Effect of Fe substitution on the thermal expansion of the high T _c superconductor Y₁Ba₂Cu₃O _y

Thermal expansion measurements have been carried out on Fe substituted superconducting compounds Y₁Ba₂(Cu_1-x Fe _x )₃O _y (0<x<4%) using a high resolution dilatometer employing the three terminal capacitance technique. The experimental set up is sensitive enough to detect changes in α of less than 10^-8K^-1. Results show that the jump Δα in the coefficient of linear thermal expansion at the superconducting transition temperature, T _c, increases almost linearly with Fe concentration. The normal state thermal expansion coefficient α first decreases, attains a minimum value around x=1% and then increases for higher Fe concentrations. The oxygen content per unit formula is almost constant up to x=1% and then increases rapidly with x. It has also been observed that the anomalous behaviour of α around 260 K observed by Meingast et al [22] for Fe concentration x=5% is due to inadequate annealing of the sample.