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

TL;DR

This study investigates how calcium substitution affects the structure and superconductivity of (Nd1-xCax)(Ba1.6La0.4)Cu3Oz compounds, revealing increased Tc and site intermixing that influence the crystal structure.

Contribution

It provides detailed neutron diffraction analysis showing site intermixing and coordination changes due to Ca doping in this superconductor system.

Findings

Superconducting transition temperature Tc increases with Ca substitution.

Ca and La ions intermix at Ba and Nd sites, affecting structure.

Coordination changes from six-fold to eight-fold at higher Ca content.

Abstract

We have investigated the influence of Ca ions 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 to 0.6. The superconducting transition temperature Tc, 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 of Ca content lead to change in its co-ordination from six-fold to eight-fold at x >= 0.4.