CHIMIE NOUVELLE N° 133 - mars 2020 1 Simon COTTON rares School of Chemistry, University of Birmingham, Edgbaston, Birmingham B15 2TT, United Kingdom - [email protected] Terres Two centuries Two centuries of the rare earths of the rare earths Abstract Back in 2017, the United Nations designated 2019 The rare earths are important in the history of as the International Year of the Periodic Table the Periodic Table, not least because no one knew of the Chemical Elements. Dmitri Mendeleev how many there were. Their separation was also first presented his periodic table in 1869. The a challenge, owing to the similarity of properties elements were arranged according to their atomic of neighbouring elements, a manifestation of the mass, with certain additional modifications lanthanide contraction. After initial uncertainty, which distinguished it from previous tables. For it was realised that these elements exhibited instance, some gaps were left blank for elements predominantly the +3 oxidation state, but over that remained to be discovered (e.g., Sc, Ge, Ga). the past century compounds of a few elements in the +2 state have been synthesised, and with recent breakthroughs all lanthanides have been 1. Discovery forced into the +2 state, with the right choice of ligand. Certain elements, notably cerium, exhibit The first five rare earth elements were known the +4 oxidation state in their chemistry. Studying when Mendeleev proposed his first periodic the structures of compounds of the elements table, starting with Carl Axel Arrhenius’s has been very rewarding, with a wide range of discovery of a black stone at a mine near Ytterby coordination numbers from 2 to 12. Among these in 1787, consequently named ytterbite. In 1794 compounds, diketonate complexes have had Johan Gadolin went further with the isolation many applications, from lanthanide shift reagents of the oxide of a new element which he named to organic light-emitting diodes. yttrium. Later on, this small village gave its Applications bring lanthanides into all homes, name to other three elements, namely erbium, whether with batteries, lighting, computer hard terbium, and ytterbium. Together with the much drives, fibre optics or wind turbines. Since the lighter scandium, yttrium and the lanthanides are discovery of yttrium, the first rare earth, over often referred to collectively as the rare earths. two centuries have passed; now, with the latest This is a double misnomer, as ‘earths’ are strictly discovery that lanthanides have a role in enzymes, the oxides of elements, and they are certainly we can say that these elements have come into not rare (Figure 1). Although not as abundant their own. as lighter elements, they are more common than well-known elements such as platinum metals, Keywords mercury, silver, and gold. Yttrium is considered Rare earth; lanthanides; Periodic Table. as a lanthanide element due to its similarities, in both atomic size and chemistry, with heavier 2 CHIMIE NOUVELLE N° 133 - mars 2020 lanthanides, most notably holmium. The reduced earlier lanthanides, espcially La and Ce; in size and mass of scandium involve important contrast, xenotime (Y, La)PO4, is richer in later differences in its chemistry. High-abundance lanthanides (e.g. Gd-Lu) and strikingly more so ores containing a specific rare earth element are in yttrium. The other very important source of missing, and therefore, the separation of mixtures these elements is a type only found in southern of similar elements into distinct species is an China, the ‘ion-absorption ores’. These are important part of the extraction process. formed by slow weathering of lanthanide- containing igneous rocks (e.g. granite) then the The term ‘lanthanide’ derives from the similarity Ln3+ ions are adsorbed by kaolinic clays. These of this series of elements to the first member, ores have a low abundance of rare earths (ca. lanthanum, which itself gets its name from 0.1%) but are very abundant and are also easy the Greek word lanqanein (lanthanein), ‘to be to mine. Their composition varies from place to hidden’, as it was first discovered as an impurity place; they generally contain significant amounts in cerium. of the middle lanthanides (e.g. Gd to Er) and also yttrium, but sometimes the lanthanum content is significant. Although their content of heavy 2. Rare earth ores lanthanides like holmium, thulium and lutetium is low, these ores have become the main source of Looking first at their overall global abundance, the heavier lanthanides in particular because of within the lanthanide series the lighter lanthanides the sheer scale of these deposits, [1, 2]. are more abundant than the heavier ones; secondly, that the elements with even atomic At the time that Mendeleev created his first number are more abundant than those with odd Periodic Table, only five of the rare earths were atomic number. known – yttrium, lanthanum, cerium, erbium and terbium. It was not until the later part of the th Among the principal ores, bastnasite, LnFCO3, 