Neutron-diffraction studies and bond valence sums of charge neutrally dopedNd1−2xCaxThxBa2Cu3O7−δ

Abstract

Equal amounts of Ca and Th have been doped into the ${\mathrm{Nd}}_{1\ensuremath{-}2x}{\mathrm{Ca}}_{x}{\mathrm{Th}}_{x}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ superconductor with $x=0,$ 0.015, 0.03, 0.05, and 0.10. This causes a fast and linear decrease of the superconducting transition temperature, ${T}_{c}.$ A single orthorhombic phase was observed with x-ray diffraction except for weak additional lines for the $x=0.10$ sample, suggesting a solubility limit below $x=0.10.$ Neutron-diffraction measurements were performed and bond valence sums (BVS's) were calculated. The results showed a small increase in the Cu2-O4 distance and a nearly constant Cu2 valence. The total hole density in the plane, ${V}_{\ensuremath{-}}$ as defined by Tallon, remains almost constant with doping in contrast to the decrease observed for Ca-Pr doping. Other results from the BVS calculations also indicate differences between Ca-Th and Ca-Pr doping in the ${\mathrm{NdBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ compound.