Magnetic order in single crystals of Na3Co2SbO6 with a honeycomb arrangement of 3d7 Co2+ ions

We have synthesized single crystals of Na3Co2SbO6 and characterized the structure and magnetic order by measuring anisotropic magnetic properties, heat capacity, and x-ray and neutron single crystal diffraction. Magnetic properties and specific heat of polycrystalline Na3Co2SbO6 were also measured for comparison. Na3Co2SbO6 crystallizes in a monoclinic structure (space group C2/m) with [Co2SbO6](3-) layers separated by Na+ ions. The temperature dependence of magnetic susceptibility shows significant anisotropic behavior in the whole temperature range 2-350 K investigated in this work. An effective moment of about 5.5 mu(B)/Co2+ from a Curie-Weiss fitting of the magnetic susceptibility is larger than the spin only value and signals significant orbital contribution. Na3Co2SbO6 single crystal undergoes a transition into a long-range antiferromagnetically ordered state below T-N = 5 K. Neutron single crystal diffraction confirmed the zigzag magnetic structure with a propagation vector k = (0.5, 0.5, 0). The ordered moment is found to be 0.9 mu(B) at 4 K and align along the crystallographic b axis. Density functional theory calculations suggest that the experimentally observed zigzag order is energetically competing with the Neel order. It is also found that the covalency between Co d and O p is quite strong and competes with the local spin-orbit coupling, suggesting a J(eff) = 1/2 ground state may not be realized in this compound.