The discovery of rare earth elements began in 1787 in the Ytterby mine, located in the Stockholm archipelago in Sweden. There, officer Carl Axel Arrhenius found an unusual black stone. The newly discovered ore was later named gadolinite, after the Finnish chemist Johan Gadolin, who in 1794 identified the first rare earth element in it, which he called “yttria.”

Over the course of the next century, the remaining rare earth elements were gradually identified. In 1803, Martin Heinrich Klaproth, along with Jöns Jacob Berzelius and Wilhelm Hisinger, simultaneously isolated the second rare earth element in a complex oxide compound: ceria. It was later discovered that these were complex mixtures containing several different elements.

Starting in 1839, the Swedish chemist Carl Gustaf Mosander began a systematic analysis of mixed rare earths and isolated lanthanum, terbium, and erbium—the latter two, like ytterbium and yttrium, were named after their first discovery site, Ytterby. The great challenge with rare earths—then as now—was separating them from one another.

The isolation of the elements was achieved through an extremely laborious and monotonous process known as chemical fractionation. This method had to be repeated thousands of times before success was achieved, as the chemical and physical properties of rare earth elements are highly similar.

In the second half of the 19th century, chemists Gustav Kirchhoff and Robert Bunsen developed spectroscopy, a new method for identifying elements, which advanced the isolation of additional rare earth elements. Lutetium, the last naturally occurring rare earth element, was discovered in 1907.

The Austrian chemist Carl Auer von Welsbach, a student of Bunsen, discovered the elements neodymium and praseodymium in 1885. Welsbach was the first to use rare earths commercially, after recognizing their glowing properties. He developed the gas mantle for gas lamps, whose bright light surpassed existing lighting technologies and was also more cost-effective.

Later, he alloyed cerium with iron, inventing the “flint” in 1903, which is still used in lighters today. In 1898, he founded Treibacher Industrie AG, which still exists today. Welsbach sourced his raw materials from Brazil, India, and the United States.

During World War II, several rare earth metals became important for phosphors in radar screens and for special glass in optical systems. The United States launched a strategic procurement program to avoid supply shortages.

Advances in nuclear physics and the Cold War in the 20th century further expanded knowledge of rare earths. The only radioactive and non-naturally occurring rare earth element, promethium, was discovered. Driven by their atomic programs, countries conducted intensive searches for thorium and uranium, leading to the discovery of many of today’s major rare earth deposits—such as Mountain Pass in California and Bayan Obo in China.

The rare earths yttrium and europium played a central role in the development of color television, which was introduced in the 1960s.

The U.S. military was also significantly involved in developing applications for rare earths. Examples include samarium–cobalt (SmCo) magnets, a revolutionary invention in magnetic technology. As the first powerful, high-performance, and miniaturized permanent rare earth magnets, they became indispensable in military and aerospace technology, high-performance motors, and medical devices.

In the 1980s, neodymium–iron–boron (NdFeB) magnets were developed, which were stronger and more cost-effective than SmCo magnets, although they were less thermally stable.

In the 1960s and 1970s, the Mountain Pass mine in California was the undisputed world leader. New chemical separation methods (ion exchange, later liquid–liquid extraction) made it possible to produce purer oxides in large quantities and at lower cost.

At the same time, in Inner Mongolia near Baotou, rare earth mining began as a byproduct of iron ore production. Initially, Chinese production served only domestic needs.

In the 1980s and 1990s, environmental laws in the United States became stricter. The mining and especially the processing of rare earths generate radioactive waste and environmentally harmful acids. Compliance costs at Mountain Pass rose sharply. In 1998, it became known that large quantities of radioactive wastewater had leaked into the environment. The Mountain Pass mine was closed in 2002.

Meanwhile, China massively expanded its production using cheap labor and lax environmental regulations. Mines and separation plants outside China could not compete with the low prices and were forced to close.

To this day, China has not only become the largest producer but has also taken over the entire value chain: mining, separation, alloy production, and magnet manufacturing. By the turn of the millennium, China controlled over 90 percent of global production.

In nature, the rare earth elements occur together — with the exception of scandium — and are often associated with the radioactive elements uranium and thorium. This makes mining and especially further processing challenging.
The most important minerals for rare earth extraction are bastnäsite, monazite, loparite, and lateritic clays (ion-adsorption clays).

Initially, monazite was the main source of rare earths, since its concentration of rare earth elements is relatively high compared to other ores. However, a disadvantage of monazite is its significantly higher thorium content, which is radioactive.

From the 1960s onward, bastnäsite deposits began to dominate. This mineral contains much lower levels of radioactive thorium than monazite, but also a smaller proportion of heavy rare earth elements (HREEs). During this period, the Mountain Pass mine in California was the most important source. Bastnäsite — a fluorocarbonate mineral — remains the principal source of rare earths today.
Most of the world’s rare earths now come from mines in China (Bayan Obo in Inner Mongolia, and the provinces of Shandong and Sichuan) and the United States (Mountain Pass, California).

From the 1970s, China steadily increased its production of rare earths from the rich bastnäsite deposits in Bayan Obo, gradually displacing U.S. production from the Mountain Pass mine, which could no longer compete on price. In addition, U.S. operators had to comply with stricter environmental regulations. After large quantities of radioactive wastewaterleaked, the mine was closed in 2002 and reopened only in 2018.

To this day, China remains the largest producer of rare earths and holds by far the largest reserves in the world.
In addition, the Mountain Pass mine (USA) and Mount Weld mine (Australia) produce substantial amounts of rare earths. Significant deposits also exist in Brazil, India, Russia, and Vietnam, and rare earths are also mined in Myanmar, Nigeria, Thailand, and Australia.

Overproduction and Market Imbalance

An economically important factor is the composition of rare earth elements in a given deposit. Deposits with higher concentrations of the heavier rare earth elements are especially valuable. The light rare earth elements (LREEs) — particularly cerium (Ce) and lanthanum (La) — usually make up the largest share of rare earth deposits (sometimes exceeding 50%).
Because market demand does not reflect their natural abundance, there is a persistent overproduction of cerium and lanthanum relative to demand.

Heavy Rare Earths and New Exploration

Ion-adsorption clays are important for the extraction of heavy rare earths and are mainly mined in southern China, particularly in the provinces of Jiangxi and Guangdong. China also increasingly imports these ores from neighboring Myanmar.

Potential deposits of heavy rare earths outside China have been identified in Brazil and Vietnam.
Western efforts to establish domestic rare earth production have led to numerous exploration and development projects in Greenland, Sweden, and Norway. Even previously non-commercial minerals such as eudialyte are being considered (notably in Greenland and Sweden).

