Abstract
Neutron and Mössbauer effect measurements have been made as a function of Fe concentration in orthorhombic (x = 0.01, 0.02) and tetragonal (x ⩾ 0.05), YBa2(Cu1 − xFex)3O7 + δ. A systematic Rietveld analysis of the neutron data for all concentrations shows that Fe principally occupies a site slightly displaced (y, y, 0) from the Cul (0, 0, 0) site in order to approach tetrahedral coordination. Neutron data for the x = 0.05 compound at 10 K show no significant difference from those obtained at 297 K. Local ordering of oxygen about an Fe substituent is conducive to placement of a second Fe atom in an adjacent site leading to local aggregation into chains of various lengths. This clustering is reflected in the character of low temperature magnetic Mössbauer spectra and in specific heat measurements. The Mössbauer spectra yield the relative changes in oxygen configurations about Fe atoms as the Fe concentration changes.
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