NEUTRON-DIFFRACTION STUDIES AND INTERATOMIC DISTANCES IN CA-PR DOPED NDBA2CU3O7-DELTA

Sintered samples of Nd1-2xCaxPrxBa2Cu3O7-delta with x = 0, 0.025, 0.05 0, and 0.10 have been studied by electrical-resistivity measurements, X-ray and neutron diffraction and from calculated bond-valence sums (B VS). Doping of equal amounts of Ca-Pr suppresses a parabolic term in t he decrease of the superconducting T-c and results in a strong linear suppression of -dT(c)/dx = 206 K. This observation indicates a single- impurity effect and suggests that Ca-Pr is a charge neutral doping in Nd-123. The c-axis length and cell volume decrease with doping and the Cu2-O4 distance increases. BVS calculations suggest that T-c in Pr do ped 123 may be governed by small changes in interatomic distances and corresponding charge transfers, rather than a contribution from magnet ic pair breaking as previously conjectured. Similar calculations of pu blished data for oxygen-deficient Y-123 and Pr doped Y-123, show that the hole density in the planes --> 0 at about the concentration where T-c vanishes. The average CuO2 plane distance supports that Pr has val ence +4 at low dopings. Results are discussed which suggest that the P r valence at higher doping levels is +3.