On the localisation of charge carriers and suppression of superconductivity by praseodymium in systems derived from YBa₂Cu₃O_7-d

Abstract

The effect of Pr substitution in suppressing T_c in LaCaBaCu₃O_7-d and Y_0.8Ca_0.2Ba₂Cu₃O_6+d has been studied. Infrared spectroscopy and a model based on ionic radii considerations were used to examine the location of Pr ions and the influence of Pr and Ca ions on hole localisation on chains and planes. For this purpose the series PrBa_2-xCa_xCu₃O_7-d was also studied. The main conclusions are that Pr ions play a role in suppressing T_c by exchange scattering, and to some extent by the hole filling mechanism involving the formation of Pr⁴⁺. The Pr ions in La_1-xPr_xCaBaCu₃O_7-d exist in both the Y and Ba sites. The magnitude of the resistivity at the insulator-metal transition for the polycrystalline samples is consistent with an anisotropic superconductor in which superconductivity accompanies metallisation. The rate of suppression of T_c is similar to that of Y_1-xPr_x Ba₂Cu₃O_7-d but it is suggested that a percolation model may explain the results more adequately than the Abrikosov-Gorkov theory.