Low temperature variation of YBa 2Cu 3O 7 − δ lattice parameters in Fe/YBa 2Cu 3O 7 − δ superconducting composites

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In order to complement a previous study of the electrical properties of xFe/ (1 − x) YBa 2Cu 3O 7 − δ composites, the thermal dependence of lattice constants and orthorhombic strain (b − a) / (a+b) in the range 15 K < T < 300 K was investigated by high resolution X-ray diffraction. For low Fe contents ( x = 2%), the main differences with the pure YBa 2Cu 3O 7 − δ (YBCO) parameters are found at temperatures lower than 100 K only; moreover, for higher concentrations ( x = 5%) the orthorhombic strain is found to be about 10 % smaller than in the pure material in the whole temperature range. The quantitative differences of the a and b lattice constants with Fe content, compared with the pure YBCO value, and their temperature dependence suggest that the Cu(1) and Cu(2) positions are preferred for substitution. The substitution, however, is small enough to retain the orthorhombic structure, as the transition to the insulating tetragonal structure is not reached. At T < 100 K, the composite with x = 2% Fe exhibits an anomalous thermal behavior of lattice constants which may be caused by a magnetostrictive response to a magnetic ordering of Fe in the lattice.