Rare earths refer to lanthanoid in periodic table of chemical elements and other two elements that is closely related to fifteen elements of lanthanides. They include La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Sc and Y.
Based on the electron structure and physicochemical property of rare earths and symbiosis situation in minerals and different property of ionic radius, the seventeen rare earths are divided into two groups.
Light Rare Earths ( or Cerium Group): Lanthanum, Cerium, Praseodymium, Neodymium, Promethium, Samarium, Europium, Gadolinium.
Heavy Rare Earths( Or Yttrium Group): Terbium, Dysprosium, Holmium, Erbium, Thulium, Ytterbium, Lutetium, Scandium, Yttrium.
Why they are called cerium group or yttrium group is cerium or yttrium take a big part in rare earth mixtures that separate from ores.
Rare earths are typical metal elements and their metal activity is just lower than alkalis and alkali metals, but higher than other metal elements. Rare earth elements can form chemically stable oxides, holides and sulfides. They can react with nitrogen, hydrogen, carbon and phosphorus, which dissolve in hydrochloric acid, sulfuric acid and nitric acid easily.
Rare earths compound with oxygen, sulfur and lead to form compounds of high melting temperature. So adding rare earths in molten steel, it can purifying steel. Due to metal atomic radius of rare earth elements are largers than steel’s atom, they fill up its crystalline grains and flaws and form film that prevent crystalline grains fro growing, which result in grain refinement and enhance the property of steel.
Rare Earths own underfill 4F electron structure that result in various electronic energy levels. So rare earths can use as excellent fluorescence, laser, electric light source and glaze of colour glass and ceramics.
When rare earth ions compound with hydroxy, azo group or sulfo group, they get wide applications in printing and dyeing industry. However there is property that the neutron capture area is larger among some rare earth elements, like samarium, europium, gadolinium, dysprosium and erbium. They could use as control material and moderator material of atomic reactor. But the neutron capture area of cerium and yttrium is small, they could use as diluent of reactor fuel.
Rare earths have similar property of microelements,which could accelerate sprouting of seeds and growing of roots and photosynthesis.