Effect of Ni doping on the thermal expansion of the high-T_c superconductor YBa₂Cu₃O_y

Thermal expansion measurements have been carried out for the Ni-substituted compounds YBa₂(Cu_1-xNi_x)3O_y, (0 ≤ x ≤ 5%) with a high-resolution dilatometer using the three-terminal capacitance technique. The present set-up is sensitive enough to detect changes in α of less than 10^-8 K^-1. Results indicate that the jump Δα in the coefficient of linear thermal expansion decreases with increase in Ni content. For x = 1.5%, Δα decreases by a factor of 3 whereas T_c does not change appreciably. There is evidence that the temperature dependence of a for compounds with high Ni concentration depends on the conditions of sample growth.