  The electronic and magnetic properties of monoclinic double perovskite
Sr$_2$CeIrO$_6$ were examined based on both experiments and first-principles
density functional theory calculations. From the calculations we conclude that
low-spin-state Ir$^{4+}$ (5$\textit{d}^5$, S=$\frac{1}{2}$) shows t$_{2g}$ band
derived anti-ferro type orbital ordering implying alternating occupations of
$\textit{d}_{yz}$ and $\textit{d}_{xz}$ orbitals at the two symmetrically
independent Ir sites. The experimentally determined Jahn-Teller type distorted
monoclinic structure is consistent with the proposed orbital ordering picture.
Surprisingly, the Ir-5$\textit{d}$ orbital magnetic moment was found to be
$\approx$ 1.3 times larger than the spin magnetic moment. The experimentally
observed AFM-insulating states are consistent with the calculations. Both
electron-electron correlation and spin-orbit coupling (SOC) are required to
drive the experimentally observed AFM-insulating ground state. This single
active site double perovskite provides a rare platform with a prototype
geometrically frustrated fcc lattice where among the different degrees of
freedom (i.e spin, orbital, and lattice), spin-orbit interaction and Coulomb
correlation energy scales compete and interact with each other.
