Structure and superconductivity of Y(Ba_1-xR_x)₂Cu₃O_7-δ (R = La, Pr, and Nd)

Powder X-ray diffraction and electrical-resistivity measurement were used to study the structure and the T_cvariation of Y(Ba_1-xR_x)₂Cu₃O_7-δ (0 ≤x ≤ 0.2; R = La, Pr, and Nd). The solubility limit was estimated as x = 0.05, 0.075, and > 0.2 with R = Nd, Pr, and La, respectively, which depends on the ionic radius of dopants. All three series undergo an orthorhombic-to-tetragonal transition at x ~ 0.15-0.2. Meanwhile, the T_c increases slightly through a maximum at x - 0.025, then decreases rapidly with x, indicating an overdoped system for YBa₂Cu₃O_7-δ. Where as, the normal-state resistivity increases with x for all temperature ranges. The similarities and differences with various dopants are discussed in conjunction with ionic size, valence change and hole-filling mechanism.