The so-called ‘rare earth elements’ (REEs) have gained an unbelievable importance in many recent technologies. The unique properties of these metals make them indispensable in various applications, for example as phosphors in screens and LEDs or as magnets in electronic devices. Although these elements are not, in a strict sense, rare, their mining is tedious and the number of sources that are suitable for mining are limited. Nevertheless, the demand for these elements is still growing. What makes the rare earth metals so unique?
The availability of REEs
The REEs group consists of the metals scandium, yttrium and the row from lanthanum to lutetium in the Periodic Table of Elements. Although not exactly rare (cerium is as common as copper), they are usually dispersed in small amounts in the Earth’s crust and not concentrated in ore deposits that are easy to mine. Unfortunately, deposits that show rare earth concentrations large enough for mining are rare.
Moreover, whether rare earth mining is viable is not only a question of metal concentrations in the deposit but also of political issues like environmental protection regulations. The latter is an especially important issue because mining rare earth elements is a severe invasion in Nature that produces a lot of excavation material and requires a huge amount of water. Thus, it is not surprising that the US reduced rare earths mining in the mid-1980s, while China is now the world leader in rare earths production. This means that the availability of rare earth metals has a strongly political component.
How to meet demand?
Despite the uncertainties in rare earths supply, the demand for these elements is still growing because they have unique properties, making their replacement by other elements impossible. Chemically, the uniqueness of the rare earth metals is based on their electronic configuration. In the Periodic Table of Elements the metals scandium, yttrium, and lanthanum belong to one group and the element lanthanum is followed by a series of 14 metals, the so-called ‘4f-elements’. All of these metals occur in Nature exclusively in the oxidation state +3. This is the reason why they chemically behave quite similarly, causing the difficulties in their separation. It is a very special situation in the periodic table that the number of electrons increases within a row of elements without severe changes in their chemical behaviour. However, the physical properties change significantly, establishing a unique situation: 17 metals favour the same oxidation state, leading to a strong chemical similarity but with different electron configurations, leading to different physical properties. Some of the physical properties are highly useful for technical applications. For example, each of the REEs shows its own characteristic absorption and emission spectrum, i.e. the interaction with visible light, is very unique. This interaction is used in energy-saving lamps, LEDs, and TV and computer screens, and there would be no mobile phone display without the special behaviour of rare earth elements.
Another important application is based on the strong magnetism of many rare earth elements. Everyone knows the small but very strong magnets to fix papers onto a metal board. These magnets are neodymium-based, and the main application of these magnets is of course not on the fridge doors in our kitchens but on permanent magnetic motors and generators; for example, each wind requires about 200kg of neodymium.
On the one hand, with respect to the unique properties of rare earth elements, they cannot be replaced by other and more easily available metals. On the other hand, there will be a growing demand for rare earth metals due to the growing need of electronic and optical rare earth-based devices. The conflict between decreasing resources and increasing demand must be solved at different stages. The search for new deposits and the exploration of their mining potential is one issue; the political discussion of fair trade regularities is another.
What role for chemists?
For chemists, two important areas of research come up in the field of rare earths: the first being the search for better, more effective compounds that allow the fabrication of devices with lower rare earth contents – requiring fundamental investigations that lead to a better understanding of the interaction of rare earths with their host lattices, and the investigation of the property-shape relation of materials (nanoparticles, thin layers) – is getting more and more important; and the second being the development of efficient recycling processes. Until now, most rare earth metals are not recycled, because the processes are economically inefficient.
Thus, chemists are encouraged to take care of the rare earth elements. There is a lot to do.
Institute of Inorganic Chemistry
Department of Chemistry
University of Cologne