The rare earth content of minerals from selected deposits is shown in the table below.

The content of individual rare earth elements varies greatly from one mineral to another and from one deposit to another. The minerals and ores are generally classified as “light” or “heavy.”
The light rare earth group includes the low–atomic-weight elements scandium, lanthanum, cerium, praseodymium, neodymium, samarium, and europium, while the heavy rare earth group comprises gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium, and yttrium.

The geochemistry of scandium differs significantly from that of the other rare earth elements. Information about its ores and minerals can be found in the article on Scandium. Essentially, scandium is not found in any of the minerals listed below.

The element promethium occurs virtually not at all in nature on Earth.

Of the roughly 160 known minerals that contain rare earth elements, only four are currently mined for rare earth production: bastnäsite, lateritic clay, monazite, and loparite. With the exception of lateritic clay, these minerals are good sources of light lanthanides and lanthanum, and they account for about 95 percent of all rare earths used. Lateritic clays are the main commercial source of the heavy lanthanides and yttrium.

Other minerals that have been used as rare earth sources include apatite, euxenite, gadolinite, and xenotime. Allanite, fluorite, perovskite, sphene (titanite), and zircon have the potential to become future sources of rare earths.
In addition, uranium and iron residues have in the past been used as sources of heavy lanthanides and yttrium, as well as light lanthanides and lanthanum.

Many of these minerals, such as apatite and euxenite, are primarily mined for other elements, with rare earths obtained as a by-product.

In addition to minerals found in the Earth’s crust, rare earths and other critical metals also occur on the ocean floor, particularly in manganese nodules.
Japan plans to begin test deep-sea mining near Minamitori Island in 2026, despite concerns from scientists and environmentalists that this could cause irreversible damage and unpredictable ecological consequences.

The idealized chemical compositions of the 13 minerals that serve as sources of rare earth elements are listed in the following table.

Composition of Primary Rare Earth Elements

Composition of Selected Rare Earth Minerals

Bastnäsite is a fluorocarbonate mineral and one of the main sources of rare earth elements.
The mineral was first described in 1838, and its name derives from the Bastnäs mine near the Swedish town of Skinnskatteberg.

The extraction of rare earth elements from bastnäsite is often more economical and simpler than from other minerals such as monazite, due to its low thorium content. This significantly reduces environmental and disposal issues during processing.

The main components of bastnäsite are the rare earth elements cerium, lanthanum, and, in smaller quantities, yttrium.
However, the proportions can vary depending on the deposit.

The rare earth content of minerals from selected deposits — including some bastnäsite samples — is shown in the table below.

The extraction and processing of individual rare earth elements has been one of the greatest challenges since their discovery—and remains so today. China dominates the global rare earth refining industry, accounting for more than 90% of total refining capacity. For heavy rare earth elements, China’s market share exceeds 95%. In terms of expertise in the complex refining and upgrading processes, China is far ahead of other countries. The country maintains specialized research institutes that work in close cooperation with industry and state authorities.

Major Rare Earth Refineries in China

• Northern Rare Earth High-Tech Group – subsidiary of the state-owned China Rare Earth Group, located in Baotou, Inner Mongolia; focuses on light rare earths: lanthanum (La), cerium (Ce), praseodymium (Pr), and neodymium (Nd).

• China Southern Rare Earth Group – based in Ganzhou, Jiangxi; specializes in heavy rare earths: dysprosium (Dy), terbium (Tb), yttrium (Y), and europium (Eu).

• Xiamen Tungsten – operates in Fujian and Jiangxi; produces heavy rare earths: dysprosium (Dy), terbium (Tb), and yttrium (Y).

• Guangdong Rare Earth Industry Group – located in Guangdong Province; processes both light and heavy rare earths.

• Shenghe Resources – based in Sichuan and Shandong; refines light and heavy rare earths, including neodymium-praseodymium (NdPr), dysprosium (Dy), and terbium (Tb).

Rare Earth Refineries Outside China

• Kuantan, Malaysia – operated by Lynas Rare Earths; produces light rare earths (La, Ce, Pr, Nd) and some heavy rare earths (Dy, Tb).

• Sillamäe, Estonia – operated by Neo Performance Materials; focuses on light rare earths.

• Mountain Pass, USA – operated by MP Materials; refines light rare earths.

Due to geopolitical tensions and supply chain vulnerabilities, the U.S., the EU, and other countries are expanding their domestic production capacities. Several additional facilities—especially in the United States and Australia—are currently under construction or scaling up operations.

Rare earth ores typically contain less than 10% rare earth oxides (REO) and must be upgraded to around 60% for further processing. The ores are first ground into a fine powder and then separated from the surrounding materials using standard techniques such as flotation, magnetic separation, and/or electrostatic separation.

For subsequent processing of the concentrated ore, separation techniques are applied that were originally developed shortly after World War II as part of the U.S. Atomic Energy Commission’s nuclear programs. Research on the ion-exchange process was carried out at Oak Ridge National Laboratory under Gerald E. Boyd and at the Ames Laboratory (now Ames National Laboratory) under Frank Harold Spedding. Both teams demonstrated that the ion-exchange process could successfully separate rare earth elements on a small scale.

In the 1950s, the Ames group further showed that it was possible to separate kilogram-scale quantities of high-purity (>99.99%) individual rare earth elements using ion exchange. This marked the beginning of the modern rare earth industry, as for the first time, significant quantities of high-purity rare earth elements became available for electronic, magnetic, phosphor-based, and optical applications.

At the Argonne National Laboratory and Oak Ridge National Laboratory, researchers Donald F. Peppard and Boyd Weaver developed liquid-liquid solvent extraction methods for rare earth separation in the mid-1950s. Using this technique, manufacturers can isolate individual rare earth elements from mixed solutions with purities ranging from 95% to 99.9%.

Although the ion-exchange process is slower, it can achieve ultra-high purities exceeding 99.99999% (“five nines” or better). For optical and phosphor materials, where purities of five to six nines are required, the individual rare earth element is first purified using the liquid-liquid extraction method to about 99.9%, and then further refined by ion exchange to reach the purity level necessary for the specific application.

In the ion exchange process, a metal ion (R³⁺) in solution is exchanged with three protons on a solid ion exchanger—either a natural zeolite or a synthetic resin, commonly referred to simply as the resin.

The strength with which the cation is held by the resin depends on the ion’s size and charge. However, separation of rare earth elements is not possible using the resin alone, as it is not sufficiently selective. Separation becomes possible through the introduction of a complexing agent; when the stability of the R³⁺ ion complex varies sufficiently between adjacent lanthanide ions, they can be separated. Two common complexing agents used for rare earth separation are ethylenediaminetetraacetic acid (EDTA) and hydroxyethylethylenediaminetriacetic acid (HEDTA).

