Interplay between crystal field splitting and Kondo effect in CeNi₉Ge_4-xSi_x

The pseudo-ternary solid solution CeNi₉Ge_4-xSi_x (0 ≤ x ≤ 4) has been investigated by means of x-ray diffraction, magnetic susceptibility, specific heat, electrical resistivity, thermopower and inelastic neutron scattering studies. The isoelectronic substitution of germanium by silicon atoms causes a dramatic change of the relative strength of competing Kondo, RKKY an crystal field (CF) energy scales. The strongest effect is the continuous elevation of the Kondo temperature T_K from approximately 3.5 K for CeNi₉Ge₄ to about 70 K for CeNi₉Si₄. This increase of the Kondo temperature is attended by a change of the CF level scheme of the Ce ions. The interplay of the different energy scales results in an incipient reduction of the ground state degeneracy from an effectively fourfold degenerate non-magnetic Kondo ground state with unusual non-Fermi-liquid features of CeNi₉Ge₄ to a lower one, followed by an increase towards a sixfold, fully degenerate ground state multiplet in CeNi9Si4 (T_K ~ Δ_CF).