Metal–insulator transition in Bi1.7Pb0.3Sr2Ca1−xGdxCu2O8+δ solid solutions

Substitution of rare earth ions for Ca in Bi2Sr2Ca1Cu2O8+δ compounds causes a depression in Tc and a metal–insulator transition. In order to understand the mechanism governing the process, we made systematic studies on Bi1.7Pb0.3Sr2Ca1−xGdxCu2O8+δ compounds in which Ca2+ is replaced with different concentrations of Gd3+ ion. The structural parameters, oxygen content and hole carrier concentration were observed to change with Gd concentration. From the X-ray diffractograms of the samples, the single phase nature of the compounds was confirmed. Lattice parameters were calculated and the results show that the c parameter decreases and the a parameter increases with increasing Gd content. At room temperature, the oxygen content of the samples was estimated using iodometric titration while the hole concentration was calculated from TEP measurements. The Tc (onset)’s obtained from AC susceptibility were found to be close to those obtained from resistance versus temperature plots. The materials exhibited superconductivity in the 0.0≤x≤0.4 concentration range and for higher concentration of Gd they became insulating. The initial susceptibility dM/dH relating to the superconducting volume, decreased gradually up to x=0.1, and sharply for higher concentrations of Gd. Both transport and magnetic properties have confirmed metal–insulator transition in the x>0.4 region.