Features of the local structure of fine-grained HTSC YBa2Cu2.985 57Fe0.015Oy

Polycrystalline high-Tc superconductors (HTSCs) with the composition YBa2Cu2.985 57Fe0.015Oy (y≈6.92, Tc=91–91.5 K) with micron and submicron grain sizes are investigated by the method of gamma resonance (Mossbauer) spectroscopy at temperatures of 293, 91, and 78 K. A correlation is found between the changes of the intensities of the components of the Mossbauer spectra and the mean grain sizes 〈D〉. It is shown that the observed changes in the spectra are caused by an interplane redistribution of oxygen wherein the oxygen content δ in the (CuOδ) planes increases and the lattice parameter c decreases. This effect is greater for smaller mean grain sizes 〈D〉. It is concluded that the observed changes in the anion subsystem of the fine-grained HTSCs as compared to bulk equilibrium samples of YBa2Cu3O6+δ are mainly due to a partial interchange of the Y3+ and Ba2+ cations in them. This interchange may be a result of the nonequilibrium synthesis conditions that must be used to produce HTSCs with micron and submic...