The 4f-ligand hybridization in the evolution of heavy-fermion behaviour inthe series CeRu2-xNixSi2

CeRu2Si2 is a heavy-fermion compound and CeNi2Si2 is a mixed-valence compou nd. We measured the L-III absorption spectra of CeRu2-xNi2Si2 (x = 0, 0.2, 0.4, 0.8, 0.9, 1.0, 1.1, 1.4, 1.6, and 2.0) at 16, 50, 100, 200, and 300 K. The linear coefficient of the specific heat y of CeRu2-xNi2Si2 is enhanced rapidly for Ce valence v < 3.08. The structure of CeRu2-xNi2Si2 is consist ent with the tetragonal space group 14/mmm. There is no critical lattice co nstant c separating the strongly mixed-valence regime from the heavy-fermio n regime for CeRu2-xNi2Si2. The value of the lattice constant c is almost a linear function of x, whereas there is a deviation from linearity for the values of the lattice constant a when x < 0.4. Judging by the L-III x-ray a bsorption spectra of CeRu2-xNi2Si2 and CeRu2Si2-xGex, the evolution from he avy-fermion to mixed-valence behaviour might be due just to the hybridizati on between the Ce 4f-electron wave function and the sp wave function of Ru.