The resin beads, about 0.1 mm (0.004 in) in diameter, are packed into a long column. The resin bed is conditioned by passing an acid through the column. It is then loaded with a mixed rare earth acid solution containing both the complexing agent and a retaining ion, such as Cu²⁺ or Zn²⁺. The retaining ion is necessary to prevent the first rare earth ion from spreading out and being lost during the separation process.

An eluting agent, typically ammonium (NH₄⁺), drives the rare earth ions through the ion exchange column. The most stable complex is eluted first—usually the copper or zinc complex—followed by lutetium, ytterbium, the other lanthanides (and yttrium, which typically appears near dysprosium and holmium depending on the complexing agent), and finally lanthanum.

The individual rare earth complexes (R³⁺) form rectangular elution bands with minimal overlap between neighboring bands. The separated rare earth solutions are collected, and the R³⁺ ion is precipitated from solution with oxalic acid. The resulting rare earth oxalate is then calcined in air at 800–1,000 °C (1,472–1,832 °F) to produce the corresponding rare earth oxide.

The liquid–liquid solvent extraction process uses two completely or partially immiscible liquids that contain dissolved rare earth elements. The two liquids are mixed, allowing the solutes to distribute between the two phases until equilibrium is reached, after which the two liquids are separated.

The concentrations of the dissolved substances in each phase depend on their relative affinity for the two solvents. By convention, the liquid containing the desired solute is called the extract, while the residue remaining in the other phase is referred to as the raffinate.

The most effective way to achieve separation of rare earth elements is by using a multi-stage countercurrent separatoroperating continuously with a series of mixer–settler tanks or cells. If substance A has a greater affinity for the organic phase and substance B has a greater affinity for the aqueous phase, the organic phase becomes enriched with A, and the aqueous phase becomes enriched with B.

In the case of rare earths, the process is much more complex, as multiple rare earth elements must often be separated simultaneously, rather than just two. Tributyl phosphate (TBP) is commonly used as the organic phase to extract rare earth ions from a strongly acidic nitric acid aqueous phase. Other extractants, such as di-(2-ethylhexyl) orthophosphoric acid and long-chain amines, are also used.

Depending on the melting and boiling points of each metal and the purity required for a specific application, three different methods are used:

• For the low-melting lanthanides—lanthanum, cerium, praseodymium, and neodymium—calciothermic and electrolytic reduction methods are employed to produce high-purity metals (99% or higher).

• For the high-melting rare earth elements—scandium, yttrium, gadolinium, terbium, dysprosium, holmium, erbium, and lutetium—the calciothermic process is used.

• For the high-vapor-pressure metals—samarium, europium, thulium, and ytterbium—the lanthanothermic process is applied.

All three methods are suitable for producing commercial-grade rare earth metals with purities of 95–98%.

The calciothermic process is used for producing all rare earth metals except for the four high-vapor-pressure metals—samarium, europium, thulium, and ytterbium.

In this process, the rare earth oxide is first converted to its fluoride by heating it with anhydrous hydrogen fluoride (HF) gas to form RF₃. The fluoride can also be prepared by dissolving the oxide in aqueous hydrochloric acid (HCl)and then adding aqueous hydrofluoric acid (HF) to precipitate the RF₃ compound from solution.

The fluoride powder is then mixed with metallic calcium and heated in a tantalum crucible to 1,450 °C (2,642 °F) or higher, depending on the melting point of R (the specific rare earth element). Calcium reacts with RF₃ to form calcium fluoride (CaF₂) and metallic R. Since these two products are immiscible, the CaF₂ floats on top of the molten metal. Upon cooling to room temperature, the CaF₂ can be easily separated from the metal.

The metal is then heated in a high vacuum in a tantalum crucible to a temperature above its melting point to evaporate excess calcium. At this stage, further purification of the metal R can be achieved through sublimation or distillation.

In China, calciothermic reduction is often carried out on a commercial scale in graphite crucibles. However, this leads to significant carbon contamination, as carbon readily dissolves in molten rare earth metals. Conventional oxide crucibles, such as alumina (Al₂O₃) or zirconia (ZrO₂), are unsuitable for calciothermic reduction of rare earths, because molten rare earth metals rapidly reduce aluminum or zirconium from their oxides.

The low-melting rare earth metals—lanthanum, cerium, praseodymium, and neodymium—can be produced from their oxides using two different electrolytic processes.

The first method involves converting the oxide into the chloride (or fluoride) and then reducing the halide in an electrolytic cell. An electric current with a current density of about 10 A/cm² is passed through the cell, reducing RCl₃ (or RF₃) to chlorine (Cl₂) or fluorine (F₂) gas at the carbon anode and forming molten rare earth metal (R) at the molybdenum or tungsten cathode. The electrolyte is a molten salt mixture of RCl₃ (or RF₃) and NaCl (or NaF). However, electrolytically produced lanthanides are less pure than those produced by the calciothermic process.

The second electrolytic method directly reduces the oxide in a RF₃–LiF–CaF₂ molten salt mixture. The main challenge with this process is the low solubility of the oxide and the difficulty in controlling the oxygen solubility in the molten salt.

The electrolytic process is limited to rare earth metals with melting points below 1,050 °C (1,922 °F), since those with higher melting points react with the electrolytic cell and electrodes.

Large-scale commercial applications use individual metals such as lanthanum for nickel–metal hydride batteries, neodymium for Nd₂Fe₁₄B permanent magnets, and mischmetal for alloying agents and lighter flints.

Mischmetal is a mixture of rare earth elements reduced directly from a rare earth concentrate, in which the ratio of individual elements is approximately the same as in the mined ores (typically about 50% cerium, 25% lanthanum, 18% neodymium, and 7% praseodymium).

Lanthanum and neodymium metals are mostly produced by direct electrolytic reduction of their oxides, while mischmetal is generally produced by electrolysis of the mixed RCl₃ halides.

The lanthanothermic process is used to produce samarium, europium, thulium, and ytterbium. The divalent metals europium and ytterbium have high vapor pressures—or lower boiling points—than the other rare earth elements, which makes their production by calciothermic or electrolytic methods difficult. Samarium and thulium also have lower boiling points compared to most other rare earth elements.

