Low Temperature Structural Phase Transition and Superconductivity in ( La Nd )-Sr-Cu-O

In (La RE)SrCuO with RE = Nd, Eu, Gd, Dy, Tb we find, besides the known tetragonal → orthorhombic transition, a further low temperature (LT) structural phase transition. Its occurance and also the low temperature structure depend on RE and Sr content. We find well defined and sharp transitions especially in La1.85-xNdxSr0.15CuO4, for which we present a detailed phase diagram including structural and superconducting properties. For a Sr content of 0.15 there is a competition between superconductivity and the low temperature phase transition: There is no bulk superconductivity in the LT phase and, vice versa, there is no complete LT-transition, if the transition temperature of the structural transition extrapolated from the Nd-concentration dependence is smaller than Tc. For Sr content sufficiently far away from 0.15 the LT-transition smears out and bulk superconductivity occurs.