Specific Heat of Lanthanum, Praseodymium, Neodymium, and Samarium Metals between 3 and 25°K

Abstract

Results of specific-heat measurements are reported for lanthanum, praseodymium, neodymium, and samarium metals in the temperature range between 3 and 25°K. The observed anomalies in the specific heat can generally be related to those found in several other physical properties of the metals. Analysis of the total specific heat $C_p$ into its component parts is based on the assumptions that for each metal the electronic term is $C_E=10.5T$ mJ/mole °K and that the lattice contribution is the same as for nonmagnetic lanthanum. The magnetic specific heat can thus be determined and is investigated in terms of the magnetic behavior of the various metals. Lanthanum displays anomalies at $T_{\alpha }=4.88\mathrm{}$ °K and $T_{\beta }=5.87\mathrm{}$ °K which are associated with superconducting transitions in the hexagonal and fcc phases of the sample. The Debye $\Theta$ below 4°K is 142°K. Praseodymium has a small hump in $C_p$ near 3.2°K, the cause of which is not yet clear. Modifications of existing theoretical models for crystalline-field splitting of the ${\mathrm{Pr}}^{3+}$ ion ground state are suggested. For neodymium the two $\lambda$-type peaks at 7.43 and 19.55°K are discussed in relation to Schottky curves for split ground states of ions in hexagonal and cubic sites. The introduction of cooperative effects and exchange interactions would allow the Schottky anomalies to reform into the observed shapes. Samarium shows a small sharp peak in $C_p$ at 9.57°K, the existence of which had not previously been established. This metal also has a large anomaly at 13.3°K associated with antiferromagnetic rearrangement of the ions; the magnetic entropy up to 25°K is very close to the value $\frac{1}{2}R \ln 2$.