These four metals are produced by mixing R₂O₃ (R = samarium, europium, thulium, or ytterbium) with fine chips of lanthanum metal and placing the mixture at the bottom of a tantalum crucible. The mixture is heated to 1,400–1,600 °C (2,552–2,912 °F), depending on the metal. The lanthanum metal reacts with R₂O₃ to form lanthanum oxide (La₂O₃), while R vaporizes and condenses on a collector at the top of the crucible, which is about 500 °C cooler than the reaction zone at the bottom. The four metals can be further purified by resublimation.

Given the limited reserves and high value of rare earth metals, recycling of these elements from end-of-life consumer products is expected to become increasingly important.

The chemical company Solvay restarted its previously idled recycling plant in La Rochelle, France, in 2025.

The chemical, metallurgical, and physical behavior of the rare earth elements is determined by their electronic configurations. In general, these elements are trivalent, R³⁺, although some of them exhibit other valence states. The number of 4f electrons for each lanthanide is listed in the table showing the number of 4f electrons and the ionic radii for the R³⁺ ion.

The 4f electrons have lower energies than the outer three valence electrons and are located radially inside these outer electrons (i.e., the 4f electrons are “localized” and form part of the ionic core). As a result, they are not directly involved in bonding with other elements when a compound is formed. Therefore, the lanthanides are chemically similar, difficult to separate, and are commonly found together in various minerals.

The outer, or valence, electrons for the 14 lanthanides and lanthanum are the same: 5d6s²; for scandium, 3d4s²; and for yttrium, 4d5s².
There are some differences in the chemical properties of the lanthanides due to the lanthanide contraction and the hybridization (or mixing) of the 4f electrons with the valence electrons.

The systematic and gradual decrease in ionic radius from lanthanum to lutetium is known as the lanthanide contraction. It is caused by the increasing nuclear charge, which is not completely shielded by the additional 4f electron as one moves across the series. This increased effective charge pulls both the core and valence electrons closer to the nucleus, accounting for the smaller radii of the higher atomic number lanthanides. The lanthanide contraction also contributes to the decrease in basicity from lanthanum to lutetium and forms the basis for several separation techniques.

As 4f electrons are added moving from lanthanum through cerium to praseodymium and so on, the electrons possessing magnetic moments due to their spin maintain the same orientation, with their moments aligned parallel until the 4f shell is half-filled — i.e., at seven 4f electrons in gadolinium. The next electron must be oriented antiparallel, according to the Pauli exclusion principle, so that two 4f electrons are paired. This continues until the 14th electron is added at lutetium, where all 4f electron spins are paired, leaving lutetium with no 4f magnetic moment.

The 4f electron configuration is extremely important and determines the magnetic and optical behavior of the lanthanide elements. For example, the special properties of powerful Nd₂Fe₁₄B permanent magnets arise from the three 4f electrons in neodymium, while the red color in optical displays and cathode-ray tubes is provided by europium ions in a host compound, and the green color by terbium.

As mentioned earlier, several lanthanides can exhibit alternative valence states — R⁴⁺ for R = cerium, praseodymium,and terbium; and R²⁺ for R = samarium, europium, and ytterbium. These additional valence states are a striking example of Hund’s rule, which states that empty, half-filled, and fully filled electronic levels are generally more stable. Thus, Ce⁴⁺ and Tb⁴⁺ lose one f electron to achieve an empty or half-filled 4f level, while Eu²⁺ and Yb²⁺ gain one f electron to achieve a half-filled or completely filled 4f level. Pr⁴⁺ and Sm²⁺ can, in rare cases, gain extra stability by losing or gaining one f electron, although they do not reach the fully empty or half-filled configuration.

By losing one 4f electron to form an R⁴⁺ ion, the radii of cerium, praseodymium, and terbium decrease to 0.80, 0.78, and 0.76 Å, respectively. Conversely, samarium, europium, and ytterbium gain one 4f electron from their valence shell to form R²⁺ ions, increasing their radii to 1.19, 1.17, and 1.00 Å, respectively. Chemists have exploited these valence changes to separate Ce⁴⁺, Eu²⁺, and Yb²⁺ from the other trivalent R³⁺ ions using relatively inexpensive chemical methods. CeO₂(where cerium is tetravalent) is the stable oxide form, whereas the oxides of praseodymium and terbium have the stoichiometries Pr₆O₁₁ and Tb₄O₇, respectively, which contain both tetra- and trivalent states — i.e., 4PrO₂·Pr₂O₃ and 2TbO₂·Tb₂O₃. The divalent ions Sm²⁺, Eu²⁺, and Yb²⁺ form dihalides such as SmCl₂, EuCl₂, and YbCl₂. Several europium oxide stoichiometries are known: EuO (Eu²⁺), Eu₂O₃ (Eu³⁺), and Eu₃O₄ (EuO·Eu₂O₃).

The ionic radius of scandium is much smaller than that of the smallest lanthanide, lutetium — 0.745 Å versus 0.861 Å. The radius of scandium is somewhat larger than that of typical metal ions such as Fe³⁺, Nb⁵⁺, U⁵⁺, and W⁵⁺. This is the main reason why scandium is not found in significant amounts in normal rare-earth minerals — typically less than 0.01 wt%. However, scandium is recovered as a by-product from processing other ores (e.g., wolframite) and from mining residues (e.g., uranium).

In contrast, the radius of yttrium (0.9 Å) is almost identical to that of holmium (0.901 Å), which explains the occurrence of yttrium in heavy lanthanide minerals.

Most rare earth metals are trivalent; however, cerium’s effective valence is about 3.2, while europium and ytterbium are divalent. This becomes evident when plotting the metallic radii as a function of atomic number. The metallic radii of the trivalent metals exhibit the normal lanthanide contraction, but a clear deviation occurs at cerium, whose radius falls below the trend line of the trivalent metals, and at europium and ytterbium, whose radii lie well above it.

The melting points of europium and ytterbium are significantly lower than those of neighboring trivalent lanthanides when plotted against atomic number, consistent with their divalent nature. Similar anomalies are observed in other physical properties of europium and ytterbium compared with the trivalent lanthanide metals (see Properties of the Metalsbelow).

The following table lists the number of 4f electrons and the radius of the R³⁺ ion for the rare earth elements.

As previously mentioned, the rare earth elements—especially the lanthanides—are quite similar. They naturally occur together, and their complete separation is difficult to achieve. However, there are some notable differences, particularly in the physical properties of the pure metallic elements. For example, their melting points differ by almost a factor of two, and their vapor pressures vary by more than a billionfold. These and other interesting facts will be explained below.

