Physical properties of the (Tl, Hg)-1223 superconductor phase substituted by lanthanum and samarium fluorides

The effect of samarium and lanthanum fluorides (SmF₃ and LaF₃) on the structural and superconducting properties of Tl_0.8Hg_0.2Ba₂Ca_2−xR_xCu₃O_9-δ-yF_y superconducting phase (Tl, Hg)-1223, where R = Sm or La and x = 0.00, 0.025, 0.050, 0.075, and 0.10 was investigated. The solid-state reaction method was used to synthesize the superconducting samples. The phase formation percentages and lattice parameters were calculated using X-ray powder diffraction (XRD). Images taken with a scanning electron microscope (SEM) show that the presence of both Sm and La increases the grain size and inter-grain connectivity up to x=0.025. The successful substitution of Sm and La elements in the crystal structure of (Tl, Hg)-1223 is confirmed by energy-dispersive X-ray (EDX) analysis. The oxidation states of the elements in the prepared samples are revealed by X-ray photoelectron spectroscopy (XPS) spectra. I–V and resistivity measurements were used to investigate the electrical properties of the (Tl, Hg)-1223 system with Sm and La substitutions. The superconducting transition temperature (T_c) and transport critical current density (J_c) have steadily improved up to x = 0.025, above which they dropped significantly. Furthermore, as temperature increased, the temperature dependence of fluctuation conductivity (Δσ), revealed distinct zones. The zero-temperature coherence length (ξ_c(0)), effective layer thickness of the two-dimensional system (d), interlayer coupling (I), Fermi velocity (v_f), and Fermi energy (E_f) were determined. ξ_c(0) and d decrease until x= 0.025, then increase as x increases. All estimated I values show weak coupling between CuO₂ planes and a transition from three-dimensional (3D) to two-dimensional (2D) regions.