Transition from a mixed-valent system to a magnetically ordered Kondo lattice in Ce(NiSi)2−x(CuGe)x

The crossover from mixed-valent to magnetically ordered Kondo or heavy-fermion behavior in Ce(NiSi${)}_{2\mathrm{-}\mathit{x}}$(CuGe${)}_{\mathit{x}}$ has been studied. Results of Ce ${\mathit{L}}_3$-edge, structural, magnetic, and transport measurements are presented. It is suggested that the abnormal increase of the lattice parameter c in the region 1.6≤x≤2.0 be irrelevant to the Ce 4f-ligand orbital dehybridizations and the associated heavy-fermion state. Our results show that the substitution for Cu (and Ge) for Ni (and Si) drives this series from a strong mixed-valent to an antiferromagnetically ordered trivalent Kondo-lattice system. Particularly, it is found that the physical properties of the x=1.4 compound are very similar to that of the heavy-fermion superconductor ${\mathrm{CeCu}}_2$ ${\mathrm{Si}}_2$. The resistivity data also indicate that there is an active interplay between the crystalline electric field, coherence effects, and Kondo scattering. Moreover, this study supports the proposal that the Kondo-type spin fluctuation can be effectively quenched by the internal 3d-host magnetic field in some ${\mathrm{CeMn}}_2$ ${\mathrm{Si}}_2$-based materials. Magnetic susceptibility data show that this compound series is antiferromagnetically ordered for x>1.6 and the spin-fluctuation temperature ${\mathit{T}}_{\mathrm{sf}}$ decreases with increasing x in this region. A phase diagram is proposed to illustrate the evolution of the series from a single-impurity Kondo to a coherent Kondo and then to an antiferromagnetically ordered system.