The coexistence of superconductivity and magnetic order seems to take place in the so called ruthenate-cuprates. Previous work [1,2] has mainly focused on RuSr2GdCu2O8 where a magnetic transition is observed at 133 K and superconductivity has been claimed below TC ~ 46K. This material has a tetragonal ?212?type structure (space group: P 4/ m m m) similar to that of YBa2Cu3O7 with [Cu-O2] chains being replaced by [Ru-O6] octahedral layers. In order to better understand this, most unusual behaviour, we have performed some substitutions in the title compound. At room pressure, we have replaced up to 10 % calcium for strontium and observed that, in consonance with the difference in ionic radii, the unit cell contracts On the other hand, the calcium doped sample shows a positive magnetic susceptibility at 5K. At room pressure, Sm, Eu and Gd seem to be the only RE elements that accept to enter into the structure of RuSr2GdCu2O8. However high pressures and temperatures allows one to replace gadolinium by other lanthanide cations, although it is difficult to get pure samples. We have performed several substitutions at the gadolinium site. We have succeeded in replacing Gd by Tb, Dy, Y, Ho and Er, Magnetic susceptibility measurements reveal the existence of very interesting phenomena. Also, structural and microstructure characterisation have been performed.

[1] L. Bauernfeind, W. Widder and H.F. Braun, Physica C, 254, 151 (1995).
[2] AC. Mc Laughlin, W. Zhou, J.P. Attfield, A.N. Fitch and J.L. Tallon, Phys. Rev. B, 60, 7512 (1999).