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

Abstract

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 (Mössbauer) spectroscopy at temperatures of 293, 91, and 78 K. A correlation is found between the changes of the intensities of the components of the Mössbauer 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 submicron grain sizes.

Disordering effects in the atomic structure of fine-crystalline HTSC YBa2Cu3O y

“…It should be noted that the oxygen content in the O5 positions noticeably (by a factor of 2-3) exceeds the corre sponding values for macrocrystalline samples. This effect was also established earlier using Mössbauer spectroscopy data [4,5].…”

“…Thus, by varying the conditions D 〈 〉 of high temperature treatment, it is possible to control the degree of cation ordering in this system. Earlier magnetic, X ray, and Mössbauer studies [3][4][5][6][7] have shown that the properties of single phase YBa 2 Cu 3 O y fine crystalline HTSCs with an orthor hombic crystal structure (both in the superconducting and normal states) noticeably differ from the proper ties of macrocrystalline HTSCs of the same composi tion and with the same superconducting transition temperatures T c , but synthesized under conditions closer to equilibrium. It was found that for fine crys talline YBa 2 Cu 3 O y , the relations between the unit cell parameters, T c , and oxygen index y established earlier for equilibrium samples (see, for example, [8]) are not observed.…”

“…6 2012 Most of the size effects observed for this compound cannot be explained by the surface effect or by com mensurability of the characteristic physical parame ters with the crystallite size. Structural features discov ered in [4,5] suggested the existence of a special type of structural inhomogeneity in YBa 2 Cu 3 O y fine crys talline HTSCs; the degree of this inhomogeneity corre lates with average crystallite size and can be respon sible for the observed changes in physical properties.…”

“…Neutron diffrac tion has made it possible to confidently analyze the effects associated with the redistribution of cations and oxygen in the cell (the possibility of taking into account the importance of these effects has been indi cated earlier). For example, a certain extent of mutual substitution of the rare earth cation and barium and simultaneous variation in the occupancy of the oxygen positions was hypothesized in [4,5,10]. However, information about these effects was obtained using macroscopic methods and Mössbauer spectroscopy, which did not provide direct answers to these ques tions.…”