Abstract
We have combined single crystal neutron and x-ray diffractions to investigate
the magnetic and crystal structures of the honeycomb lattice $\rm Na_2IrO_3$.
The system orders magnetically below $18.1(2)$~K with Ir$^{4+}$ ions forming
zigzag spin chains within the layered honeycomb network with ordered moment of
$\rm 0.22(1)~\mu_B$/Ir site. Such a configuration sharply contrasts the
N{\'{e}}el or stripe states proposed in the Kitaev-Heisenberg model. The
structure refinement reveals that the Ir atoms form nearly ideal 2D honeycomb
lattice while the $\rm IrO_6$ octahedra experience a trigonal distortion that
is critical to the ground state. The results of this study provide
much-needed experimental insights into the magnetic and crystal structure
crucial to the understanding of the exotic magnetic order and possible
topological characteristics in the 5$d$-electron based honeycomb lattice.
