Crystal structure and Kondo lattice behavior of ${\mathrm{CeNi}}_9{\mathrm{Si}}_4$

We have studied the crystal chemistry and magnetic, thermodynamic, and transport properties of $R{\mathrm{Ni}}_9{\mathrm{Si}}_4$ with $R=\mathrm{La}$ and Ce. These compounds crystallize in a fully ordered tetragonal (space group $I4/mcm)$ variant of the cubic ${\mathrm{NaZn}}_{13}$ type. The low-temperature properties characterize ${\mathrm{CeNi}}_9{\mathrm{Si}}_4$ as a Kondo lattice with a large Sommerfeld value $\gamma =155\mathrm{}\left(5\right)\mathrm{}{\mathrm{m}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{}\mathrm{K}}^2$ as compared to $\gamma =33\mathrm{}{\mathrm{m}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{}\mathrm{K}}^2$ of Pauli paramagnetic ${\mathrm{LaNi}}_9{\mathrm{Si}}_4.$ The temperature dependencies of the specific heat and susceptibility are well described by the degenerate $\mathrm{}(J=5/2\mathrm{})\mathrm{}$ Coqblin-Schrieffer model with a characteristic temperature $T_0\simeq 180\mathrm{K}.$ The large Ce-Ce spacing in ${\mathrm{CeNi}}_9{\mathrm{Si}}_4$ ${(d}_{\mathrm{C}\mathrm{e}-\mathrm{C}\mathrm{e}}\approx 5.6\mathrm{\AA })$ implies very weak Ce-Ce intersite exchange interactions which is corroborated by the thermoelectric power $S\left(T\right)\mathrm{}$ showing close agreement with theoretical results of the degenerate Anderson lattice without intersite interactions. ${\mathrm{CeNi}}_9{\mathrm{Si}}_4$ appears to be a model type Kondo lattice system with $T_0>{\Delta }_{\mathrm{CEF}}\gg T_{\mathrm{RKKY}}$ where $T_0,$ ${\Delta }_{\mathrm{CEF}}$ (crystalline electric field), and $T_{\mathrm{RKKY}}$ (Ruderman-Kittel-Kasuya-Yosida) are the characteristic energy scales of the Kondo interaction, crystal field splitting, and Ce-Ce intersite exchange coupling, respectively. ${\mathrm{CeNi}}_9{\mathrm{Si}}_4$ shows a remarkably low ratio $A/{\gamma }^2=0.83\mathrm{}\mathrm{}\left(8\right)\mathrm{}\times {10}^{-6}\mu \Omega \mathrm{cm}(\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{K}/\mathrm{m}\mathrm{J}{)}^2$ which is one order-of-magnitude smaller than the usual Kadowaki-Woods ratio of heavy-fermion systems.