Effect of fluorine on the phase formation and superconducting properties of Tl-1223 superconductors

Fluorination of (Tl_0.5, Pb_0.5)Sr_1.6Ba_0.4Ca₂Cu₃O_y high-temperature superconductors has been performed through partial replacement of CuO by CuF₂ in the starting materials. Seven samples with nominal fluorine content x atom per formula unit (0≤x≤3.6) were examined by X-ray diffraction (XRD), scanning electron microscopy (SEM), resistivity and magnetization M(H) measurements. Fluorine addition has raised the onset transition temperature to 128 K, and increased the critical current density to 300%, the value of the fluorine free sample. XRD, SEM and energy dispersive X-ray (EDX) results showed that the formation of the Tl-1223 phase was enhanced as fluorine was introduced, and the formation of large grains was observed. EDX spot analysis revealed the presence of fluorine as a species of the structure of the Tl-1223 grains. The c-axis of Tl-1223 decreased with increasing x, indicating that fluorine had replaced oxygen in the structure. The behavior of the normal state resistivity with temperature indicated that fluorine addition changed this material from overdoped to optimally doped, and upon further fluorine addition, changes it to slightly underdoped state. That provided an additional evidence that fluorine addition changes the density of charge carrier concentration in the Cu-O planes and improves the superconducting properties of this material.