Introducing the monoclinic polymorph of the honeycomb magnet Na2Co2TeO6

Recent theoretical studies have suggested that the low-energy Hamiltonian of honeycomb cobaltate systems could be dominated by anisotropic Kitaev interactions. Motivated by the theory, a honeycomb layered material Na2Co2TeO6 with a hexagonal unit cell has been studied and found to exhibit antiferromagnetic (AFM) ordering at 27 K with two spin reorientation transitions at 15 and 5 K. Here we report a monoclinic polymorph of Na2Co2TeO6, also with honeycomb layered structure but with a single AFM transition at 9.6 K and without spin reorientation transitions at lower temperatures. Using neutron diffraction, we identify an in-plane zigzag AFM order in the ground state with the spins canted out of the honeycomb planes and ferromagnetically coupled between them. The zigzag order is suppressed by a magnetic field of 6 T. The lower critical temperature and field, positive Curie-Weiss temperature, and out-of-plane canting of spins in the monoclinic Na2Co2TeO6 suggest enhanced frustration in this polymorph compared to the hexagonal one.