Enhancement of superconducting properties in Cu0.5Tl0.5Ba2Ca0.5M1.5Cu1.5Ni1.5O10−δ (M = Mg, Be) superconductors

Polycrystalline samples of Cu0.5Tl0.5Ba2Ca0.5M1.5Cu1.5Ni1.5O10−δ (M = Mg, Be) superconductor have been synthesized by solid-state reaction method. The structure and physical properties were investigated by powder X-ray diffraction (XRD), resistivity, ac-susceptibility and Fourier transform infrared absorption spectroscopy (FTIR). The X-ray diffraction data indicates that the lattice parameters 'a' and 'c' vary but structure of the samples remain tetragonal with predominant CuTl-1223 phase. The c-axis length observed by X-ray diffraction analysis has been found to decrease with the increased Be or Mg doping contents in the unit cell. Electrical resistivity and ac-susceptibility measurements showed that the Tc(R = 0) values increase for the sample with M = Mg and got even higher values for M = Be, as the level of doping increases from 0.0 to 1.5. It is most likely arising from the improved inter-plane coupling promoted by more electronegative and small ionic radii of Mg and Be substituted at the Ca sites. The enhanced inter-plane coupling increases the interaction among the carriers in neighboring NiO2/CuO2 planes. The increased correlation between the carriers increases the coherence length along c-axis and hence lowers the anisotropy resulting in enhanced superconducting properties. Most likely reasons for the enhanced superconductivity have been explored by FTIR absorption measurements. The effects of variation in carrier concentration in the conducting CuO2 planes have been explored by carrying out annealing in oxygen atmosphere.