Thermodynamic behavior of the heavy-fermion compounds Ce₃X (X=Al,In,Sn)

We have measured the resistivity ρ(T) and susceptibility χ(T) of Ce₃Al, Ce₃In, and Ce₃Sn in the temperature range 1–350 K, the specific heat C(T) for 1–25 K and the pressure dependence of the resistivity ρ(P,T) for 0<P<16 kbar and 1<T<300 K. These are heavy-fermion systems that show no superconductivity above 0.4 K. In the ground state the linear coefficients of the specific heat γ are 0.70 and 0.26 J/mol Ce K² for Ce₃In and Ce₃Sn, respectively. The magnetic specific heat of Ce₃In shows two separated maxima: one at 4.3 K due to the heavy fermions and a second Schottky peak at 23 K arising from a Γ₇-Γ₈ crystal-field splitting of order T_CF=65 K. For Ce₃Sn the crystal-field splitting is comparable. From χ(0) we obtain values of the Wilson ratio of 11.5 and 7.0 for Ce₃In and Ce₃Sn. We argue that these large values represent the presence of ferromagnetic correlations in the ground state.

For Ce₃In the enhancement of the susceptibility and specific-heat coefficient and the rapid decrease of the resistivity all occur below the same temperature (7 K), suggesting that the onset of the heavy mass coincides with the onset of magnetic correlations and coherence. In addition, for Ce₃In an inflection point occurs in ρ(T) at T_inf=2.2 K, below which ρ varies as T², and there may be a peak in C(T)/T at 2 K. Thus, it appears that there are two temperature scales for the onset of interaction effects: One coincides with the single-ion Kondo temperature T_K, and the other, a low-temperature scale T_L, obeys a rule T_L=T_K/N_deg, where N_deg is the degeneracy of the ground-state multiplet. The ground state of Ce₃Al is antiferromagnetic with T_N=2.5 K. The specific-heat anomaly makes it impossible to determine γ but for 10<T<20 K γ=0.085 J/mol Ce K². From χ and the magnetic entropy we estimate T_K=10–15 K.

The magnetic entropy remains smaller than R ln2 even at 25 K; we conclude that the ordering occurs in a regime where the moment (at least on some sites) is strongly compensated by the Kondo effect. With application of pressure, the characteristic temperature increases in Ce₃Sn; it increases initially for Ce₃In, but for P>9 kbar there is a structural transition with the unusual feature that the low-temperature phase has smaller characteristic energy than the high-temperature phase. A related transition occurs in Ce₃Al at ambient pressure, which disappears at higher pressure.