Specific Heat of Europium and Ytterbium Metals between 3 and 25°K

Abstract

The specific heats of europium and ytterbium metals have been measured, using a germanium resistance thermometer, between 3 and 25°K. For europium a sharp peak was found at 16.1°K, indicating that a co-operative transformation takes place in the sample at this temperature. The entropy under the peak is so large that the anomaly must be caused by the bulk specimen and not by impurities. The origin of this transition, which has not been observed before, is not clear, but various possibilities are discussed on the basis of available experimental information. According to our earlier measurements, the electronic specific heat of europium is $C_E=5.8T$ mJ/mole °K ($T$ in °K); the lattice and magnetic contributions, $C_L$ and $C_M$, respectively, could not be separated. It is clear, however, that $C_M$ is large, particularly at low temperatures. For ytterbium, which is nonmagnetic, there are only two terms in $C_p$, namely $C_L$ and $C_E$. Our earlier measurements below 4°K gave $C_E=2.90T$ mJ/mole °K, and by employing this result $C_L$ was calculated. The Debye temperature $\Theta$ decreases from an initial value of 118°K below 3°K to a minimum of 109°K at 14°K and then rises again slowly. The $\Theta$-versus-$T$ curve is rather similar to that observed for many other metals.