  Li3Co2SbO6 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 Na3Co2SbO6 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 TN about 6.7 K and 9.9 K in Na3Co2SbO6 and Li3Co2SbO6, 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
Na3Co2SbO6 and 2D AFM magnons in Li3Co2SbO6. 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 A3Co2SbO6 (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 Li3Co2SbO6 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.
