Specific Heat of Cerium and Europium Metals between 0.4 and 4°K

The specific heats of cerium and europium metals have been measured between 0.4 and 4^∘{}K in a He³ cryostate. For cerium, two experiments were made with samples consisting of 62 and 39% of the α phase (fcc), the balance being β cerium (hex). Due to a pronounced peak at 12.5^∘{}K the specific heat is quite large already at 4^∘{}K. A previously unknown flat anomaly in Cₚ was found around 0.8^∘{}K. By combining the present measurements with those of Parkinson and Roberts (3% of α cerium) and by assuming CL=0.612T³ and CE=10.5T (specific heats always given in mJ/mole^∘{}K) for the lattice and electronic specific heats of the trivalent β phase an analysis gave Cₚ=x(0.50T³+21T)+(1-x)(0.612T³+10.5T+CM). Here x is the fraction of α cerium in the sample and CM is the magnetic specific heat of the β phase. CE is thus about twice as large for the quadrivalent α phase as for β cerium. In the former modification CM=0 because there is no magnetic $4f$ electron. Since all of the stable isotopes of cerium are even-even nuclei the nuclear specific heat CN=0.Below 1.2^∘{}K the specific heat of europium can be represented with 0.8% maximum deviation from experimental points by Cₚ=27.96T³+6.18T+2.333T^-2. Above 1.2^∘{}K the temperature dependence of CM gradually becomes less than T³ and the results no longer can be expressed as a simple power law. This points to an anomaly in CM, centered probably somewhere between 4 and 10^∘{}K. As averages of two least-squares analyses we adopt CE=(5.8±1.0)T, CN=(2.36±0.07)T^-2. CE is thus in between the values ~10.5T and $2.9T$ observed for many trivalent lanthanides and divalent ytterbium, respectively. Europium metal appears to be divalent and the ground state of Eu²⁺ ions is ⁸S7/2. Since the orbital angular momentum of the $4f$ electrons is zero in an S state, CN is rather small. The effective magnetic field at europium nuclei becomes Heff=260±4 kG in excellent agreement with Heff=264±8 kG deduced recently by Barrett and Shirley from M\"ossbauer measurements. Another interpretation of the experimental results, suggesting that europium at low temperatures might be a mixture of di- and trivalent phases, is also briefly discussed.