Abstract
The effect of Pr substitution in suppressing T c in LaCaBaCu 3O 7− d and Y 0.8Ca 0.2Ba 2Cu 3O 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− x Ca x Cu 3O 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 Pr 4+. The Pr ions in La 1− x Pr x CaBaCu 3O 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− x Pr x Ba 2Cu 3O 7− d but it is suggested that a percolation model may explain the results more adequately than the Abrikosov-Gorkov theory.
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