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

Abstract

The normal and superconducting properties of Bi1.7Pb0.30Sr2Ca2−xLaxCu2Oy (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 Tm 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 Tc 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 Hv is increased by La up to 0.30, but it is decreased by the applied load F. Additionally, a negative linear relation between Tm or HV and Tc is obtained as due to cooperative interactions, (Tc ∝ –Tm) i.e. there are an extended forces acting along such a wide temperature range. The Hv–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 de is decreased, and the resistance pressure Fo is increased. But, the values of Fo and de 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.