Synthesis and physical properties of the Li_1-xTM_xFeAs, TM=Cu, Mn

Polycrystalline Li_1-xTM_xFeAs with a degree of substitution x = 0, 0.05, 0.1, 0.3. 0.5 for TM = Cu and x = 0, 0.05, 0.1, 0.3. 0.5, 0.7, 1 for TM = Mn were prepared by two-stage solid state synthetic approach. According to the SEM, specimens consisted of partially sintered lamellar microcrystals having dimensions of 5-20 μm. The EDX analysis of individual microcrystals showed that for all samples the atomic iron content was equal to the content of arsenic, and the content of TM deviated from the nominal content by not more than 2.5%.

Full-profile analysis of diffraction data by the Rietveld method confirmed the partial replacement of lithium by TM cations. Along with the main phase isostructural to LiFeAs (P4/nmm), the impurity phase FeAs is present in the samples. Increase of the degree of substitution of lithium by TM is accompanied by a decrease of the unit cell parameters, which corresponds to decreasing of the interlayer distance.

The study of the isothermal magnetization curves and the T-dependence of the magnetic susceptibility curves for Li_1-xCu_xFeAs samples (x≤0.3) showed that they exhibit paramagnetic properties. Treatment of the c(T) curves based on the Curie-Weiss model allows us to conclude that the samples contain only diamagnetic Cu⁺ cations. Approximation of the magnetization curves by a Langevin function allowed us to determine the effective magnetic moments (4.3 - 5.5 μB) and the concentration of the magnetic centers (≈0.3%). Thus, the magnetic properties of Li_1-xCu_xFeAs (x≤0.3) samples can be described within the model of non-interacting magnetic moments, which may be impurity iron cations in the lithium positions.

The features of the electronic and magnetic state of iron cations of the Li_1-xTM_xFeAs were studied by Mössbauer spectroscopy on ⁵⁷Fe. The main spectral component over the entire temperature range is a slightly enlarged broadened, quadrupole doublet, which corresponds to the localisation of iron in a tetrahedral environment of arsenic atoms within conducting layers. In addition to the main component, the second signal is present in the spectra, which corresponds to the FeAs impurity. In contrast to TM=Cu, Li_1-xMn_xFeAs samples exhibited complex magnetic behavior and spontaneous magnetization observed up to room temperature.