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