The manganese tellurate Li 2 MnTeO 6 consists of trigonal spin lattices made up of Mn 4+ (d 3 , S = 3/2) ions. The magnetic properties of this compound were characterized by several experimental techniques, which include magnetic susceptibility, specific-heat, dielectric permittivity, electron-spin-resonance, nuclear magnetic resonance (NMR), and neutron powder-diffraction measurements, and by density functional theory calculations. The magnetic susceptibility χ (T ) demonstrates very unusual behavior. It is described by the Curie-Weiss law at high temperature with Curie-Weiss temperature of = −74 K and exhibits no obvious anomaly indicative of a long-range magnetic ordering at low magnetic fields. At high magnetic fields, however, the character of χ (T ) changes showing a maximum at about 9 K. That this maximum of χ (T ) reflects the onset of an antiferromagnetic order was confirmed by specific-heat measurements, which exhibit a clear λ-type anomaly at T N ≈ 8.5 K even at zero magnetic field, and by 7 Li NMR and dielectric permittivity measurements. The magnetic structure of Li 2 MnTeO 6 , determined by neutron powder-diffraction measurements at 1.6 K, is described by the 120 • noncollinear spin structure with the propagation vector k = (1/3, 1/3, 0). Consistent with this finding, the spin-exchange interactions evaluated for Li 2 MnTeO 6 by density functional calculations are dominated by the nearest-neighbor antiferromagnetic exchange within each triangular spin lattice. This spin lattice is strongly spin frustrated with f = | |/T N ≈ 8 and exhibits a two-dimensional magnetic character in a broad temperature range above T N .

Long-range magnetic ordering and short-range spin correlations in layered noncentrosymmetric orthogermanate Li 2 MnGeO 4 were studied by means of polarized and unpolarized neutron scattering. The combined Rietveld refinement of synchrotron and neutron powder diffraction data at room temperature within the Pmn2 1 space group allowed us to specify the details of the crystal structure. According to the additional Bragg peaks in low-temperature neutron diffraction patterns a long-range antiferromagnetic ordering with the propagation vector k = (1/2 1/2 1/2) has been found below T N ≈ 8 K. Symmetry analysis revealed the model of the ground state spin structure within the C a c (no. 9.41) magnetic space group. It is represented by the noncollinear ordering of manganese atoms with a refined magnetic moment of 4.9 μ B /Mn 2+ at 1.7 K, which corresponds to the saturated value for the high-spin configuration S = 5/2. Diffuse magnetic scattering was detected on the neutron diffraction patterns at temperatures just above T N . Its temperature evolution was investigated in detail by polarized neutron scattering with the following XYZ-polarization analysis. Reverse Monte Carlo simulation of diffuse scattering data showed the development of short-range ordering in Li 2 MnGeO 4 , which is symmetry consistent on a small scale with the long-range magnetic state below T N . The reconstructed radial spin-pair correlation function S(0) S(r) displayed the predominant role of antiferromagnetic correlations. It was found that spin correlations are significant only for the nearest magnetic neighbors and almost disappear at r ≈ 12 Å at 10 K. Temperature dependence of the diffuse scattering implies short-range ordering long before the magnetic phase transition. Besides, the spin arrangement was found to be similar in both cases above and below T N . As a result, an exhaustive picture of the gradual formation of magnetic ordering in Li 2 MnGeO 4 is presented.

The trigonal layered quaternary tellurate Na 2 MnTeO 6 has been studied by means of various techniques to clarify its magnetic properties. The crystal structure of this compound is based on the triangular arrangement of all cations in the parallel layers with the space group P 31c. By using symmetry analysis of the magnetic neutron scattering data, we have found that the solution for the magnetic structure corresponds to the magnetic Shubnikov group R 3 c (No. 167.4.1337). Mn 4+ ions in an octahedral environment form a triangular network where all spins are directed from the center of each triangle. Overall magnetic structure in Na 2 MnTeO 6 is commensurate 120 • spin helix with propagation vector k = (1/3, 1/3, 1/3) in variance with planar spin structure in structurally equivalent Li 2 MnTeO 6 with magnetic propagation vector k = (1/3, 1/3, 0). The magnetization measurements show that Na 2 MnTeO 6 experiences an antiferromagnetic order at T N = 5.5 K. NMR, electron spin resonance, and thermodynamics experiments demonstrate the extended temperature region of 2D short-range correlations well above the ordering temperature.

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