Effect of Co substitution on the thermal expansion of the high-T_c superconductor Y₁Ba₂Cu₃O_y

Measurement of the coefficient of linear thermal expansion, α, has been carried out for the high-T_c superconducting compounds Y₁Ba₂(Cu_1-xCo_x)₃O_y using a high resolution capacitance dilatometer in the temperature range 77-300 K. The studies reveal that the superconducting and normal state values of α, increase almost linearly with increase in Co content. All the superconducting samples exhibit the expected second order phase transition (from normal to superconducting state) characterized by discontinuity in α, at their respective transition temperatures, T_c. The jump Δα, is also found to increase almost linearly with the increase in Co concentration. There is evidence that the temperature dependence of α for compounds having large Co concentration (≥3%) depends on the conditions of specimen growth.