Light Rare Earth Elements (LREE)

The light rare earth elements, light-group rare earths, or LREE are a subset of the lanthanide series of the rare earth elements, which are themselves a special set of transition metals. Like other metals, the light rare earths have a shiny metallic appearance. They tend to produce colored complexes in solution, conduct heat and electricity, and form numerous compounds. None of these elements occurs in pure form naturally. Although the elements are not that "rare" in terms of element abundance, they are extremely difficult to isolate from each other. Also, the minerals that bear rare earth elements are not uniformly distributed across the globe, so the elements are uncommon in most countries and must be imported.

Elements That Are Light Rare Earth Elements

You'll see different sources site slightly different lists of elements classified as LREEs, but the US Department of Energy, US Department of the Interior, US Geological Survey, and national labs use a very specific set of criteria to assign elements to this group.

The light-group rare earth elements are based on the configuration of 4f electrons. LREEs have no paired electrons. This makes the LREE group consist of 8 elements with atomic number 57 (lanthanum, with no unpaired 4f electrons) through atomic number 64 (gadolinium, with 7 unpaired 4f electrons):

• lanthanum (La) - used in high-end optical lenses and in lanthanum nickel-metal hydride (NiMH) rechargeable batteries
• cerium (Ce) - the 25th most abundant element in the Earth's crust (so not rare at all), used in catalytic converters and the oxide as a polishing powder 
• praseodymium (Pr) - oxide is used as a catalyst in plastic manufacturing and is combined with zirconium oxide to produce a vivid yellow pigment used in ceramics
• neodymium (Nd) - used to make super-strong magnets; neodymium-iron-boron (NeFeB) magnets are used to make cell phones vibrate
• promethium (Pm) - used to make a phosphorescent pigment and to make the starter switch for fluorescent lamps
• samarium (Sm) - used in high strength magnets and to make servo-motors
• europium (Eu) - used to make phosphors, notably the reddish-orange color of screens and monitors
• gadolinium (Gd) - used in a reactor to control rods to control fission reaction and as a contrast agent to improve magnetic resonance imaging (MRI)

Uses of the LREE

All of the rare earth metals have great economic importance. There are many practical applications of the light rare earth elements, including:

• laser
• magnets
• phosphors
• luminous paints
• catalysts
• metallurgy
• superconductors
• sensors
• flat panel displays
• medical tracers
• microphones and speakers
• rechargeable batteries
• fiber optics
• numerous defense applications

The Special Case of Scandium

The element scandium is considered to be one of the rare earth elements. Although it is the lightest of the rare earths, with atomic number 21, it is not classified as a light rare earth metal. Why is this? Basically, it's because an atom of scandium does not have an electron configuration comparable to that of the light rare earths. Like other rare earths, scandium typically exists in a trivalent state, but its chemical and physical properties don't warrant grouping it with either the light rare earths or the heavy rare earths. There are no middle rare earths or other classification, so scandium is in a class by itself.