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We have combined single crystal neutron and x-ray diffractions to investigate the magnetic and crystal structures of the honeycomb lattice Na2IrO3. The system orders magnetically below 18.1(2) K with Ir 4+ ions forming zigzag spin chains within the layered honeycomb network with an ordered moment of 0.22(1) µB/Ir site. Such a configuration sharply contrasts with the Néel or stripe states proposed in the Kitaev-Heisenberg model. The structure refinement reveals that the Ir atoms form a nearly ideal two-dimensional honeycomb lattice while the IrO6 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 that are crucial to the understanding of the exotic magnetic order and possible topological characteristics in the 5d-electron-based honeycomb lattice.

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