All rare earth metals except europium crystallize in one of four close-packed structures (crystal lattices). While one of these progresses along the lanthanide series from lanthanum to lutetium, the crystal structures change from face-centered cubic (FCC, also called “cubic close-packed” or copper/Cu-type) to hexagonal close-packed (HCP, also called “hexagonal close packing” or magnesium/Mg-type), with two intermediate structures composed of a mixture of FCC and HCP layers. One intermediate consists of 50% of each layer (double hexagonal close packed / DHCP), and the other consists of one-third FCC and two-thirds HCP (close packed rhombohedral / samarium/Sm-type). These two intermediate structures are unique among all metallic elements’ crystal structures, while the FCC and HCP structures are relatively common.

Several elements exhibit two close-packed structures: lanthanum and cerium have both FCC and DHCP structures, samarium has Sm- and HCP-structures, and ytterbium has FCC and HCP structures. The existence of these structures depends on temperature. In addition to the close-packed structures, most rare earth metals (scandium, yttrium, lanthanum to samarium, and gadolinium to dysprosium) exhibit a body-centered cubic (BCC or tungsten/W-type) high-temperature polymorphism. Exceptions are europium, which is BCC from 0 K (-273 °C / -460 °F) up to its melting point at 822 °C (1,512 °F), and holmium, erbium, thulium, and lutetium, which are monomorphic with the HCP structure. Cerium, terbium, and dysprosium undergo low-temperature transformations (below room temperature). Cerium’s transformation is due to a valence change, whereas terbium’s and dysprosium’s are of magnetic origin.

The melting points of the lanthanide metals increase rapidly with rising atomic number, from 798 °C (1.468 °F) for cerium to 1.663 °C (3.025 °F) for lutetium, effectively doubling. The melting points of scandium and yttrium are comparable to those of the heavier trivalent lanthanides. The relatively low melting points of the light to middle lanthanides are attributed to a contribution of the 4f electrons to bonding, which peaks at cerium and decreases to near zero by erbium. The low melting points of europium and ytterbium are due to their divalent state.

The boiling points of the rare earth metals vary by almost a factor of three. Those of lanthanum, cerium, praseodymium, yttrium, and lutetium are among the highest of all chemical elements, while those of europium and ytterbium rank among the metals with the lowest boiling points. This large difference arises from the variation in the electronic structures of the atoms in the solid metal versus the corresponding gaseous atoms. For the trivalent solid metals with the highest boiling points, the gaseous atom has three outer electrons (5d¹6s²), whereas the divalent solid metals with low boiling points have gaseous atoms with only two outer electrons (6s²). The lanthanides with intermediate boiling points are trivalent solids, but their gaseous forms possess only two outer electrons (6s²). This difference in the electronic states of the solid metals compared to their corresponding gaseous atoms explains the observed behavior.

The electrical resistivities of the rare earth metals range between 25 and 131 microohm-centimeters (μΩ·cm), which fall in the middle of the electrical resistivity values for metallic elements. Most trivalent rare earth metals have values at room temperature of about 60 to 90 μΩ·cm. The low value of 25 μΩ·cm corresponds to divalent FCC ytterbium metal, while the two highest values, gadolinium (131 μΩ·cm) and terbium (115 μΩ·cm), are attributed to a magnetic contribution to the specific electrical resistivity that arises near the magnetic ordering temperature of a material.

Lanthanum metal is the only rare earth metal that is superconducting at atmospheric pressure (i.e., conducting without electrical resistance), while scandium, yttrium, cerium, and lutetium only exhibit superconductivity under high pressure.
The FCC modification of lanthanum becomes superconducting at a temperature of Tc = 6.0 K (-267.2 °C or -448.9 °F), whereas the DHCP polymorph has a Tc = 5.1 K (-268.1 °C or -450.5 °F).

The magnetic properties of rare earth metals, alloys, and compounds strongly depend on the number of unpaired 4f electrons. Metals without unpaired electrons (scandium, yttrium, lanthanum, lutetium, and divalent ytterbium) are only weakly magnetic, similar to many non-rare earth metals. The rest of the lanthanides (cerium to thulium) are strongly magnetic due to their unpaired 4f electrons. Thus, the lanthanides form the largest family of magnetic metals. The magnetic ordering temperature generally depends on the number of unpaired 4f electrons. It is about 13 K (-260 °C, or -436 °F) for cerium, which has one unpaired electron, and reaches room temperature for gadolinium, which has seven unpaired electrons—the maximum possible number. All other magnetic ordering temperatures of the lanthanides fall between these two values. Gadolinium is the only element that orders ferromagnetically at room temperature—besides the 3d-electron elements iron, cobalt, and nickel. The magnetic strength, measured by the effective magnetic moment, has a more complex correlation with the number of unpaired 4f electrons because it also depends on their orbital motions. Considering this, the maximum effective magnetic moment is found in dysprosium and holmium, very close to each other, with values of 10.64 and 10.60 Bohr magnetons, respectively; gadolinium’s value is 7.94.

Rare earth metals exhibit exotic (and sometimes complex) magnetic structures that change with temperature. Most lanthanides have at least two magnetic structures. At room temperature, gadolinium has the simplest structure: all 4f spins are aligned parallel in one direction, known as ferromagnetic gadolinium. Most other lanthanide metals have 4f spins aligned antiparallel to each other (i.e., parallel but in opposite directions), sometimes fully and often only partially. They are all called antiferromagnetic metals, regardless of whether the spins are fully or partially compensated. In many antiferromagnetic structures, the spins form spiral arrangements.

When comparing the linear coefficient of thermal expansion (LCTE) of hexagonally packed metals, the thermal expansion is always greater in the close-packed direction than within the planes (A, B, and C layers). The anomalously high expansion values for europium and ytterbium once again confirm the divalent nature of these two metals.

As with most other properties of the rare earth metals, the elastic moduli of the rare earth metals fall into the middle percentile range of other metallic elements. The values for scandium and yttrium are roughly comparable to those of the heavier lanthanides (erbium to lutetium). There is a general increase in elasticity with increasing atomic number, with cerium (due to 4f bonding) as well as europium and ytterbium (due to divalency) standing out as notable exceptions with lower values.

The rare earth metals are neither weak nor particularly strong metallic elements mechanically, and they exhibit low ductility. Since these properties strongly depend on the purity of the metals and the environmental temperatures they have been exposed to in the past, it is difficult to compare the values reported in the literature directly. The tensile strength varies from about 120 to approximately 160 MPa (megapascal), and the ductility ranges from around 15 to 35 percent. Ytterbium’s strength is much lower, at 58 MPa, and its ductility is higher, about 45 percent, as expected for a divalent metal.

