Crystal structure and Kondo lattice behavior of CeNi9Si4

We have studied the crystal chemistry and magnetic, thermodynamic, and transport properties of RNi₉Si₄ with R=La and Ce. These compounds crystallize in a fully ordered tetragonal (space group I4/mcm) variant of the cubic NaZn₁₃ type. The low-temperature properties characterize CeNi₉Si₄ as a Kondo lattice with a large Sommerfeld value γ=155(5) mJ/mol K² as compared to γ=33 mJ/mol K² of Pauli paramagnetic LaNi₉Si₄. The temperature dependencies of the specific heat and susceptibility are well described by the degenerate (J=5/2) Coqblin-Schrieffer model with a characteristic temperature T₀≃180 K. The large Ce-Ce spacing in CeNi₉Si₄ (d_Ce-Ce≈5.6 Å) implies very weak Ce-Ce intersite exchange interactions which is corroborated by the thermoelectric power S(T) showing close agreement with theoretical results of the degenerate Anderson lattice without intersite interactions. CeNi₉Si₄ appears to be a model type Kondo lattice system with T₀>Δ_CEF≫T_RKKY where T₀, Δ_CEF (crystalline electric field), and T_RKKY (Ruderman-Kittel-Kasuya-Yosida) are the characteristic energy scales of the Kondo interaction, crystal field splitting, and Ce-Ce intersite exchange coupling, respectively. CeNi₉Si₄ shows a remarkably low ratio A/γ²=0.83 (8)×10^-6 μΩ cm(molK/mJ)² which is one order-of-magnitude smaller than the usual Kadowaki-Woods ratio of heavy-fermion systems.