Synthesis, Crystal Structure, and Structural Features of Solid Solutions Ba(Cr1–xCox)2As2 - New Layered Arsenides with the ThCr2Si2 Structure

A series of Ba(Cr_1–xCo_x)₂As₂ solid solutions is obtained for the entire range of compositions as polycrystalline samples (x = 0, 0.2, 0.3, 0.4, 0.6, 0.7, 0.8, 1.0) and single crystals (x = 0.8). By single crystal X-ray diffraction it is found that mixed arsenides Ba(Cr_1–xCo_x)₂As₂ belong to the ThCr₂Si₂ structure type. They consist of antifluorite-like [TAs]^– layers separated by Ba²⁺ cations. The layers are formed by [TAs₄] tetrahedra with common edges, T = Cr, Co atoms statistically occupy sites of the square planar network at T⋯T distances of 2.80-2.82 Å, which assumes the possible magnetic interaction. As cobalt substitutes for chromium, the T–As bond length gradually decreases, which decreases the [TAs]^– layer thickness, with the distance between As atoms from the neighboring layers remaining almost unchanged. The crystal chemical characteristics of Ba(Cr_1–xCo_x)₂As₂ solid solutions are in general well consistent with those of isoelectronic representatives of the 122 family, except BaMn₂As₂ having a number of significant distinctions in the structure, which is due to a large radius of the Mn²⁺ cation (d⁵, hs). Based on the obtained structural data, the magnetic behavior of Ba(Cr_1–xCo_x)₂As₂ solid solutions is theoretically evaluated.