Effect of cooling rate and strontium doping on the formation of (Hg, Tl)-1234 phase

Superconductor samples of nominal composition Hg1-xTlxBa2Ca3Cu4O10+delta (0 .3 less than or equal to x less than or equal to 1.0) are prepared at ambie nt pressure using a closed quartz tube at 850 degreesC. X-ray powder diffra ction for the prepared samples is carried out and the results show that the structure of this system is a tetragonal. The lattice parameters a and c a re calculated, and the data indicate that a is not affected by the substitu tion of Tl3+ into Hg2+, whereas c decreases as the Tl content increases. Th e superconductivity is investigated using the electrical resistance and ac magnetic susceptibility measurements. All the samples have a superconductin g behaviour except when x = 0.3. To improve the superconducting properties of this system, the cooling rate for the sample with x = 0.7 is studied; al so the replacement of Ca by Sr for both x = 0.3 and x = 0.5 have been inves tigated. The excess conductivity for x = 0.3 shows that the dimensionality is changed from two-dimensional in the high-temperature region to three-dim ensional in the mean field region.