ИСТИНА

Substitution in iron-based superconductors is an effective way of optimizing the superconducting and other physical properties of these compounds. Among the various options, there are poorly investigated variants for substituting one alkali metal for another in families 122(As) and 122(Se) (A2->A1) and replacing lithium with a 3d element (TM->Li) in the 111 family. In the report, we will show how substitution of this type affects the structure and properties of representatives of families 122 and 111. 1. Substitution of potassium for sodium and cesium in KFe2As2 It was found by x-ray analysis (XRD) and energy-dispersive x-ray analysis in a scanning electron microscope (EDX) of the polycrystalline samples K1-xAxFe2As2 (A = Na, Cs), that for A = Na the maximum degree of substitution x is about 0.3, whereas in the case of A = Cs solid solutions is unlimited (0<x<1). Single-crystal samples of K1-xAxFe2As2 were synthesized by the self flux method from solution in the melt of KAs-AAs (A = Na, Cs). The compositions and the phase purity of the investigated samples were determined by EDX and by x-ray analysis. The temperature dependence of the magnetic susceptibility shows that even at small substitution levels x, the transition temperature to the superconducting state decreases rapidly, which may indicate a d-symmetry of the superconducting gap. Important information about the features of the electronic structure is obtained by the ARPES method. 2. Substitution of potassium for sodium in KxFe2-ySe2 In contrast to the cases described above, the substitution of potassium for sodium practically does not lead to a change in the physical properties. of the substance KxFe2Se2 without substitution. It was shown that the maximum level of substitution x in the samples (K1-zNaz)xFe2-ySe2 is about 30%. 3. Substitution of lithium for Cu and Mn in LiFeAs Of all the alkaline metals, lithium cation has a minimum radius comparable to the radii of cations of some 3d elements. Due to this, among the variety of ways of substitution in iron superconductors there is only one example, when a 3d-element (Fe) replaces an alkali metal (Li) to form Li1-xFexFeAs (x≤0.04) [JACS 2010, 132, 10567]. In our work we show that not only Fe, but also Mn or Cu atoms may be incorporated in the tetragonal-pyramidal polyhedra [LiAs5] of the LiFeAs structure instead of lithium. In the case of Li1-xMnxFeAs a continuous series of solid solutions (0 <x <1 ) are formed, and in the case of Li1-xCuxFeAs the maximum level of substitution x was found to be about 0.3. In all cases, the replacement of lithium by the 3d element leads to a rapid suppression of superconductivity (at a level of 1-2% doping). With further substitution in the case of manganese, Li1-xMnxFeAs samples exhibit magnetic properties corresponding to the behavior of spin glass, while Li1-xCuxFeAs samples demonstrated paramagnetic behavior. The work was done by the financial support of the Volkswagen Foundation.