  The synthesis, structure, and basic magnetic properties of Na2Co2TeO6 and
Na3Co2SbO6 are reported. The crystal structures were determined by neutron
powder diffraction. Na2Co2TeO6 has a two-layer hexagonal structure (space group
P6322) while Na3Co2SbO6 has a single-layer monoclinic structure (space group
C2/m). The Co, Te, and Sb ions are in octahedral coordination, and the edge
sharing octahedra form planes interleaved by sodium ions. Both compounds have
full ordering of the Co2+ and Te6+/Sb5+ ions in the ab plane such that the Co2+
ions form a honeycomb array; the stacking of the honeycomb arrays differ in the
two compounds. Both Na2Co2TeO6 and Na3Co2SbO6 reveal antiferromagnetic ordering
at low temperatures, with a metamagnetic transition displayed by Na3Co2SbO6.
