  We have investigated the MoSr2R1.5Ce0.5Cu2O10-d (Mo-1222R, R=rare earth)
system by several complementary experimental techniques. In contrast to the
iso-structural RuSr2R1.5Ce0.5Cu2O10-d (Ru-1222) system, in which
superconductivity (SC) in the CuO2 planes and weak-ferromagnetism in the Ru
sub-lattice coexists, in Mo-1222, displays a competition between the two
states, namely, SC vanishes when the magnetic order sets in. The contraction in
the R elements leads to a change of the physical states. The light R ions (Pr
and Nd) are paramagnetic down to 5 K, whereas the middle R ions (Sm and Eu) are
SC at TC 18-23 K respectively. The SC charge carriers originate from the CuO2
planes, and annealing under oxygen pressures does not affect TC. A simple model
for the SC state is proposed. For the heavy R elements Ho-Lu and Y, the
pentavalent Mo layers are antiferromagnetically (AFM) ordered at TN ranging
from 13-26 K. For R=Gd, the sample is not SC and exhibit two magnetic
transitions at 11 and 184 K. Both the SC or AFM states depend strongly on the
R/Ce ratio and for R/Ce=1, both states are suppressed.
