NEUTRON-DIFFRACTION STUDIES AND BOND VALENCE SUMS OF CHARGE NEUTRALLYDOPED ND1-2XCAXTHXBA2CU3O7-DELTA

Equal amounts of Ca and Th have been doped into the Nd1-2xCaxThxBa2Cu3 O7-delta superconductor with x = 0. 0.015. 0.03, 0.05. and 0.10. This causes a fast and linear decrease of the superconducting transition te mperature, T-c. A single orthorhombic phase was observed with x-ray di ffraction except for weak additional lines for the x = 0.10 sample, su ggesting a solubility limit below x = 0.10. Neutron-diffraction measur ements were performed and bond valence sums (BVS's) were calculated. T he results showed a small increase in the Cu2-O4 distance and a nearly constant Cu2 valence, The total hole density in thf plane, V- as defi ned by Tallon, remains almost constant with doping in contrast to the decrease observed for Ca-Pr doping. Other results from the BVS calcula tions also indicate differences between Ca-Th and Ca-Pr doping in the NdBa2Cu3O7-delta compound.