The atomic heats of the rare-earth elements

Abstract The atomic heats of lanthanum, cerium, praseodymium and neodymium have been measured from 2 to 180° K. Lanthanum shows an anomaly corresponding to superconductivity at 4⋅37° K, and the atomic heat (CD) rises to 6⋅2 cal./g. atom at 180° K. The free electronic specific heat deduced from the low-temperature results appears to explain this high value satisfactorily. Cerium, praseodymium and neodymium all behave anomalously. A specimen of cerium of face-centred cubic structure shows an anomaly between 90 and 180° K which exhibits large hysteresis effects. Taking into account the results of other research workers which have been published since this work was begun, this anomaly appears to correspond to the transition of the 4f electron to a 5d state. A second specimen of cerium in which both face-centred cubic and hexagonal close-packed structures were present did not show this anomaly. Both specimens, however, showed large anomalous humps in the low-temperature region at approximately 12° K. Praseodymium shows a very large distributed anomaly which produced a maximum in the atomic heat curve at 65° K. Neodymium shows two anomalous peaks, one at 7⋅5° K and one at 19° K. These anomalies in praseodymium and neodymium, together with the low-temperature anomaly in cerium, can be explained qualitatively by the view that the electronic states attributable to the 4f.electrons are split by the electric fields existing within the metallic crystals. This effect is more complicated than with the magnetically dilute hydrated rare-earth salts, as magnetic interaction is probably very important.