Low temperature variation of Yba(2)Cu(3)O(7-delta) lattice parameters in Fe/YBa2Cu3O7-delta superconducting composites

In order to complement a previous study of the electrical properties of xFe /(1-x) YBa2Cu3O7-delta composites, the thermal dependence of lattice consta nts and orthorhombic strain (b-a)/(a+b) in the range 15 K < T < 300 K was i nvestigated by high resolution X-ray diffraction. For low Fe contents (x = 2%), the main differences with the pure YBa2Cu3O7-delta (YBCO) parameters a re found at temperatures lower than 100 K only; moreover, for higher concen trations (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 wit h the pure YBCO value, and their temperature dependence suggest that the Cu (1) and Cu(2) positions are preferred for substitution. The substitution, h owever, is small enough to retain the orthorhombic structure, as the transi tion to the insulating tetragonal structure is not reached. At T < 100 K, t he composite with x = 2% Fe exhibits an anomalous thermal behavior of latti ce constants which may be caused by a magnetostrictive response to a magnet ic ordering of Fe in the lattice.