Cooperative effects due to Ca substitution by La on the normal and superconducting states of (Bi, Pb):2223 system

The normal and superconducting properties of Bi_1.7Pb_0.30Sr₂Ca_2−xLa_xCu₂O_y (Bi, Pb):2223 system with various x values (0.00 ≤ x ≤ 0.30) have been reported. It is found that the replacement of Ca²⁺ by La³⁺ up to 0.30 does not influence the phase purity of the pure system, while the orthorhombic distortion, excess oxygen, effective Cu valance and hole carriers/Cu ions are clearly affected. Further, the doping distance and crystal geometry factor are decreased, but the distance between neighboring Cu-atoms and density of excess doping are increased. Furthermore, the DTA diagrams show strong endothermic peaks T_m at 844.1, 848, 850.1 and 857.4 °C for pure and La samples, respectively. The mass loss determined by TGA displayed a sharp mass loss started at about 800 °C and extended to 1000 °C for all samples. Although the critical temperatures T_c of the samples are decreased by La insertion from 119 to 104, 71, and 53 K, the critical concentration for quenching superconductivity may be extended above 0.30. On the other hand, the Vickers hardness H_v is increased by La up to 0.30, but it is decreased by the applied load F. Additionally, a negative linear relation between T_m or H_V and T_c is obtained as due to cooperative interactions, (T_c ∝ –T_m) i.e. there are an extended forces acting along such a wide temperature range. The H_v–F characters are divided into two linear parts; the first at lower loads (0.00–1.96 N) and the second at higher loads (2.94–4.90 N). The variation of surface energy ϒ against La is different according to the range of applied loads, while the elastic indentation d_e is decreased, and the resistance pressure F_o is increased. But, the values of F_o and d_e at higher loads are generally more than that of lower loads, while the vice is versa for the ϒ. Our results are discussed in terms of the balance between the hole carriers lost by La³⁺ with that introduced by excess oxygen in the Cu–O₂ planes.