  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.
