A series of ${\mathrm{YBa}}_2{\mathrm{Cu}}_{3-x}{\mathrm{Fe}}_x{\mathrm{O}}_y$ $(x=0\mathrm{}–0.50)$ samples has been studied by means of positron annihilation technology, scan electron microscope and x-ray diffraction. The oxygen contents of the samples have been measured using a volumetric method. The positron short-lifetime component ${\tau }_1$ decreases abruptly between $x=0.12\mathrm{}$ and 0.15 where the compound undergoes an O-T phase transition and the tweed microstructure disappears. We proposed a simple model to describe the dependency of ${\tau }_1$ on oxygen vacancy and twin (and tweed) boundary densities. The experimental results can be satisfactorily explained using this model. The positron lifetime ${\tau }_1$ depends not only on the oxygen vacancy density, but also on the twin and tweed densities. Therefore, the positron can be used as a sensitive probe for the O-T phase transition in this system. In addition, analysis of the experimental results also gives certain indication for Fe clustering when $x>~\mathrm{0.20.}$

• Received 27 November 2000

DOI:https://doi.org/10.1103/PhysRevB.65.054513