Synthesis, structure and magnetic properties of honeycomb-layered Li 3 Co 2 SbO 6 with new data on its sodium precursor, Na 3 Co 2 SbO 6

Li 3 Co 2 SbO 6 is prepared by molten salt ion exchange and its structure refined by the Rietveld method confirming the honeycomb-type Co/Sb ordering of its Na precursor. Monoclinic rather than trigonal symmetry of Na 3 Co 2 SbO 6 is directly demonstrated for the first time by peak splitting in the high-resolution synchrotron XRD pattern. The long-range antiferromagnetic order is established at T N ≈ 6.7 K and 9.9 K in Na 3 Co 2 SbO 6 and Li 3 Co 2 SbO 6 , respectively, confirmed by both the magnetic susceptibility and specific heat. Spin-wave analysis of specific heat data indicates the presence of 3D AFM magnons in Na 3 Co 2 SbO 6 and 2D AFM magnons in Li 3 Co 2 SbO 6 . The field dependence of the magnetization almost reaches saturation in moderate magnetic fields up to 9 T and demonstrates characteristic features of magnetic field induced spin-reorientation transitions for both A 3 Co 2 SbO 6 (A = Na, Li). Overall thermodynamic studies show that the magnetic properties of both compounds are very sensitive to an external magnetic field, thus predicting a non-trivial ground state with a rich magnetic phase diagram. The ground state spin configuration of Li 3 Co 2 SbO 6 has been determined by low-temperature neutron powder diffraction. It represents a ferromagnetic arrangement of moments in the honeycomb layers with antiferromagnetic coupling between adjacent layers.