Neutron structural studies on the superconducting (Nd1-xCax)(Ba1.6La0.4)Cu3Oz system

We have investigated the influence of Ca ion substitution on the structural and superconducting properties of (Nd1-xCax)(Ba1.6La0.4)Cu3Oz system. Magnetization, x-ray diffraction, and neutron diffraction studies have been carried out on a series of compounds with x=0.0-0.6. The superconducting transition temperature T-c, determined from magnetization measurements, increases with increasing Ca2+ substitution. Neutron diffraction studies reveal that these compounds crystallize in a tetragonal structure (space group P4/mmm). A detailed analysis of the neutron diffraction data reveals that Ca and La ions are intermixed at the nominal Ba and Nd sites. While a major fraction of Ca ions occupy the usual Nd site, a small fraction occupies the Ba site. Consequently, the corresponding amount of La substitutes at the nominal Nd site. The intermixing of Ca and La sites randomizes the chain site oxygens leading to a tetragonal structure despite an oxygen content close to 7.0 for all the Ca doped samples. Further increase in Ca content leads to change in its coordination from sixfold to eightfold at x >= 0.4.