Structural aspects and superconductivity in oxygen-deficient Y 1− xCa xBa 2Cu 3O 7− y ( y ≈ 0.3) system A neutron-diffraction study

Abstract

We have studied the superconductivity and structural aspects of substituting Y by Ca in oxygen-deficient YBa 2Cu 3O 7− y (Y:123) high- T c cuprate superconductor near y ≈ 0.3. Unlike for fully oxygenated systems [1,2], the superconducting transition temperature T c, determined from AC susceptibility measurements, increases with increasing Ca 2+ substitution at the Y 3+ site in this system. A detailed structural analysis of the system has been carried out through neutron diffraction. On replacement of Y by Ca the oxygen content of the Y 1− x Ca x Ba 2Cu 3O 7− y ( y ≈ 0.3) system remains nearly unchanged, till x = 0.10, and drops sharply for x = 0.15. The in-plane bond distances in the CuO planes decreases with increasing Cu(2)O(2)Cu(2) angle, while the bond distances in the CuO chains are found to be intact again till x = 0.10. These structural-refinement results indicate that there is an increase in the number of holes with the substitution of Y 3+ by Ca 2+ in Y 1− x Ca x Ba 2Cu 3O 7− y ( y ≈ 0.3) system for x = 0.05 and 0.10. Nearly intact bond distances in the CuO chains and decreased Cu(2)O(2) bond distances in the CuO planes indicate that Ca 2+ substitution at Y 3+ directly dopes the mobile holes in the CuO planes without affecting the CuO chains for the low level of substitution, i.e., till x = 0.10. The increased number of holes in the system seems responsible for an increase in the T c of the oxygen-deficient YBa 2Cu 3O 7− y system with Y 3+ site Ca 2+ substitution. For higher concentration ( x = 0.15) of Ca the increased number of holes due to Y 3+ by Ca 2+ substitution are compensated with a corresponding decrease in oxygen content.