ON THE RARE-EARTH IONIC SIZE EFFECT IN THE R(1-X)PR(X)BA2CU3O(Y) SYSTEM (R = RARE-EARTH ELEMENT)

Superconducting and magnetic properties of the R1-xPrxBa2Cu3Oy system can be consistently explained within the framework of the percolation model in which localization of holes by two neighbouring Pr ions and the resultant local destruction of the superconductivity are assumed. A tendency of the system to phase-separate into RBa2Cu3Oy and PrBa2Cu3Oy assumed to be stronger the smaller the difference in the ionic size, shifts the percolation threshold at which Pr ions form an infinite cluster and macroscopic superconductivity gives way to long-range antiferromagnetic ordering of Pr moments. The enhanced Neel temperature T(N) for lighter R ions is ascribed to the increase in the number of neighbouring Pr-Pr pairs due to the clustering tendency. Results of Monte Carlo simulations are presented to support these arguments.