Chapter 2 Preparation and basic properties of the rare earth metals

Abstract

Summary For many years scientists have considered the rare earth elements as a group of nearly identical elements, and if something was known about one of them it was generally assumed to be the same for all of them. As rare earth scientists continue to work on these metals more exceptions are found each day. Indeed the differences in many cases are greater than those found for elements which lie in the same group, e.g. the alkaline earth metals, or the noble metals - Cu, Ag and Au. The differences due to the divalency of Eu and Yb as compared to the normal trivalent rare earth metals are the most obvious in consideration not only of their physical properties, but also in the method of preparing the pure metals. For the trivalent rare earths there are four major methods used to prepare these 14 metals (sections 1.1.1 to 1.1.3 and 1.2), and within each group there are minor but significant differences. A comparison of the basic physical properties shows significant and widely varying behaviors in their crystal structures (fcc→dhcp→Sm-type→hcp), melting points, (a difference of a factor of two between Ce and Lu), boiling points (the “saw-tooth” characteristic so vividly displayed in fig. 2.10), and vapor pressures (a difference of a factor of one million in the vapor pressures of La and Tm at 1300°C). Even the smooth, regular and systematic variation as implied in the term “lanthanide contraction” is not found in the metallic radii which exhibit anomalies at La, Gd and Lu, and between the light and heavy lanthanides (fig. 2.7a). Although, as noted, wide differences exist in the properties between certain rare earth elements, the systematic variation from one neighboring rare earth to the next (especially for the lanthanides) does occur, and allows one to predict properties which have not been determined experimentally. In making these predictions one should be aware that anomalies can occur and exercise caution. In so doing the application of systematics to rare earth materials can be a powerful tool for understanding their nature and predicting their behaviors in new surroundings.