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

Abstract

We have measured the resistivity \ensuremath{\rho}(T) and susceptibility \ensuremath{\chi}(T) of ${\mathrm{Ce}}_{3}$Al, ${\mathrm{Ce}}_{3}$In, and ${\mathrm{Ce}}_{3}$Sn in the temperature range 1--350 K, the specific heat C(T) for 1--25 K and the pressure dependence of the resistivity \ensuremath{\rho}(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 \ensuremath{\gamma} are 0.70 and 0.26 J/mol Ce ${\mathrm{K}}^{2}$ for ${\mathrm{Ce}}_{3}$In and ${\mathrm{Ce}}_{3}$Sn, respectively. The magnetic specific heat of ${\mathrm{Ce}}_{3}$In 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 ${\ensuremath{\Gamma}}_{7}$-${\ensuremath{\Gamma}}_{8}$ crystal-field splitting of order ${T}_{\mathrm{CF}=65}$ K. For ${\mathrm{Ce}}_{3}$Sn the crystal-field splitting is comparable. From \ensuremath{\chi}(0) we obtain values of the Wilson ratio of 11.5 and 7.0 for ${\mathrm{Ce}}_{3}$In and ${\mathrm{Ce}}_{3}$Sn. We argue that these large values represent the presence of ferromagnetic correlations in the ground state.For ${\mathrm{Ce}}_{3}$In 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 ${\mathrm{Ce}}_{3}$In an inflection point occurs in \ensuremath{\rho}(T) at ${T}_{\mathrm{inf}=2.2}$ K, below which \ensuremath{\rho} varies as ${T}^{2}$, 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 ${T}_{L}$=${T}_{K}$/${N}_{\mathrm{deg}}$, where ${N}_{\mathrm{deg}}$ is the degeneracy of the ground-state multiplet. The ground state of ${\mathrm{Ce}}_{3}$Al is antiferromagnetic with ${T}_{N}$=2.5 K. The specific-heat anomaly makes it impossible to determine \ensuremath{\gamma} but for 10<T<20 K \ensuremath{\gamma}=0.085 J/mol Ce ${\mathrm{K}}^{2}$. From \ensuremath{\chi} 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 ${\mathrm{Ce}}_{3}$Sn; it increases initially for ${\mathrm{Ce}}_{3}$In, 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 ${\mathrm{Ce}}_{3}$Al at ambient pressure, which disappears at higher pressure.