Physical and Chemical Properties of the Rare Earths

Abstract This chapter discusses the chemical and physical properties of the lanthanides, some of which are in a certain way peculiar. It discusses the oxidation states of the REE, and the phenomenon called the lanthanide contraction (meaning that the atomic radius decreases with increasing atomic number in the series lanthanum-lutetium). It lists the isotopes known per element, and explains the radioactivity of promethium, the only element of the rare earths that has only radioactive isotopes and no stable isotopes. Magnetism and luminescence also are discussed. 

The rare earths are divided into the lanthanide group, and the elements scandium and yttrium. The lanthanides constitute a special group of elements that have an atomic structure different from the other elements, although they are somewhat akin to the actinide series. The lanthanides make up a group of elements ranging from atomic number 57 (lanthanum) to 71 (lutetium). They are all very similar to lanthanum, which is the reason for the name lanthanide. The other two rare earths, scandium and yttrium, are somewhat apart from the lanthanide series, and will be treated separately. 

Voncken, The Rare Earth Elements, SpringtaBriefs in Earth Sciences, DOI 10.1007/978-3-319-26809-5 3 

The similarity may be explained by the electronic configuration of the atoms. This is discussed later. 

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Lanthanides properties and general references. For a systematic treatment and general references of the physical and chemical properties of the rare earths and their compounds and alloys mention can be made to a periodical publication in which several contributions to these subjects are being collected. See for instance Gschneidner and Eyring (1978) and Gschneidner etal. (2005). We would also like to quote a sentence, included in the prefaces of all these books, which hints at the complexity and richness of the rare earth behaviour and the ever-increasing interest in their properties and applications. The mentioned sentence is as follows ... 

The elements with atomic numbers from 57 (l thanum) to 71 (lutetium) are referred to as the lanthanide elements. These elements and two others, scandium and yttrium, exhibit chemical and physical properties very similar to lanthanum. They are known as the rare earth elements or rare earths (RE). Such similarity of the RE elements is due to the configuration of their outer electron shells. It is well known that the chemical and physical properties of an element depend primarily on the structure of its outermost electron shells. For RE elements with increasing atomic number, the first electron orbit beyond the closed [Xe] shell (65 remains essentially in place while electrons are added to the inner 4f orbital. Such disposition of electrons about the nucleus of the rare earth atoms is responsible for the small effect an atomic number increase from 57 to 71 has on the physical and chemical properties of the rare earths. Their assignment to the 4f orbital leads to slow contraction of rare earth size with increasing atomic number. The 4f orbitals of both europium and gadolinium are half occupied [Xe] (4F6s and [Xe] (4F5d 6s, so that there... 

In the first half of this book, chemical stability, reactivity, structural features, and chemical bonding including band calculation of the rare earth oxides, have been examined from the viewpoints of the fundamental characterization and appearance mechanism of the properties. Particularly, further development of high resolution electron microscopy (HREM) and quantum band calculation will be of great aid for us to understand the unique characteristics of binary rare earth oxides from both the experimental and theoretical approaches. In addition, physical and chemical properties of the rare earth oxides such as electrical, magnetic, optical, and diffusion properties are also analyzed in details, leading to find relationships between basic science and applications in several functional materials. 

The elements scandium and yttrium, which are also considered to belong to the rare earth elements (because of their similar chemical behaviour) also have a 3+ oxidation state. The atomic stmcture of the REE is further discussed in Chap. 3 (Physical and Chemical Properties of the Rare Earths). 

Structural, physical and chemical properties of bulk rare earth oxides can be found in Chapter 27, Volume 3 of this series and have been compiled with a view towards catalysis in a recent review by Rosynek (1977). An important parameter for the catalytic behavior of rare earth oxides is their basicity. The basicities of rare earth oxides resemble those of the alkaline-earth oxides, and scale directly with the respective cation radii. Thus, La203 shows the strongest basicity and SC2O3 the weakest, with sesquioxide basicities decreasing smoothly along the lanthanide series going from La to Lu. This periodic trend allows one to study the influence of subtle variations in basicity on catalytic behavior in a class of related materials with similar electronic and geometrical structure. 

Solid state physical and chemical properties of binary rare earth compounds involving non-metallic elements have attracted considerable scientific interest in recent years (see volumes 3 and 4 of this series). Some of the compounds exhibit... 

Nickel is a well-known catalyst component and is thought to play an important role in the transformation of the hydrogen of steam to hydrocarbons. Hence, a part of the rare earth metal in REY is exchanged with Ni to become prepared Ni and the rare earth metal-exchanged Y-type zeolite catalyst (Ni-REY) [14, 15]. The physical and chemical properties of the catalysts are listed in Table 6.8. The polyethylene plastics-derived heavy oil shown in Table 6.2 was used as the feed oil. 

Electronic Structures. Almost all the physical properties and chemical behavior of the rare earth elements find a logical explanation in terms of their electronic structures. Scandium, yttrium, lanthanum, and actinium are the first members, respectively, of the first, second, third, and fourth transition sequences of elements. In other words, each such element marks the beginning of an inner building where a stable group of 8 electrons is expanding to a completed (or more nearly complete) group of IS. This situation is illustrated for the first transition sequence. 

