Neutron Powder Diffraction Studies of YBa₂(Cu_1-xM_x)₃O_7-δ with M= ⁵⁷Fe, ⁵⁸Ni

Abstract

Neutron powder diffraction studies of YBa2(Cu_1.xM_x)O_7-δ with M=⁵⁷Fe,⁵⁸Ni were carried put for series of samples with 0 ≤ x ≤ 0.27 for ⁵⁷Fe and 0 ≤ x 0.09 for ⁵⁸Ni. Analysis of the data was focused on the problem of the crystal structures, site occupancy of dopant atoms as well as solubility limits. The obtained results suggest that in the case of iron the structure transforms from orthorhombic (space group Pmmm) to tetragonal (space group P4/mmm) already for x ≤ 0.05, while for nickel it resembles that of undoped YBa₂Cu₃O_7-δ in the whole range of composition x studied. Taking into account the localization, for the case of iron, for 0 ≤ x ≤ 0.10 the whole amount of iron is predominantly localized at the Cu(l) position, while for x > 0.10 Fe substitutes for Cu at both, Cu(1) and Cu(2) sites with equal probabilities. Furthermore, for every two atoms of Fe substituted YBa₂Cu₃O_7-δ approximately one extra oxygen is incorporated into the Cu(1)-0 plane. On the other hand, the Ni atoms substitute the Cu cations located both in CuO chains and in the CuO₂ planes with equal probabilities from the very beginning. Moreover, our results confirm that the solubility of Ni in the structure of the YBa₂Cu₃O_7-δ phase is limited and much lower than that of Fe, i.e., starting from x = 0.05 Ni is found to precipitate as NiO.