Two synthetic routes—ion-exchange preparation from layered Na₃Ni₂SbO₆ at 300 °C and direct solid-state synthesis at 1150 °C resulted in layered Li₃Ni₂SbO₆ , a cation-ordered derivative from the rocksalt type. The Fddd form reported earlier could not be reproduced. According to the XRD Rietveld analysis, Li₃Ni₂SbO₆ 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, Li₃Ni₂SbO₆ 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 Ni²⁺ (S = 1). Electron spin resonance (ESR) spectra show single Lorentzian shape line attributed to Ni²⁺ 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 Li₃Ni₂SbO₆ , the superexchange interaction between Ni²⁺ ions through Ni–O–Ni pathways within Ni₂SbO₆ layers are assumed to be ferromagnetic, while the dominant interaction between layers is antiferromagnetic.

Structure, electrochemical, magnetic and resonance properties of new layered antimonate Li₄FeSbO₆ were comprehensively studied using powder X-ray diffraction, cyclic voltammetry, magnetic susceptibility, heat capacity, electron spin resonance and Mössbauer spectroscopy. In the crystal structure the iron ions form the triangular network within (LiFeSbO₆)³⁻ layers alternating with nonmagnetic lithium layers. The electrochemical activity studied implies an Fe³⁺/Fe⁴⁺ redox couple at 4.3 V (ox.) and 3.9 V (red.) thereby revealing that Li can be reversibly extracted. The long-range antiferromagnetic order was found to occur at the Néel temperature, T_N ≈ 3.6 K, confirmed both by the magnetic susceptibility data and specific heat ones. The effective magnetic moment is estimated to be 5.93 μ_B/f.u. and satisfactorily agrees with theoretical estimations assuming high-spin configuration of Fe³⁺ (S = 5/2). In the magnetically ordered state, though, the magnetization demonstrates rather peculiar behavior. An additional anomaly on the M(T) curves appears at T₂ < T_N in moderate magnetic field. The positions of transitions at T_N and T₂ separate increasingly with increasing external field. Multiple measurements consistently demonstrated field-sensitive moving of magnetic phase boundaries constituting a unique phase diagram for the compound under study. The complex low-dimensional (2D) nature of magnetic coupling was confirmed by the dynamic magnetic properties study. Electron spin resonance from Fe³⁺ ions in paramagnetic phase is characterized by a temperature independent effective g-factor of 1.99 ± 0.01. However, the distortion and broadening of the ESR line were found to take place upon approaching the magnetically ordered state from above. The divergence of the temperature-dependent linewidth is analyzed in terms of both critical behavior close to long-range magnetic order and the Berezinskii–Kosterlitz–Thouless (BKT)-type transition. Heat capacity measurements even at zero field manifested an appearance of the additional anomaly at temperatures below the Néel temperature. The temperature dependence of ESR intensity, linewidth and shift of the resonant field imply an extended region of short-range order correlations in the compound studied. The rich variety of the anomalies in magnetic and resonance properties makes this new antimonate a very interesting system to investigate the multiple phase transitions and competing exchange interaction due to the critical role of the layered structure organization accompanied by the frustration effects in triangular antiferromagnets.

The magnetic structure of $\mathrm{L}{\mathrm{i}}_3\mathrm{N}{\mathrm{i}}_2\mathrm{Sb}{\mathrm{O}}_6$ has been determined by low-temperature neutron diffraction, and the crystal structure has been refined by a combination of synchrotron and neutron powder diffraction. The monoclinic ($C2/m$) symmetry, assigned previously to this pseudohexagonal layered structure, has been unambiguously proven by peak splitting in the synchrotron diffraction pattern. The structure is based on essentially hexagonal honeycomb-ordered $\mathrm{N}{\mathrm{i}}_2\mathrm{Sb}{\mathrm{O}}_6$ layers alternating with $\mathrm{L}{\mathrm{i}}_3$ layers, all cations and anions being in an octahedral environment. The compound orders antiferromagnetically below $T_{\mathrm{N}}=15\mathrm{K}$, with the magnetic supercell being a $2a\times 2b$ multiple of the crystal cell. The magnetic structure within the honeycomb layer consists of zigzag ferromagnetic spin chains coupled antiferromagnetically. The ordered magnetic moment amounts to $1.62\left(2\right){\mu }_{\mathrm{B}}/\mathrm{Ni}$, which is slightly lower than the full theoretical value. Upon cooling below $T_{\mathrm{N}}$, the spins tilt from the $c$ axis, with a maximum tilting angle of $15.6^{\circ }$ at $T=1.5\mathrm{K}$. Our data imply non-negligible ferromagnetic interactions between the honeycomb layers. The observed antiferromagnetic resonance modes are in agreement with the two-sublattice model derived from the neutron data. Orthorhombic anisotropy shows up in zero-field splitting of $\mathrm{\Delta }=198\pm 4$ and $218\pm 4\mathrm{GHz}$. Above $T_{\mathrm{N}}$, the electron spin resonance data imply short-range antiferromagnetic order up to about 80 K.