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

Fluorination of (Tl-0.5, Pb-0.5)Sr1.6Ba0.4Ca2Cu3Oy high-temperature superco nductors has been performed through partial replacement of CuO by CuF2 in t he starting materials. Seven samples with nominal fluorine content x atom p er formula unit (0 less than or equal to x less than or equal to 3.6) were examined by X-ray diffraction (XRD), scanning electron microscopy (SEM), re sistivity and magnetization M(H) measurements. Fluorine addition has raised the onset transition temperature to 128 K, and increased the critical curr ent density to 300%, the value of the fluorine free sample. XRD, SEM and en ergy dispersive X-ray (EDX) results showed that the formation of the Tl-122 3 phase was enhanced as fluorine was introduced, and the formation of large grains was observed. EDX spot analysis revealed the presence of fluorine a s a species of the structure of the Tl-1223 grains. The c-axis of Tl-1223 d ecreased with increasing x, indicating that fluorine had replaced oxygen in the structure. The behavior of the normal state resistivity with temperatu re indicated that fluorine addition changed this material from overdoped to optimally doped, and upon further fluorine addition, changes it to slightl y underdoped state. That provided an additional evidence that fluorine addi tion changes the density of charge carrier concentration in the Cu-O planes and improves the superconducting properties of this material. (C) 1999 Pub lished by Elsevier Science B.V. All rights reserved.