Long-range magnetic ordering in Li2MnGeO4 and precursor short-range spin correlations

Abstract

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.

Aqueous synthesis of Li2MnAO4/C (A = Si, Ge) as positive electrode active materials for lithium-ion batteries by acid–base reaction

“…[4][5][6][7] Here, the Li 2 MGeO 4 (M = Mn, Fe) system, whose structure is similar to that of Li 2 MSiO 4 , is also an attractive material; however, there are very few reports. 8,9 These materials are usually synthesized by the sol-gel method, flux method, hydrothermal method, and solid-state reactions. However, these methods have the some problems for mass production, such as cost-efficiency and environmental impact, and do not afford satisfactory battery performance.…”