The Effect of Varying Ca-Content on the Structure of High-T_c Superconductor (Ca_xLa_(1-x))(Ba_(1.75-x)La_(0.25+x))Cu_3O_(7-δ) (x = 0.5, 0.6, and 0.8) Studied by Neutron Powder Diffraction

摘要

We have performed neutron powder diffraction (NPD) experiments on polycrystalline powders with nominal compositions (Ca_(0.5)La_(0.5))(Ba_(1.25)La_(0.75))Cu_3O_(7-δ), (Ca_(0.6)La_(0.4))(Ba_(1.15)La_(0.85))Cu_3O_(7-δ) and (Ca_(0.8)La_(0.2))(Ba_(0.9)5La_(1.05))Cu_3O_(7-δ). The diffraction patterns, analysed by the Rietveld method, show that all samples consist mainly of a tetragonal Y-123 type phase. Unit cell parameters a and c shorten as the calcium content increases: a = 3.8660(2), 3.8634(3), and 3.8624(5) A; c = 11.6325(11), 11.6143(14), and 11.5822(20) A for x-values 0.5, 0.6, and 0.8, respectively. For the x = 0.6 and 0.8 samples the Rietveld refinement of calcium occupancies and EDX analysis suggest that the actual composition is closer to x ≈ 0.5. However, since the lattice parameters do change, it is also suggested that at these higher doping levels calcium does enter the Y-site to a larger extent than for the x = 0.5 composition. This is also in accordance with previously reported values for the T_c, which decreases slighty as x changes from 0.5 to 0.6 and has a pronounced change from 80 K to 73 K for x = 0.6 and 0.8, respectively.