19 century that improved separation techniques, and monazite, (Ln,Th)PO4, are both richer in along with spectroscopic analysis, allowed the Figure 1. Abundances of the chemical elements. Source: http://upload.wikimedia.org/wikipedia/commons/0/09/Elemental_abundances.svg [from WikiCommons] Two centuries of the rare earths 3 discovery and isolation of all but one of the 1869, 12, 405-406 and https://web.lemoyne.edu/ elements by 1907 (Lutetium). giunta/EA/MENDELEEVann.HTML Initially, Mendeleev faced some problems in positioning these elements [3], as he assumed By 1871 he had changed his ideas [4], taking the that like many metals they typically had oxidation formulae of the oxides to be M2O3, leading to states of +2 in their compounds, leading to atomic masses in line with today’s values, though incorrect atomic masses (Fig. 2). he appeared to believe that lanthanum had an oxidation state of (+4). Whilst in the latter part of the 19th century new lanthanides were rapidly being discovered, no one knew how many there were until H. G. J. Moseley (1887-1915) and his study of X-ray spectra of the elements introduced the concept of atomic number. Moseley’s Law showed that the square root of the frequency of the emitted X-ray is proportional to the atomic number, and revealed that the only rare earth remaining to be discovered was element 61. [5] Several claims for this element were made by researchers, notably in 1926 when “Illinium” and “Florentium” were simultaneously reported by American and Italian researchers. [6] None of these claims could be supported by other researchers. Figure 2. Mendeleev’s paper (1869) entitled “On the Relationship of the Properties of the Elements to their Atomic Weights”. Lines in spectra claimed to be from element 61 were found to be due to other elements, present as ‘One orders the elements according to increasing impurities. atomic weight in vertical rows so that the horizontal rows contain analogous elements, still It was realised that Element 61 would be ordered by increasing atomic weight, one obtains radioactive and too short-lived to be found on the following arrangement, from which a few earth, and it was finally recognised in 1946 among general conclusions may be derived.’ the fission products of uranium. [7] In points 1-3 he says: - 1. If arranged according to their atomic weights, the elements show an evident gradual variation of properties. 2. Chemically 3. Oxidation states and separating the elements analogous elements have either similar atomic weights (Pt, Ir, Os), or weights which increase by All the lanthanides form stable Ln3+ ions in equal increments (K, Rb, Cs). 3. The arrangement aqueous solution, and since there is very little according to atomic weight corresponds to significant aqueous chemistry in other states, this the valency of the element and to a certain extent affects separating and purifying the individual the difference in chemical behaviour, for example elements. Neighbouring elements have very Li, Be, B, C, N, O, F. He lists (in order of increasing similarly sized ions, so that their compounds mass) erbium, yttrium, cerium, lanthanum and have similar solubilities. Initially, however, didymium. Note that Di refers to ‘didymium’, fractional crystallisation, which takes advantage the mixture of Pr, Nd and Sm which was at that of small differences in solubility of compounds time considered to be an element. Yt was then of neighbouring metals, was employed, though the symbol for yttrium; a space has been left for multiple stages were necessary, most famously in an element of atomic mass 45, now known to be the case of thulium, where 15 000 operations were scandium. D. Mendeleev, Zeitschrift für Chemie, necessary to remove the last traces of erbium.[8] 4 CHIMIE NOUVELLE N° 133 - mars 2020 Lanthanides are prominent among the fission 4. The lanthanide contraction products of uranium, so that during the Manhattan project new separation methods were developed, These separations work because of the lanthanide notably cation-exchange chromatography, where contraction (a term coined in 1925 by the the elements are eluted from ion-exchange Norwegian mineralogist Victor Goldschmidt), the resins. Initially complexing agents like citrate decreasing size of the lanthanide atoms and ions or a-hydroxy-isobutyrate were employed, with increasing atomic number [11]. This accounts but subsequently the complexes of EDTA for subtle and progressive changes often observed (Ethylenediaminetetraacetate) were found to in properties of lanthanide compounds, though have much higher stability constants. [9] the ionic radii are the most obvious manifestation. This impinges on bond lengths in lanthanide Such separations work very well on the small compounds, so that in a series of isostructural scale to produce high-purity samples of individual compounds with the same coordination number, lanthanides, but on the industrial scale solvent the lanthanide – bond length decreases steadily extraction is the preferred technique.