Nm. Hamdan et al., Effect of fluorine on the phase formation and superconducting properties of Tl-1223 superconductors, PHYSICA C, 314(1-2), 1999, pp. 125-132
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.