Rare Earth Chemistry

Rainer Pöttgen, Thomas Jüstel, Cristian A. Strassert

• Publisher: De Gruyter
• Year: 2020
• ISBN:
  9783110653724
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Inorganic Chemistry

This work introduces into the chemistry, materials science and technology of Rare Earth Elements. The chapters by experienced lecturers describe comprehensively the recent studies of their characteristics, properties and applications in functional materials. Due to the broad range of covered topics as hydrogen storage materials, LEDs or permanent magnets this work gives an up-to-date presentation of this fascinating research.

1 The elements

1.1 Discovery of the rare-earth elements

Jean-Claude G. Bünzli

1.1.1 The protagonists

The rare earths (REs) are a homogeneous group of 17 elements. According to the IUPAC nomenclature rules, they correspond to the elements 21 (Sc), 39 (Y) and 57–71 (Ln = La-Lu). The Ln subgroup should be called “lanthanoids” but “lanthanides” (corresponding in principle to Ce-Lu) is still the most used designation for these metallic elements and their compounds. In the long form of the periodic table, Ln elements are inserted between Ba (56) and Hf (72) with Sc, Y and Lu, forming a column to the left of Ti, Zr and Hf (Figure 1.1.1).

Figure 1.1.1: Rare-earth elements with the two generic minerals, cerite (left) and gadolinite (right) and the reproduction of a photograph of Johan Gadolin (middle, © 1910, Acta Societatis Scientarium Fennicae). Element 61, first artificially produced, is only found as minute traces in some uranium ores.

When it comes to subdividing the lanthanides, chemists rely on the electronic structure (see Section 1.6) of the most common oxidation state, the trivalent ions Ln^III, [Xe]4fⁿ: the lighter lanthanides (LREs) are those which have no paired 4f electrons (n = 0–7, La-Gd), while the heavier lanthanides (HREs) correspond to n = 8–14, Dy-Lu. Geochemists use different definitions, excluding Eu which has “anomalous” properties as compared to the LREs, leaving it alone in a special group. In metallurgy and industry, LREs correspond to La-Nd (also called ceric REs), middle REs (MREs) to either Sm-Gd or Sm-Dy, and HREs to Dy-Lu or Ho-Lu; finally yttric REs are those from Sm to Lu, plus Y. Fortunately, everybody agrees with the nonlanthanoid elements: yttrium has chemical properties very similar to Dy-Ho, so it is included in HREs, while scandium has geochemical and chemical behaviors so different from of all the other REs that it is not listed in any of these groups.

1.1.2 The search for REs: an intricate problem

The discovery of RE elements has been an extraordinary endeavor spanning over a century, from 1794 (Y) to 1907 (Lu) for the naturally occurring elements and extending to 1945 for the radioactive Pm. This quest has been a tremendous success for crystal and analytical chemistry at first and, later, in the mid-1850s, for spectroscopy, since pure elements had to be separated from intricate mixtures: indeed, due to the similar chemical properties of REs (with the exception of Sc), RE minerals contain several of these elements, usually almost the entire LRE or HRE series. It is also a tortuous story reflecting both on the little amount of information available at that time on chemical bonding and element systematics, as well as on the unsophisticated methods of analysis available. Different compositions of inhomogeneous minerals, depending on where they were found, increased the difficulty. To this should be added the hefty competition and rivalry between scientists, mirroring the race of nations for gaining supremacy in science and technology. The thrilling story of the isolation of RE elements is, therefore, full of incorrect claims and heated disputes among would-be discoverers, whilst reflecting the developments in separation, analytical, and spectroscopic techniques which took place during the nineteenth century [1, 2].

Before starting the description of RE discovery, a note on the concept of element discovery may be useful. In principle, the discoverer should prepare the new element in its elemental form, that is metallic form for REs. But these elements are highly electropositive and their metallic form is difficult to obtain unless special reduction techniques are used, that were not always available at the time when the new RE elements were separated and identified. In particular, electrochemical methods only started to be mastered with Sir Humphry Davy’s (1778–1829) experiments in the first decade of the nineteenth century. As a consequence, the discovery of a new RE was often attributed to the scientist who was able to produce a pure oxide of the element. These oxides are termed “earths” and their denomination takes the ending “a” instead of “um” for the element: yttria for yttrium oxide, terbia for terbium oxide and so on. Moreover, it is also noteworthy that element names were sometimes changed or interchanged, for instance, erbium and terbium, adding to the confusion. The denomination “rare earths” is not quite correct since the abundance of some RE elements is larger than several other technological metals such as silver or indium and comparable to that of lead. RE resources are also found in many places around the world in Europe, Asia, Australia, and North and South Americas. The term “rare earths” stems from the fact that while the RE content in specific minerals is relatively large (35% to 75%), these minerals are rather dispe...

Table of contents

• Title Page
• Copyright
• Contents
• 1 The elements
• 2 Reactivity and compounds
• 3 Characterization and properties
• 4 Materials and applications
• Subject Index
• Formula Index