Competition of Kondo Spin Fluctuations and RKKY Interactions in CeRh 2 Si 2- x Ge x and CeM 2 X 2 Compounds: a Kondo Necklace Problem

We present the results of our measurements of the lattice constants and magnetic susceptibility of the pseudo-ternary system CeRh 2 Si 2- x Ge x which crystallizes in the tetragonal ThCr 2 Si 2 structure. Both of the cell constants a and c increase linearly with x . The magnetic ordering temperature T N exhibits initially an enhancement with the increase in x ( T N = 36.9 K for x = 0 and 38.3 K for x = 0.125) and then decreases as x continues to increase further. These results, along with those on the pressure dependence of T N in CeRh 2 Si 2 , can be understood on the basis of the Doniach's model of a Kondo necklace. We discuss also the applicability of this model to describe the strong correlation between the structural aspects and the ground-state properties of the whole series of Ce-based ternaries CeM 2 X 2 (M = 3 d , 4 d and 5 d elements; X = Si, Ge) of the ThCr 2 Si 2 structure.