Rare Earth Elements (REE) or Rare Earth Metals (REM) is a collection of seventeen chemical elements in the periodic table, namely scandium, yttrium, and the fifteen lanthanides. Scandium and yttrium are considered rare earths since they tend to occur in the same ore deposits as the lanthanides and exhibit similar chemical properties.
REE are incorporated into many modern technological devices, including superconductors, samarium-cobalt and neodymium-iron-boron high-flux rare-earth magnets, electronic polishers, refining catalysts and hybrid car components (primarily batteries and magnets). Rare earth ions are used as the active ions in luminescent materials used in optoelectronics applications, most notably the Nd:YAG laser. Erbium-doped fibre amplifiers are significant devices in optical-fibre communication systems. Phosphors with rare earth dopants are also widely used in cathode ray tube technology such as television sets. The earliest color television CRTs had a poor-quality red; europium as a phosphor dopant made good red phosphors possible. Yttrium iron garnet (YIG) spheres have been useful as tuneable microwave resonators. Rare earth oxides are mixed with tungsten to improve its high temperature properties for welding, replacing thorium, which was mildly hazardous to work with.