Hole density of (La_1-xCa_x)(Ba_1.75-xLa_0.25+x)Cu₃ O_7+d synthesized by amorphous metal-complex method

We measured the resistivity and the optical reflectivity spectra, and performed the coulometric titration to examine the average hole density in tetragonal (La_1-xCa_x)(Ba_1.75-xLa_0.25+x)Cu₃ O_7+d superconductors. High-purity polycrystalline samples were synthesized for Ca cation content 0≤ x ≤ 1 by the amorphous metal-complex method. With increasing x T_c increases until x = 0.5-0.6, then decreases over x = 0.6. Temperature dependence of the resistivity in 0.4 ≤ x ≤ 0.9 shows a linear relation like a metallic behavior. The resistivity at room temperature decreases until x = 0.5, and increases over x = 0.5. The x dependence of the optical conductivity within the low energy region is consistent with the results of electrical dc conductivity. The results of the plasma frequency and the coulometric titration also show the tendency that the hole density increases until x = 0.5 and decreases over x=0.5 with increasing x.