Antiferromagnetic Kondo-lattice systems ${\mathrm{Ce}}_2{\mathrm{Rh}}_3{\mathrm{Ge}}_5$ and ${\mathrm{Ce}}_2{\mathrm{Ir}}_3{\mathrm{Ge}}_5$ with moderate heavy-fermion behavior

The results of electrical resistivity, magnetic susceptibility, specific heat, isothermal magnetization, and magnetoresistance measurements on ${\mathrm{Ce}}_2{\mathrm{Rh}}_3{\mathrm{Ge}}_5$ and ${\mathrm{Ce}}_2{\mathrm{Ir}}_3{\mathrm{Ge}}_5$ are reported. For both systems, the magnetic contribution to the resistivity shows the logarithmic T dependence in two separate T ranges being typical of a Kondo-lattice system in the presence of crystalline field. A marked peak in the susceptibility and a sharp drop in the resistivity are observed at $T_N=5.5\mathrm{}$ and 9.5 K for ${\mathrm{Ce}}_2{\mathrm{Rh}}_3{\mathrm{Ge}}_5$ and ${\mathrm{Ce}}_2{\mathrm{Ir}}_3{\mathrm{Ge}}_5,$ respectively. The total drop in resistivity is as large as 300 μΩ cm. Isothermal magnetization curves at 1.3 K reveal a field induced transition at 23 T in ${\mathrm{Ce}}_2{\mathrm{Rh}}_3{\mathrm{Ge}}_5$ and two transitions at 10 and 15 T in ${\mathrm{Ce}}_2{\mathrm{Ir}}_3{\mathrm{Ge}}_5.$ The saturation moment of $0.8{\mu }_B/\mathrm{Ce}$ is suggestive of the ${\Gamma }_7$ doublet ground state in both compounds. The specific heat exhibits a λ-type anomaly around $T_N,$ while the values of magnetic entropy at $T_N$ are reduced to 50 and 65% of $R\ln 2$ for ${\mathrm{Ce}}_2{\mathrm{Rh}}_3{\mathrm{Ge}}_5$ and ${\mathrm{Ce}}_2{\mathrm{Ir}}_3{\mathrm{Ge}}_5,$ respectively. The linear coefficients of specific heat are 148 and 152 ${\mathrm{m}\mathrm{J}/\mathrm{K}}^2$ mol Ce above $T_N.$ Thus both compounds are classified into moderate heavy-fermion antiferromagnets.