The reactivity of rare earth metals with air shows a significant difference between the light and heavy lanthanides. The light lanthanides oxidize much faster than the heavy lanthanides (gadolinium to lutetium), as well as scandium and yttrium. This difference is partly due to the variation in the type of oxide formed.

The light lanthanides (lanthanum to neodymium) form the hexagonal R₂O₃ structure of the A-type; the middle lanthanides (samarium to gadolinium) form the monoclinic R₂O₃ phase of the B-type; while the heavy lanthanides, scandium, and yttrium form the cubic R₂O₃ modification of the C-type.

The A-type oxide reacts with water vapor in the air to form an oxyhydroxide that flakes off as a white coating, exposing fresh metal surface and allowing further oxidation.
The C-type oxide forms a dense, coherent coating that prevents further oxidation, similar to the behavior of aluminum. Samarium and gadolinium, which form the B-type R₂O₃ phase, oxidize somewhat faster than the heavy lanthanides, scandium, and yttrium but still develop a dense coating that prevents further oxidation.

For this reason, the light lanthanides must be stored under vacuum or in an inert gas atmosphere, while the heavy lanthanides, scandium, and yttrium can remain outdoors for years without oxidation.

Europium metal, with a BCC structure, is the rare earth metal that oxidizes fastest in moist air and therefore must always be kept under inert atmosphere. The reaction product of europium with moist air is unusually a hydrated hydroxide, Eu(OH)₂·H₂O — all other rare earth metals form oxides.

The metals react vigorously with all acids except hydrofluoric acid (HF), releasing H₂ gas and forming the corresponding rare earth anion compounds. When introduced into hydrofluoric acid, rare earth metals form an insoluble RF₃ coating that prevents further reaction.

Rare earth metals react easily with hydrogen gas to form RH₂ and, under strong hydrogenation conditions, the phase RH₃, except for scandium, which does not form a trihydride.

The rare earth elements form tens of thousands of compounds with all elements from group 7 and beyond on the right side of the periodic table, including metals such as manganese, technetium, and rhenium, as well as with beryllium and magnesium, which are located on the far left of group 2. Important series of compounds and some individual compounds with unique properties or unusual behavior are described below.

The largest family of inorganic rare earth compounds studied so far are the oxides. The most common stoichiometry is the R₂O₃ composition, but since some lanthanide elements exhibit valence states other than 3+, other stoichiometries exist—for example, cerium oxide (CeO₂), praseodymium oxide (Pr₆O₁₁), terbium oxide (Tb₄O₇), europium oxide (EuO), and Eu₃O₄. Most of the discussion will focus on the binary oxides, but ternary and other higher-order oxides will also be briefly addressed.

All rare earth metals form the sesquioxide (oxides with an oxygen-to-metal ratio of 3 to 2, from the Latin sesqui meaning one and a half) at room temperature, although this may not be the stable equilibrium composition. There are five different crystal structures for the R₂O₃ phase. These are designated as types A, B, C, H, and X, and their existence depends on the rare earth element and the temperature.

• The A-type exists for the light lanthanides, which transform into the H-type above 2.000 °C (3.632 °F), and then at an additional 100–200 °C (180–360 °F) higher temperatures into the X-type.

• The B-type is found in the middle lanthanides, which also transform into the H-type above 2.100 °C (3.812 °F), and then into the X-type near their melting points.

• The C-structure occurs for the heavy lanthanides as well as for Sc₂O₃ and Y₂O₃. The C-type R₂O₃ compounds convert into the B-type between 1,000 and 2.000 °C (1.832 and 3.632 °F), and then into the H-type before melting.

The R₂O₃ phases are refractory oxides with melting points between 2.300 and 2.400 °C (4.172 and 4.352 °F) for the light and heavy R-oxides, respectively, but due to the structural transformations mentioned above, their use as refractory materials is limited.

Sesquioxides are among the most stable oxides in the periodic table; the more negative the Gibbs free energy of formation (ΔG), the more stable the oxide. Notably, the oxides of europium (Eu₂O₃) and ytterbium (Yb₂O₃) show anomalous free energies of formation. Although europium and ytterbium are trivalent in these compounds, their ΔG values are less negative. This is because both metals are divalent in their elemental form, and energy is required to convert them to the trivalent state during oxidation.

There are several important applications involving R₂O₃ compounds, usually in combination with other compounds or materials. Oxides without unpaired 4f electrons — lanthanum oxide (La₂O₃), lutetium oxide (Lu₂O₃), and gadolinium oxide (Gd₂O₃) — are added to glasses used as lenses in the optics industry, increasing the refractive index. These same oxides, plus yttrium oxide (Y₂O₃), are used as host materials for rare earth-based phosphors, usually mixed with other oxides to optimize optical properties. Yttrium orthovanadate (YVO₄) and yttrium oxysulfide (Y₂O₂S) are among the most commonly used host materials.

Some lanthanide ions with unpaired 4f electrons have electronic transitions that produce intense and bright colors when excited by electrons or photons. These are used in cathode ray tubes, optical displays, and fluorescent lamps. The important activators are Eu³⁺ (red), Eu²⁺ (blue), Tb³⁺ (green), and Tm³⁺ (blue). These activator R₂O₃ oxides are added to the host materials in amounts of 1–5% to produce the respective phosphor colors. The Eu³⁺ ion produces an intense red color; its discovery in 1961 revolutionized the TV industry. Before europium phosphors, color TV images were dull and faint. The introduction of europium phosphors made the colors brighter and more vivid, greatly enhancing the viewing experience. This marked the beginning of the modern rare earth industry. The annual production of individual rare earth elements grew significantly, with products of ever-higher purity and dramatically increased mining of rare earths in subsequent years.

Y₂O₃ is added to ZrO₂ to stabilize the cubic form of zirconia and introduce oxygen vacancies, resulting in a material with high electrical conductivity. These materials (5–8% Y₂O₃ in ZrO₂) are excellent oxygen sensors used to determine oxygen content in air and to control the fuel/air mixture in combustion engines.

Adding about 2 wt% R₂O₃ (R = lanthanum, cerium, and unseparated rare earth mixture) to zeolites (3SiO₂/Al₂O₃) doubles or triples the catalytic activity of fluid catalytic cracking (FCC) catalysts compared to zeolites without rare earths. Since their invention in 1964, FCC catalysts have become one of the largest applications for rare earths (15–18%). The primary roles of rare earths are to stabilize the zeolite structure, extending its lifetime, and to improve selectivity and effectiveness of the FCC catalyst.

