Results are presented of single crystal structural, thermodynamic, and reflectivity measurements of the double-perovskite ${\mathrm{Ba}}_2{\mathrm{NaOsO}}_6$. These characterize the material as a $5d^1$ ferromagnetic Mott insulator with an ordered moment of $\sim 0.2{\mu }_B$ per formula unit and $T_C=6.8(3)\mathrm{K}$. The magnetic entropy associated with this phase transition is close to $R\ln 2$, 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.

• Received 29 September 2006

DOI:https://doi.org/10.1103/PhysRevLett.99.016404