These metallic elements — yttrium, scandium and the lanthanides — now have a wide range of uses, including strong permanent magnets (from Nd, Pr and Sm), superalloys, phosphors and ceramics; yttrium is the key to the most commonly used high-temperature superconducting copper oxide. But the rare earths remain in relatively short supply throughout the world, the primary sources being in China (38%), Vietnam, Brazil and Russia.

That combination of utility and uncertainty of supply was what led to the rare earths being classified as “critical” metals in a 2006 report by the US National Research Council¹. Graedel et al. subsequently showed that several of the rare earths, including La, Eu, Dy, Tm and Yb, lack any good substitutes in their major areas of application, and that most have some degree of vulnerability in this regard². As a result, they concluded, economic and population growth mean that “scientists will be increasingly challenged to maintain and improve product utility by designing new and better materials, but doing so under potential constraints in resource availability.”