Structural transition in an unusual $5d$-electron ferromagnetic insulator*

Double-perovskite barium sodium osmate (Ba$_2$NaOsO$_6$) is a ferromagnetic (FM) insulator ($T_c\sim6.8$ K, ordered moment $\sim$0.2 $\mu_B$ per formula unit) with $<110>$ easy axis. We present precision x-ray diffraction studies on single crystals to understand structural symmetry in this rare FM compound. At room temperature there is a subtle splitting of crystal Bragg peaks indicating the global symmetry to be weakly tetragonal. At or slightly above $T_c$, the material becomes orthorhombic. These changes are crucial in lifting the degeneracy of $d$-state manifold and are likely associated with orbital ordering.