One of the oldest applications of rare earth oxides, dating back to 1912, is glass coloration: neodymium oxide (Nd₂O₃) produces colors ranging from delicate pink at low concentrations to violet-blue at higher concentrations; samarium oxide (Sm₂O₃) imparts yellow; and erbium oxide (Er₂O₃) imparts light pink. Didymium oxide, Di₂O₃ (a mixture of approximately 25% praseodymium and 75% neodymium), is used in glassblowers’ and welders’ goggles because it effectively absorbs the intense yellow light produced during glassworking and welding. (The use of CeO₂-Ce₂O₃ for decolorizing glass is discussed in the next section.)

Due to the tendency toward completely empty or half-filled 4f orbitals (see above: Electronic Structures and Ionic Radius), cerium, praseodymium, and terbium tend to form tetravalent or partially tetravalent compounds—namely CeO₂, Pr₆O₁₁, and Tb₄O₇. However, the Gibbs free energies of formation for the R₂O₃ sesquioxides of cerium, praseodymium, and terbium are close to those of their higher oxides. A number of oxide intermediate phases, ROₓ (where 1.5 < x < 2), have been observed depending on temperature, oxygen partial pressure, and the thermal history of the sample. In the CeOₓ system, at least five intermediate phases exist. CeOₓ compounds have been used as portable oxygen sources. By far the most important application of CeOₓ compounds is in automotive catalytic converters, which largely eliminate the harmful exhaust gases carbon monoxide and nitrogen oxides from gasoline-powered vehicles.

Another important application of CeO₂ is as a polishing agent for glass lenses, monitor screens, semiconductors, mirrors, gemstones, and automobile windshields. CeO₂ is much more effective than other polishing agents (e.g., iron oxide [Fe₂O₃], zirconium dioxide [ZrO₂], and silicon dioxide [SiO₂]) because it is three to eight times faster, while the quality of the final result is equal to or better than that of other oxide polishes. The exact mechanism of the polishing process is not fully understood, but it is believed to involve a combination of mechanical abrasion and chemical reaction between CeOₓ and the SiO₂ glass, with water playing an active role.

CeO₂ is an important glass additive suitable for various applications. It is used for decolorizing glass. It prevents browning of glass caused by X-rays, gamma rays, and cathode rays, and it absorbs ultraviolet radiation. These applications exploit the redox behavior of CeO₂-Ce₂O₃. Since iron oxide is always present in glass, CeO₂ serves to oxidize Fe²⁺, which imparts a bluish tint to the glass, to Fe³⁺, which has a weak yellow color. Selenium is added to the glass as a complementary colorant to “neutralize” this Fe³⁺ color. Glass easily browns due to the formation of color centers when exposed to various types of radiation. The Ce⁴⁺ ions act as electron traps in the glass, absorbing electrons released by high-energy radiation. Cerium is found in non-browning glasses for televisions and other cathode ray screens as well as in radiation-shielding windows used in the nuclear industry. CeO₂ is added to glass containers to protect them from aging caused by long-term exposure to ultraviolet radiation from sunlight, again utilizing the Ce⁴⁺-Ce³⁺ redox pair.

In the PrOₓ and TbOₓ systems, seven and four intermediate phases respectively have been found with compositions between 1.5 < x < 2.0. Some of the compositions and crystal structures are the same as those in the CeOₓ system. However, since the abundance of praseodymium and especially terbium in common ore sources is much lower than that of cerium, few or no commercial applications have been developed for the PrOₓ and TbOₓ systems.

A NaCl-like RO phase has been reported for practically all rare earth elements, but it has been shown that these are ternary phases stabilized by nitrogen, carbon, or both. The only true binary RO compound is europium oxide (EuO), which is a ferromagnetic semiconductor below its Curie temperature Tc = 77 K (-196 °C or -321 °F). This discovery had a profound impact on solid-state magnetism theory, as there are no overlapping conduction electrons, which were previously considered necessary for the occurrence of ferromagnetism. It is believed that the ferromagnetism in EuO is due to oxygen-mediated cation-cation (Eu²⁺–Eu²⁺) superexchange interactions. Subsequently, ferromagnetism was also found in EuS and EuSe, as well as antiferromagnetism in EuTe.

Europium also forms another suboxide, Eu₃O₄, which can be regarded as a mixed-valence material containing both Eu³⁺ and Eu²⁺ ions — i.e., a combination of Eu₂O₃ and EuO.

The rare earth oxides form tens of thousands of ternary and higher oxides with other oxides such as aluminum sesquioxide (Al₂O₃), iron oxide (Fe₂O₃), cobalt sesquioxide (Co₂O₃), chromium sesquioxide (Cr₂O₃), gallium sesquioxide (Ga₂O₃), and manganese sesquioxide (Mn₂O₃). The two most common structures formed by the ternary rare earth oxides are the perovskite, RMO₃, and the garnet, R₃M₅O₁₂, where M represents a metal atom.

The perovskite structure is a closed lattice with R atoms located at the eight corners of the unit cell. The M atoms, which are smaller than the R atoms and generally trivalent, are located at the center of the unit cell, and oxygen atoms occupy the centers of the six faces. The basic structure is a primitive cube, but there are tetragonal, rhombohedral, orthorhombic, monoclinic, and triclinic distortions. Other elements can be partially or fully substituted, resulting in a wide range of properties with M and R: conductors, semiconductors, insulators, dielectrics, ferroelectrics, ferromagnets, antiferromagnets, and catalysts. Some of the more interesting applications are epitaxial layers of LaGaO₃, LaAlO₃, or YAlO₃ for high-temperature oxide superconductors, magnetoresistive layers and GaN layers; cathodes and compounds of (La,M)MnO₃ and (La,M)CrO₃ for solid oxide fuel cells; lanthanum-modified lead zirconate titanate (commonly known as PLZT) as transparent ferroelectric ceramics for heat and flash protection devices, data recorders, and protective glasses; and (Pr,Ca)MnO₃, which exhibits colossal magnetoresistance and is used in switches.

Garnets have a much more complex crystal structure than perovskites: 96 oxygen sites, while the metal atoms occupy 24 tetrahedral, 16 octahedral, and 24 dodecahedral sites (64 total). The general formula is R₃M₅O₁₂, where R occupies the dodecahedral sites and M atoms occupy the other two sites. M is generally a trivalent ion of aluminum, gallium, or iron. One of the most important rare earth garnets is yttrium iron garnet (YIG), which is used in a variety of microwave devices such as radars, attenuators, filters, circulators, isolators, phase shifters, power limiters, and switches. YIG is also used in integrated microwave circuits where thin films are applied on garnet substrates. The properties of these materials can be modified by substituting gadolinium for yttrium and aluminum or gallium for iron.

