On the localisation of charge carriers and suppression of superconductivity by praseodymium in systems derived from YBa<SUB>2</SUB>Cu<SUB>3</SUB>O<SUB>7-d</SUB>

The effect of Pr substitution in suppressing Tc in LaCaBaCu3O7-d and Y0.8Ca0.2Ba2Cu3O6+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 PrBa2-xCaxCu3O7-d was also studied. The main conclusions are that Pr ions play a role in suppressing Tc by exchange scattering, and to some extent by the hole filling mechanism involving the formation of Pr4+. The Pr ions in La1-xPrxCaBaCu3O7-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 Tc is similar to that of Y1-xPrx Ba2Cu3O7-d but it is suggested that a percolation model may explain the results more adequately than the Abrikosov-Gorkov theory