Ferromagnetism in the Mott insulator Ba2NaOsO6

Results are presented of single crystal structural, thermodynamic, and reflectivity measurements of the double-perovskite Ba2NaOsO6. These characterize the material as a 5d(1) ferromagnetic Mott insulator with an ordered moment of similar to 0.2 mu(B) per formula unit and T-C=6.8(3) K. The magnetic entropy associated with this phase transition is close to Rln2, indicating that the quartet ground state anticipated from consideration of the crystal structure is split, consistent with a scenario in which the ferromagnetism is associated with orbital ordering.