Evidence for 4f-ligand dehybridization in the evolution of heavy-fermion behavior in the series ${\mathrm{CeCu}}_{2\mathrm{-}\mathrm{x}}$${\mathrm{Ni}}_{\mathrm{x}}$${\mathrm{Si}}_2$

Evolution of heavy-fermion behavior in ${\mathrm{CeCu}}_2$ ${\mathrm{Si}}_2$ from strong mixed valence in ${\mathrm{CeNi}}_2$ ${\mathrm{Si}}_2$ is demonstrated in the ${\mathrm{CeCu}}_{2\mathrm{-}\mathrm{x}}$ ${\mathrm{Ni}}_{\mathrm{x}}$ ${\mathrm{Si}}_2$ system. Heavy-fermion behavior, as probed by the low-temperature susceptibility, χ(O), is destroyed at xapeq20.65. There is an anomalous increase in the Ce-ligand distances, whithout any anomaly in the Ce-Ce separation, for x<0.65, tracking the x dependence of χ(O). This observation shows explicitly for the first time that the tendency for dehybridization of the ligand valence orbitals with the Ce 4f orbital is essential to attain the heavy-fermion ground state.