The magnetic structure of quasi-two-dimensional (2D) honeycomb lattice Na2Ni2TeO6 has been determined by low-temperature neutron diffraction and the crystal structure fine details at room temperature have been established by a combination of synchrotron and neutron powder diffraction. The atomic structure is described by the P6(3)/mcm space group, but the strong presence of stacking faults defects in layered ordering is found for the first time. Both magnetization and specific heat data indicates an establishment of a long-range antiferromagnetic order with T-N = 25 +/- 1 K, preceded by a short-range order on 2D honeycomb lattice at about 34 K. Determined effective magnetic moment mu(eff) = 4.35 mu(B) is in excellent agreement with numerical estimation using effective g-factor g = 2.19 directly measured by electron spin resonance. The ground magnetic state at T = 1.5 K is represented by the commensurate zigzag-type magnetic order. Magnetic moments of Ni are almost perpendicular to the honeycomb layers, which antiferromagnetically coupled along the c-direction. Besides, the coherent magnetic scattering area has a disk shape that is homogeneous over the ab plane and compressed along the c-axis that indicates 2D nature of magnetic correlations in the compound. The magnetic diffuse neutron scattering related to the presence of strong short-range spin-spin correlations above T-N was observed. Based on our experimental measurements we propose magnetic phase diagram for Na2Ni2TeO6. (C) 2019 Elsevier B.V. All rights reserved.

Crystallographic and magnetic properties of new layered honeycomb-lattice Li3Co2SbO6 antimonate were studied and compared with its sodium precursor Na3Co2SbO6.

The structure and the magnetic properties of layered Li3Cu2SbO6 are investigated by powder X-ray diffraction, static susceptibility, and electron spin resonance studies up to 330 GHz. The XRD data experimentally verify the space group C2/m with halved unit cell volume in contrast to previously reported C2/c. In addition, the data show significant Li/Cu-intersite exchange. Static magnetic susceptibility and ESR measurements show two magnetic contributions, i.e., quasi-free spins at low-temperature and a spin-gapped magnetic subsystem, with about half of the spins being associated to each subsystem. The data suggest ferromagnetic–antiferromagnetic alternating chains with JFM = −285 K and JAFM = 160 K with a significant amount of Li-defects in the chains. The results are discussed in the scenario of fragmented 1D S = 1 AFM chains with a rather high defect concentration of about 17% and associated S = 1/2 edge states of the resulting finite Haldane chains.