition metal oxides offer new opportunities to test our understanding\nof the interplay of correlation effects and spin-orbit interactions in\nmaterials in the absence of a single dominant interaction. The subtle balance\nbetween solid-state interactions can result in new mechanisms that minimize the\ninteraction energy, and in material properties of potential use for\napplications. We focus here on the 5d transition metal oxide NaOsO3, a strong\ncandidate for the realization of a magnetically driven transition from a\nmetallic to an insulating state exploiting the so-called Slater mechanism.\nExperimental results are derived from non-resonant and resonant x-ray single\ncrystal diffraction at the Os L-edges. A change in the crystallographic\nsymmetry does not accompany the metal-insulator transition in the Slater\nmechanism and, indeed, we find no evidence of such a change in NaOsO3. An\nequally important experimental observation is the emergence of the (300) Bragg\npeak in the resonant condition with the onset of magnetic order. The intensity\nof this space-group forbidden Bragg peak continuously increases with decreasing\ntemperature in line with the square of intensity observed for an allowed\nmagnetic Bragg peak. Our main experimental results, the absence of crystal\nsymmetry breaking and the emergence of a space-group forbidden Bragg peak with\ndeveloping magnetic order, support the use of the Slater mechanism to interpret\nthe metal-insulator transition in NaOsO3. We successfully describe our\nexperimental results with simulations of the electronic structure and, also,\nwith an atomic model based on the established symmetry of the crystal and\nmagnetic structure.\n