The Effect of Varying Ca-Content on the Structure of High-T_C Superconductor (Ca_xLa_1-x)(Ba_1.75-xLa _0.25+x)Cu₃O_7-δ (x = 0.5, 0.6, and 0.8) Studied by Neutron Powder Diffraction

We have performed neutron powder diffraction (NPD) experiments on polycrystalline powders with nominal compositions (Ca_0.5La _0.5)(Ba_1.25La_0.75)Cu₃O _7-δ, (Ca_0.6La_0.4)(Ba _1.15La_0.85)Cu₃O_7-δ and (Ca_0.8La_0.2)(Ba_0.95La_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) Å; c = 11.6325(11), 11.6143(14), and 11.5822(20) Å 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.