Rare Earth Elements: Properties, Uses and Occurrence

Rare earth elements are a set of seventeen indistinguishable silvery-white and luscious soft heavy metals. These are also commonly known as rare earth metals or rare earth oxides. These rare earth metals are spread across the periodic table as lanthanides. Scandium and Yttrium are also considered as part of these rare earth metals as they are found in the same ore deposits as lanthanides and exhibit similar chemical properties. But the magnetic and electronic properties of Scandium and Yttrium differ from lanthanides or rare earth metals. 

Rare Earth Elements in Periodic Table

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Rare earth elements, also commonly known as lanthanides are basically the elements present in the top extended row placed below the main body of the periodic table and the rare earth metals stretched from Cerium (Ce) to Lutetium (Lu). The other rare earth metals are situated in the third row as twenty-first and thirty-ninth metals. These are Scandium and Yttrium. They exhibit the same chemical properties as other lanthanides. The rare earth metals are generally trivalent in nature but sometimes fewer lanthanides show different valencies. For example, Cerium, Praseodymium and Terbium are tetravalent. On the other hand, samarium, europium and ytterbium are divalent in nature. All these rare earth materials are so similar in their chemical properties that 25% of their uses are the same but 75% of their usage is based on their unique properties.

Properties of Rare Earth Metals

Though rare earth materials exhibit similar properties chemically but are very different in physical aspects and possess different electronic and magnetic properties. For example, Lanthanum being the prototype of the lanthanide series exhibits a melting point of 9180C or 16840F which is much lower than the melting point of Lutetium which is the last rare earth element of the series. The melting point of lutetium is observed to be 16630C or 30250F. Such wide differences of physical properties are not found in other groups of elements. For instance, the melting point of copper, silver and gold vary by merely 1000C. Thus few of the common properties of rare earth metals are as follows:

1. The metals are either silver, silvery-white or gray in colour.

2. They have high luster but tarnish easily when they come in contact with the air.

3. The metals have high electrical conductivity.

4. They share many common properties with each other and therefore it gets very difficult to separate or distinguish them.

5. The rare earth metals naturally occur together in minerals. For example, monazite is a mixed rare earth phosphate.

6. There is very little difference in the solubility and complex formation ability of the rare earth metals.

7. Rare earth metals are found with non-metals in their third oxidation state (3+). There is very little tendency of these metals to exhibit different oxidation states. An exception to this is Europium a with valency (2+) and Cerium with a valency of 4+.

Occurrence and Abundance of Rare Earth Elements

Though the abundance of the rare earth metals is very high, their availability is very low as their percentage of concentrations in ores are extremely low, as low as 5% availability by weight. Among all 83 elements that occur naturally these 16 rare earth elements fall in the 50 percentile of natural occurrence. Among them, Promethium, a radioactive element with the most stable isotope and a half-life of 17.7 years is not considered to be naturally occurring, although some traces of the element was found in ores. 

Among all the naturally occurring rare earth elements, Cesium is the most abundant with the 28th ranking and Thulium, the least abundant with the 83rd ranking. It has been observed over the years that lighter lanthanides are in more abundance than heavy lanthanides. Though rare earth elements are distributed all over the world, the major concentrations are found in China, America, Australia and Russia. The other viable ores are found in Canada, India, South Africa and Southeast Asia. 

The major rare earth materials found in these ores are bastnasite (fluorocarbonate), monazite (phosphate), loparite [(R, Na, Sr, Ca)(Ti, Nb, Ta, Fe3+) O3] and laterite clay (SiO2, Al2O3 and Fe2O3).  As it was estimated in 2017, the total rare earth reserves are about 120 million metric tons. Most of these reserves are situated in China with about forty-four million metric tons of rare earth metal ores. America holds about 1.5 million metric tons of rare earth reserves. After China a decent amount of reserves are found in Russia, Brazil followed by Vietnam. Many of these countries use these rare earth metals for many key technologies in the medical and energy division. China and America use these metals for the manufacturing of lasers, battery electrodes, MRI contact agents, magnets, catalysts, alloys etc.