Origin of Magnetism in a Supposedly Nonmagnetic Osmium Oxide

A supposedly nonmagnetic 5d1 double perovskite oxide is investigated by a combination of spectroscopic and theoretical methods, namely, resonant inelastic x-ray scattering, x-ray absorption spectroscopy, magnetic circular dichroism, and multiplet ligand-field calculations. We found that the large spin-orbit coupling admixes the 5d t2g and eg orbitals, covalency raises the 5d population well above the nominal value, and the local symmetry is lower than Oh. The obtained electronic interactions account for the finite magnetic moment of Os in this compound and, in general, of 5d1 ions. Our results provide direct evidence of elusive Jahn-Teller distortions, hinting at a strong electron-lattice coupling.