Looking at the rare earth elements

Introduction

President Trump said in reference to a proposed minerals deal with Ukraine:

Over the past few decades, the technological revolution has expanded the applications of the rare earth elements (REEs) from modern electronics to renewable energy sources. Despite the name, the REEs are relatively abundant in the Earth's crust, but their perceived scarcity is centred around difficulties in extracting and processing. As REE refining is currently monopolised by China, access to these materials is a constant source of geopolitical tension.

The REEs comprise the lanthanide series as well as scandium (Sc) and yttrium (Y), and are characterised by the similarity of their chemical properties. Therefore, this article aims to introduce some of the fundamental chemistry of the rare earth elements to contextualise their role in modern technologies.

Chemical properties of the REEs

Scandium and yttrium are considered “honorary lanthanides,” as they form highly ionic, charge‑dense +3 cations when ionised. However, as they are transition metals, their properties cannot be explained by considering the f‑orbitals.

The f‑orbitals are a set of seven orbitals which can hold a maximum of 14 electrons. For the lanthanides, each element has a set of 4f and 6s valence orbitals, with cerium (Ce),lanthanum (La), gadolinium (Gd), and lutetium (Lu) also having an occupied 5d¹ orbital.

The 4f orbitals are generally contracted because of the nuclear charge felt by the electrons in these orbitals. As the atomic radius across the period decreases, this contraction is felt more strongly, meaning the resulting ions become more charge‑dense. This phenomenon is known as the lanthanide contraction.

The contracted nature of the 4f orbitals explains why the lanthanides preferentially adopt a +3 oxidation state (O.S). The 4f electrons are strongly attracted to the nucleus, making them energetically unfavourable to remove. Therefore, once the two 6s electrons and one 4f (or sometimes 5d) electron are removed, further ionisation becomes much more difficult. This is reflected by the ionisation potentials of the lanthanides (Figure 1).

However, some lanthanides can form stable +2 O.S (samarium (Sm), europium (Eu), and ytterbium (Yb)), while Ce can form a +4 O.S (Figure 2). This is because of the electronic configurations of these elements. For example, Eu has an electronic configuration of [Xe] 4f⁷ 6s²; therefore, by removing two electrons, the ion becomes exchange‑energy stabilised (Eu²⁺ [Xe] 4f⁷).

Another notable property of the lanthanides is their large magnetic moments. This again is a consequence of the 4f orbitals. Magnetism is determined by the number of unpaired electrons an element has and its orbital angular momentum. Orbital angular momentum is an intrinsic property and becomes more prevalent with larger elements. Therefore, as the 4f orbitals can hold up to seven unpaired electrons, coupled with the intrinsic heaviness of the lanthanides, they often exhibit strong magnetic behaviour.

Applications

1. Catalytic Converters

As previously mentioned, most lanthanides preferentially adopt a +3 O.S, Ce being a key exception due to its ability to cycle between +3 and +4. This property makes Ce particularly valuable in catalytic converters — vehicle exhaust devices which help reduce emissions of toxic pollutants such as carbon monoxide (CO) and nitric oxide (NO). Using CeO₂ as a catalyst, CO₂ and N₂ are generated as less harmful by‑products (Figure 2).

2. Chemical Reagents

The redox flexibility of certain lanthanides is also exploited in organic chemistry. Ce(IV) and Sm(II) compounds serve as effective oxidising and reducing agents respectively. Reagents such as ceric ammonium nitrate (CAN) and cerium ammonium sulphate (CAS) are frequently used as selective oxidants, while samarium bromide (SmBr₂) is an effective reductant.

3. MRI & Chiral Shift Reagents

The magnetic properties of the lanthanides can be exploited in medical imaging, particularly in magnetic resonance imaging (MRI). Prior to an MRI scan, patients may be injected with a gadolinium (Gd³⁺) complex, such as [Gd(DTPA)]²⁻ (Figure 4), to enhance image contrast. By coordinating water molecules and increasing the proton relaxation rate, these complexes cause certain regions of tissue to appear brighter and more easily distinguishable.

Chemically, this principle is utilised when NMR spectroscopy is conducted in the laboratory. Fundamentally, MRI and NMR machines work in the same way, so by adding small quantities of paramagnetic lanthanide reagents to a proton NMR sample, changes in the chemical shift can be induced.

These “lanthanide shift reagents” increase the proton relaxation rate, which reduces signal overlap and allows specific proton environments to be more easily identified. Commonly used lanthanide reagents include Eu³⁺ and Pr³⁺ complexes.

Conclusion

In conclusion, the advent of recent technology has driven a surge in the use of the REEs. While chemically similar, each element has a broad range of diverse applications, whether as magnets, reagents, or even phosphors in TV sets. Certain to dominate geopolitics for the foreseeable future, understanding the chemistry and applications of the REEs has never been more important.

Written by Antony Lee