Ever since high Tc superconductivity was discovered in La-based mixed oxide system by Bednorz and Muller, enormous efforts have been put in by several researchers around the world in understanding the origin and mechanism of superconductivity in these, as well as in systems derived from them. It is a proven fact that the superconductivity in RE-123 superconductors is governed by the oxygen content, which in turn is responsible for the carrier concentration in the system. Due to their dependence on oxygen content, RE-123 superconductors undergo structural transformation from orthorhombic to tetragonal as a function of oxygen content making them very difficult compounds to work with, in terms of technological applications, such as device fabrication. Hence, it would be interesting to obtain a stable compound whose superconducting properties, are not only insensitive directly to oxygen content but, having dependence of its carrier concentration and Tc on the nature and amount of the substituted cation. In the present work, we focus our investigations for such a compound, which has been derived from a tetragonal RE-123 superconducting system. In this chapter, we present a brief review of the studies carried out on La-2125 compounds to elucidate the role of dopants in modifying the superconducting properties and establish a structure-property correlation in them.
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