This work includes a number of the more important physical and chemical properties of the nine transition elements included in Groups 4, 5, and 6 of the 4th, 5th, and 6th periods. The three actinide systems, Th-C, U-C, and Pu-C, have also been discussed. While this limited selection does not include all of the high melting carbides (many Group 3 and rare earth systems fall into this category), it does include the more refractory and the more useful ones. Besides, only for these systems has sufficient information been generated to make a critical review worthwhile. 

Einsteinium has homologous chemical and physical properties of the rare-earth holmium (g Ho), located just above it in the lanthanide series in the periodic table. 

The pronoimced similarity between the chemical and physical properties of the rare-earth elements made their isolation a difficult task. The traditional methods of chemical analysis were to no avail and chemists of... 

Despite Brauner s belief in the validity of the Mendeleev methodology, he also had to admit that he had not yet succeeded in resolving the rare-earth crisis. Thus Brauner wrote in 1901 with reference to praseodymium that its maximum valency was tetravalent, like that of cerium but that no place had been found in the periodic table for an element possessing the physical and chemical properties of praseodymium and its compounds (Brauner, 1901b). He also admitted that the difficulties of finding a place for neodymium in the periodic table were even greater than in the case of praseodymium. 

According to Bohr s opinion, the rare-earth group consisted of elements where the four-quantum level was gradually filled up from 18 to 32 electrons. The number of electrons in the five- and six-quantum levels on the other hand remained imchanged. Bohr s quantum theory thus served as a useful explanation for the pronoimced similarity between the chemical and physical properties of the rare-earth elements. He mentioned that their mutual similarity must be ascribed to the fact that we have here to do with the development of an electron group that lies deeper in the atom. He moreover emphasized that lutetium (Z = 71) had to be considered the last rare-earth element. Element 72 on the other hand did not belong to... 

We anticipated the need for other volumes to follow the first four, both to cover topics already mature but not included and others whose time had not yet come. We underestimated both aspects of this need. There is a time for a review of past work when the review itself promotes understanding and stimulates new lines of research. In response to this the publisher will continue the series to assure an evolving authoritative and comprehensive Handbook on the Physics and Chemistry of Rare Earths. Volume 5 contains new topics concerning thin films of alloys and compounds, transport properties of intermetallic compounds, catalytic behavior of metallic and non-metallic surfaces, defects and transformations in oxides, physical and chemical properties of fluorides, and spectroscopic properties of lanthanide ions in a variety of ionic hosts. 

To better imderstand the physical nature of the rare earth perovskite materials availability of detailed information about their crystal structures is the key. The above-mentioned physical and chemical properties of rare earth aluminates... 

The Rare Earths Fundamentals and Applications provides the knowledge of fundamental REE chemistry necessary to understand how the elements are currently being used and how they might be used in the future. The book is organized to provide a comprehensive description of the breadth of REE chemistry in four sequential sections fundamental chemistry (Chapters 1 12), important representative compounds (Chapters 13-30), examples of solid-state materials (Chapters 31-36), and current and potential new applications (Chapters 37-45). It is designed to provide students, instmctors, academic researchers, and industrial personnel with a fundamental understanding of the electronic, chemical, and physical properties of the rare earth... 

Therefore, our purpose is to demonstrate that the physical and chemical properties of rare earth metallic and rare earth alloy thin films or single-crystals must be cautiously analyzed. Particularly, the lack of crystallographic spectra (X-ray or electron diffraction), of chemical analyses (absorbed or adsorbed gases, surface contamination, impurities,...), of structural investigations (grain size, defects,. ..) for example, is truly detrimental to precise characterization of the materials. In this way one can claim that numerous... 

Gas-phase chemistry studies of atomic and molecular rare-earth and actinide ions have a deep-rooted history of more than three decades. In gas phase, physical and chemical properties of elementary and molecular species can be studied in absence of external perturbations. Due to the relative simplicity of gas-phase systems compared to condensed-phase systems, solutions or solids, it is possible to probe in detail the relationships between electronic structure, reactivity, and energetics. Most of this research involves the use of a variety of mass spectrometry techniques, which allows one exerting precise control over reactants and products. Many new rare earth and actinide molecular and cluster species have been identified that have expanded knowledge of the basic chemistry of these elements and provided clues for understanding condensed-phase processes. Key thermodynamic parameters have been obtained for numerous atomic and molecular ions. Such fundamental physicochemical studies have provided opportunities for the refinement and validation of computational methods as applied to the particularly challenging lanthanide and actinide elements. Among other applications, the roles of... 

Lanthanum is the fourth most abundant of the rare-earths found on the Earth. Its abundance is 18 ppm of the Earth s crust, making it the 29th most abundant element on Earth. Its abundance is about equal to the abundance of zinc, lead, and nickel, so it is not really rare. Because the chemical and physical properties of the elements of the lanthanide series are so similar, they are quite difficult to separate. Therefore, some of them are often used together as an alloy or in compounds. 

But matters do not always proceed in so orderly a manner. Some elements have an electron in an outer shell when an inner shell is not completely full. This is what happened in the case of the rare earths. They all had the same number of electrons in the outermost shell, giving them similar physical and chemical properties. But except for the last rare earth there were gaps in the inner shells. 

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