Magnetic Properties of A 2 Ni 2 TeO 6 (A = K, Li): Zigzag Order in the Honeycomb Layers of Ni 2+ Ions Induced by First and Third Nearest-Neighbor Spin Exchanges.

The static and dynamic magnetic properties and the specific heat of K 2 Ni 2 TeO 6 and Li 2 Ni 2 TeO 6 were examined and it was found that they undergo a long-range ordering at T N = 22.8 and 24.4 K, respectively, but exhibit a strong short-range order. At high temperature, the magnetic susceptibilities of K 2 Ni 2 TeO 6 and Li 2 Ni 2 TeO 6 are described by a Curie-Weiss law, with Curie-Weiss temperatures Θ of approximately -13 and -20 K, respectively, leading to the effective magnetic moment of about 4.46 ± 0.01 μ B per formula unit, as expected for Ni 2+ ( S = 1) ions. In the paramagnetic region, the ESR spectra of K 2 Ni 2 TeO 6 and Li 2 Ni 2 TeO 6 show a single Lorentzian-shaped line characterized by the isotropic effective g -factor, g = 2.19 ± 0.01. The energy-mapping analysis shows that the honeycomb layers of A 2 Ni 2 TeO 6 (A = K, Li) and Li 3 Ni 2 SbO 6 adopt a zigzag order, in which zigzag ferromagnetic chains are antiferromagnetically coupled, because the third nearest-neighbor spin exchanges are strongly antiferromagnetic while the first nearest-neighbor spin exchanges are strongly ferromagnetic, and that adjacent zigzag-ordered honeycomb layers prefer to be ferromagnetically coupled. The short-range order of the zigzag-ordered honeycomb lattices of K 2 Ni 2 TeO 6 and Li 2 Ni 2 TeO 6 is equivalent to that of an antiferromagnetic uniform chain, and is related to the short-range order of the ferromagnetic chains along the direction perpendicular to the chains.