The four compounds Na 2 M 2 TeO 6 (M 2þ = Ni, Co, Zn, Mg) have been prepared by solid-state reactions in air at 600-820 °C and characterized by powder X-ray diffraction, redox titration, impedance, and polarization measurements on ceramic samples. All of them are superstructures of the well-known hexagonal layered P2-type with ordering of M and Te in octahedral brucite-like layers. They have similar parameters of the hexagonal cells: a = 5.20-5.28 Å, c = 11.14-11.31 Å, but different stacking sequences along c. With M = Co, Zn, Mg (P6 3 22), there are columns Te M Te M and M M M M, but with M = Ni (P6 3 /mcm), there are columns Te Te Te Te and Ni Ni Ni Ni. As little as 5% Li substitution for Ni induces transformation to the P6 3 22 structure. Sodium ions in the interlayer gaps are disordered over a number of trigonal prisms sharing faces and exhibit high conductivity: 4-11 S/m at 300 °C, despite relatively low densities of the ceramics. The materials are purely ionic conductors; the largest electronic contribution (0.1% at 300 °C) has been found for the Co compound, presumably due to a minor admixture of Co(3þ).

No abstract

No abstract

Two synthetic routes-ion-exchange preparation from layered Na(3)Ni(2)SbO(6) at 300 °C and direct solid-state synthesis at 1150 °C resulted in layered Li(3)Ni(2)SbO(6), a cation-ordered derivative from the rocksalt type. The Fddd form reported earlier could not be reproduced. According to the XRD Rietveld analysis, Li(3)Ni(2)SbO(6) is a pseudohexagonal monoclinic structure, C2/m, with a = 5.1828(2) Å, b = 8.9677(3) Å, c = 5.1577(2) Å, β = 109.696(2)°. No Li/Ni mixed occupancy was detected. At high temperatures, the magnetic susceptibility follows the Curie-Weiss law with a positive value of Weiss temperature, ∼8 K, indicating a predominance of ferromagnetic interactions. However, Li(3)Ni(2)SbO(6) orders antiferromagnetically at T(N)∼ 15 K. The effective magnetic moment is 4.3 μ(B)/f.u. which satisfactorily agrees with theoretical estimations assuming high-spin configuration of Ni(2+) (S = 1). Electron spin resonance (ESR) spectra show single Lorentzian shape line attributed to Ni(2+) ion in octahedral coordination. The absorption is characterized by isotropic temperature independent effective g-factor g = 2.150 ± 0.005. In accordance with the layered honeycomb crystal structure determined for Li(3)Ni(2)SbO(6), the superexchange interaction between Ni(2+) ions through Ni-O-Ni pathways within Ni(2)SbO(6) layers are assumed to be ferromagnetic, while the dominant interaction between layers is antiferromagnetic.

We present a comprehensive experimental and theoretical study of the electronic and magnetic properties of two quasi-two-dimensional (2D) honeycomb-lattice monoclinic compounds A 3 Ni 2 SbO 6 (A=Li, Na). Magnetic susceptibility and specific heat data are consistent with the onset of antiferromagnetic (AFM) long range order at low temperatures with Néel temperatures ~ 14 and 16 K for Li 3 Ni 2 SbO 6 and Na 3 Ni 2 SbO 6 , respectively. The effective magnetic moments of 4.3  B /f.u. (Li 3 Ni 2 SbO 6 ) and 4.4  B /f.u. (Na 3 Ni 2 SbO 6 ) indicate that Ni 2+ is in a high-spin configuration (S=1). The temperature dependence of the inverse magnetic susceptibility follows the Curie-Weiss law in the high-temperature region and shows positive values of the Weiss temperature ~ 8 K (Li 3 Ni 2 SbO 6 ) and ~12 K (Na 3 Ni 2 SbO 6 ) pointing to the presence of nonnegligible ferromagnetic interactions, although the system orders AFM at low temperatures. In addition, the magnetization curves reveal a field-induced (spin-flop type) transition below T N that can be related to the magnetocrystalline anisotropy in these systems. These observations are in agreement with density functional theory calculations, which show that both antiferromagnetic and ferromagnetic intralayer spin exchange couplings between Ni 2+ ions are present in the honeycomb planes supporting a zigzag antiferromagnetic ground state. Based on our experimental measurements and theoretical calculations we propose magnetic phase diagrams for the two compounds. 75.30.Kz; 75.10.Dg; 75.30.Gw; 75.30.Et