Low temperature variation of YBaCuO lattice parameters in Fe/YBaCuO superconducting composites

In order to complement a previous study of the electrical properties of xFe/(1 − x) YBa2Cu3O7 − 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 YBa2Cu3O7 − 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.