.Synthesis and characterization of Mg and Ge doped (Cu0.5Tl0.5)Ba2Can-1CunO2n+4-δ (n = 3, 4, 5) superconductors

Abstract

Different series of polycrystalline superconductor, Cu0.5Tl0.5Ba2Ca4-xMgxCu5O14-δ (x = 0, 1, 2 and 3), (Cu0.5Tl0.5)Ba2Ca2Cu3-yGeyO10- (y = 0, 0.5, 0.75 and 1.0) and (Cu0.5Tl0.5)Ba2Ca3(Cu4-yGey)O12- (y = 0, 0.3, 0.6 and 0.9) 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). X-ray diffraction scans of polycrystalline samples of Cu0.5Tl0.5Ba2Ca4-xMgxCu5O14-δ (x = 0, 1, 2 and 3) samples show tetragonal structure following P4/mmm space group. The dominant phase in x = 0 sample is CuTl- 1234 while the remaining all Mg doped samples show the projected CuTl-1245 phase with a small inclusion of impurity phases. It suggests that the presence of Mg plays a very vital role in synthesis of five planar CuTl-1245 compounds. With the partial substitution of Ca with Mg in the samples, the CuO2 planes become uniformly doped due to improved inter-planer coupling which results in the enhancement of Tc (R = 0) as well as the magnitude of diamagnetism. Further superconducting properties of the samples have been enhanced by carrying out annealing experiments in nitrogen air and oxygen atmospheres. The enhanced superconductivity in different annealing environments have been attributed to the optimization of carrier density in the OPs and the IPs. Oxygen annealed samples show the best superconductivity among all, which have been attributed to the optimization of carrier concentration in the CuO2 planes and improved weak link behavior due to the intercalation of the oxygen in the unit cell and at the intra-grain and inter-grain sites. Superconductivity in the Cu0.5Tl0.5Ba2Ca2Mg2Cu5O14- superconductor has been optimized by varying both time and temperature during the air annealing experiments by attaining Tc ~ 132 K. In (Cu0.5Tl0.5)Ba2Ca2Cu3-yGeyO10- (y = 0, 0.5, 0.75 and 1.0) samples, synthesized at 860oC, y = 0 and 0.5 show the CuTl-1223 tetragonal phase but y = 0.75 and 1.0 develop CuTl-1234 as the dominant phase. It indicates that the substitution of Ge lowers the synthesis temperature and for single phase sample, temperature less than 860oC seems more plausible. Four planar Ge doped (Cu0.5Tl0.5)Ba2Ca3(Cu4-yGey)O12- (y = 0, 0.3, 0.6 0.9) superconductors were prepared at 870oC rather than 880oC and investigated for their superconducting properties. X-ray diffraction scans of the samples confirm the CuTl-1234 phase. Increased Ge substitution at the Cu planar sites has been found to suppress the superconducting properties of the samples. Possible causes include the impurity scattering and the carrier depletion due the replacement of Cu2+ ions with Ge4+ ions. The superconductivity has been restored by carrier doping through annealing processes. FTIR analysis has also been conducted and the role Ge in the variations of different phonon modes has been discussed. Also the effect of oxygen annealing on these phonon modes have been observed and analyzed. The fluctuation induced conductivity of Cu0.5Tl0.5Ba2Ca4−xMgxCu5O14−δ (x = 1, 2) and Cu0.5Tl0.5Ba2Ca2Cu2M1O10- (M =Si, Sn, Ge) superconductor samples has been done. FIC data of the as-prepared Cu0.5Tl0.5Ba2Ca4−xMgxCu5O14−δ (x = 1) sample does not fit with 2D, 3D AL equations, but for x = 2 and the oxygen annealed samples seem to fit well with 2DAL, 3D AL equations with two distinct cross over temperatures. In the samples the interlayer coupling strength J is found to increase with increased Mg content and oxygen post annealing. Higher concentration of Mg seems to lower the anisotropy along the c-axis and thus improves the inter-planer coupling. It results in 2D to 3D cross over of the conductivity and enhancement of the interlayer coupling strength J. The 2D to 3D cross over and higher J values in post annealed samples can be attributed to the change in the carrier concentration in the CuO2 planes, and improved weak link behavior, brought about by the intercalation of the oxygen in the charge reservoir layer. The excess- conductivity data of Cu0.5Tl0.5Ba2Ca2Cu2M1O10- (M = Si, Sn and Ge) samples also shows two cross over temperatures and three distinct exponents. Si and Sn doped samples with almost same critical temperature have not shown much diversity in different parameters such as room temperature resistivity, transition width,  (N 0 K) etc. Similarly not much difference is seen in the dimensionality exponents and the corresponding temperature ranges. The Ge doped sample with lower critical temperature has shown different FIC parameters with large transition width. Ge doped sample has shown higher 3D character; in this sample 2D fluctuations are found in very small temperature range. Here a possible link between the confinement of the 2D fluctuations to a narrow temperature window and the lower critical temperature of the Ge doped sample can be conjectured but not claimed.