Rare earth elements (REEs) hold a significant status, being part of the list of 35 minerals recognized as critical to the economic and national security of the United States, a designation first published by the U.S. Department of the Interior on May 18, 2018. REEs encompass the lanthanide series, which includes lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium, along with scandium and yttrium. These elements are categorized as "light" and "heavy" based on their atomic numbers, with light REEs (LREEs) encompassing lanthanum through gadolinium (atomic numbers 57 through 64). Heavy REEs (HREEs) consist of terbium through lutetium (atomic numbers 65 through 71) and yttrium (atomic number 39), which shares similar chemical and physical characteristics with the HREEs. Notably, neodymium and praseodymium are essential materials in the production of magnets known for their exceptional magnetic strength, contributing to high energy density and efficiency in a wide range of applications. Dysprosium and terbium are also critical elements often added to magnet alloys to extend their operating temperature ranges. HREEs are typically less abundant and more costly than LREEs.
