  We used single-crystal x-ray and neutron diffraction to investigate the
crystal and magnetic structures of trigonal lattice iridate Ca2Sr2IrO6. The
crystal structure is determined to be $R\bar3$ with two distinct Ir sites. The
system exhibits long-range antiferromagnetic order below $T_N = 13.1$ K. The
magnetic wave vector is identified as $(0,0.5,1)$ with ferromagnetic coupling
along the $a$ axis and antiferromagnetic correlation along the $b$ axis. Spins
align dominantly within the basal plane along the [1,2,0] direction and tilt
34$^\circ$ towards the $c$ axis. The ordered moment is 0.66(3) $\mu_B$/Ir,
larger than other iridates where iridium ions form corner- or edge-sharing $\rm
IrO_6$ octahedral networks. The tilting angle is reduced to $\approx19^\circ$
when a magnetic field of 4.9 Tesla is applied along the $c$ axis. Density
functional theory calculations confirm that the experimentally determined
magnetic configuration is the most probable ground state with an insulating gap
$\sim0.5$~eV.
