Mössbauer Spectroscopic Study of Superconducting Y–Ba–Cu(Fe)–O Ceramics and Gamma-Ray Irradiation Effect

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Abstract
Mössbauer spectra of YBa2(Cu1−xFex)3O7−δ ceramics (x=0.003, 0.008, and 0.017) measured at room temperature show a superposition of three quadrupole doublets. An outermost doublet (δ=0–0.04 mm s−1, Δ=1.94–1.98 mm s−1) is ascribed to high-spin Fe4+ ions present at substitutional sites of Cu2+ ions (Cu(2) site). An intermediate doublet (δ=−0.03–0.02 mm s−1, Δ=1.12–1.17 mm s−1) is ascribed to intermediate-spin Fe3+ ions present at substitutional sites of Cu3+ ions (Cu(1) site). An innermost doublet is ascribed to Fe3+ ions (high spin) with tetrahedral symmetry, present along the b-axis. A decrease in Tc from 72 to 60 K, observed when iron content (x) is increased from 0.008 to 0.017, is explained by a lowered layer structure composed of “dimpled” CuCO4 planes, owing to a displacement of O(4) oxygen atoms sharing CuO5 tetragonal pyramids and CuO4 square planars. One-dimensional chains composed of CuO4 square planars proved to be primarily affected by γ-ray irradiation, whereas no change of Tc was observed. Low-temperature Mössbauer measurements revealed that “partial Debye temperatures” are 330 and 740 K for the chains and layer structure, respectively.