The quaternary oxide YBa₂Cu₃O₇ is the best-known of the higher oxides and has a layered perovskite-like structure. It was found in 1987 to exhibit superconductivity at 77 K (-196 °C or -321 °F), meaning it has zero electrical resistance. This discovery triggered a revolution because the Tc of 77 K allowed cooling with inexpensive liquid nitrogen (before 1986, the highest known superconducting transition temperature was 23 K [-250 °C]). YBa₂Cu₃O₇ (YBCO, also known as Y-123) broke the temperature record, and the fact that it was an oxide was also a surprise since all previous good superconductors were metallic materials. This material was quickly commercialized and is now used to generate high magnetic fields in research devices, magnetic resonance imaging (MRI) scanners, and electrical power transmission lines.

The rare earth metals readily react with hydrogen to form RH₂, and by increasing the hydrogen pressure, the trivalent R metals (except scandium) also form the RH₃ phase. Both the RH₂ and RH₃ phases are non-stoichiometric (i.e., the number of atoms of the elements present cannot be expressed as a ratio of small integers). The RH₂ phase has the CaF₂ fluorite structure for trivalent R, while for divalent europium and ytterbium, the dihydride crystallizes in an orthorhombic structure, which is the same as that of alkaline earth dihydrides. The RH₃ phases exhibit two different crystal structures. For the light lanthanides (lanthanum through neodymium), RH₃ has a fluorite-like structure and forms a continuous solid solution with RH₂. For the heavy lanthanides (samarium through lutetium) and yttrium, RH₃ crystallizes in a hexagonal structure. The rare earth hydrides are air-sensitive and must be handled in gloveboxes.

The electrical resistivity of RH₂ is about 75 percent lower than that of the pure metals. However, it increases as more hydrogen is added beyond RH₂, approaching that of a semiconductor at RH₃. Compounds such as lanthanum hydride (LaH₃) are diamagnetic and also semiconductors. Most RH₂ compounds, where R is a trivalent rare earth, are either antiferromagnetic or ferromagnetic. However, the divalent europium dihydride EuH₂ is ferromagnetic at 25 K (-248 °C or -415 °F).

In 2001, a phenomenon known as the switchable mirror effect was described for YHₓ or LaHₓ when x approaches 3:
When a thin film of YHₓ or LaHₓ, protected by a thin palladium metal layer, was hydrogenated, the metal phase with x < 2.9 remained reflective, but became transparent as x approached 3.0. By reducing the hydrogen content, the transparent YHₓ (LaHₓ) film reverted back to a mirror. Since then, a number of other hydrogen-containing switchable mirror materials have been developed — including all trivalent rare earth elements, R–magnesium alloys, and magnesium alloys with vanadium, manganese, iron, cobalt, and nickel additives.

The three most important stoichiometries in the halide systems (X = fluorine, chlorine, bromine, and iodine) are trihalides (RX₃), tetrahalides (RX₄), and reduced halides (RXᵧ with y < 3). The trihalides are known for all rare earths except europium. The only known tetrahalides are the RF₄ phases, where R = cerium, praseodymium, and terbium. The dihalides RX₂, where R = samarium, europium, and ytterbium, have long been known; they are stable compounds and easy to prepare. A number of supposed RX₂ compounds have been reported in the literature for most lanthanides, but subsequent studies have shown that these phases were actually ternary compounds stabilized by interstitial impurities such as hydrogen and carbon. This also applies to other reduced halides (2 < x < 3) — e.g., Gd₂Cl₃.

The RF₃ compounds behave very differently from RCl₃, RBr₃, and RI₃. The fluorides are air-stable, non-hygroscopic (i.e., they do not readily absorb water), insoluble in water and mild acids. The fluorides are produced by converting the oxide into RF₃ via reaction with ammonium bifluoride (NH₄HF₂). The RF₃ phases crystallize in two modifications — the trigonal LaF₃ structure (lanthanum to promethium) and the orthorhombic YF₃ structure (samarium to lutetium and yttrium).

The RF₃ compounds are alloyed with other earth-free fluorides — namely ZrF₄ and ZrF₄-BaF₂ — classified as heavy metal fluoride glasses (HMFGs). Many HMFGs are transparent from the ultraviolet to the mid-infrared range and are used as fiber optic materials for sensors, communication, windows, light guides, and prisms. These materials are characterized by good glass-forming properties, chemical resistance, and thermal stability. One of the most important compositions is 57% ZrF₄, 18% BaF₂, 3% LaF₃, 4% AlF₃, and 17% NaF (with slight deviations from these percentages) and is known as ZBLAN.

The compounds RCl₃, RBr₃, and RI₃ behave quite differently from the RF₃ compounds, as they are hygroscopic and rapidly hydrolyze in air. As expected, the RX₃ compounds (X = chlorine, bromine, and iodine) are highly soluble in water. The trihalides are generally prepared from the respective oxide by dissolving R₂O₃ in an HX solution and crystallizing the RX₃ compound from the solution by dehydration. The dehydration process must be carefully controlled, as the RX₃ phase otherwise contains some oxygen. The dehydration becomes increasingly difficult with higher atomic numbers of the lanthanide and also depends on X.

The RCl₃ and RBr₃ compounds exhibit three different crystal structures from the light to the middle and heavy lanthanides (including YX₃), while the RI₃ compounds show only two different crystal structures along the series.

Among the many intermetallic compounds formed by the rare earth elements, some stand out due to unusual applications or interesting scientific facts. Six of these applications are explained below.

The best-known intermetallic compound of the rare earth elements is Nd₂Fe₁₄B, which is ferromagnetic and becomes the hardest known magnetic material after appropriate heat treatment. Therefore, this intermetallic compound is used as a permanent magnet in many applications. The main areas of use include electric motors (for example, modern cars contain up to 35 electric motors), spindles for computer hard drives, speakers for mobile phones and portable media players, directly driven wind turbines (to avoid gearboxes), actuators, and MRI devices. SmCo₅ and Sm₂Co₁₇ are also permanent magnets. Both have higher Curie temperatures (magnetic ordering) than Nd₂Fe₁₄B but are not quite as magnetically strong.

Another important compound, serving as a hydrogen absorber for green energy, is LaNi₅. It is a key component in nickel-metal hydride batteries used in hybrid and fully electric vehicles. LaNi₅ absorbs and releases hydrogen quite easily near room temperature and can absorb six hydrogen atoms per LaNi₅ molecule at low hydrogen pressure. This represents one of the most significant markets for rare earth elements.

The next compound, lanthanum hexaboride (LaB₆), has only a small market but is crucial for electron microscopy. It has an extremely high melting point (>2500 °C or >4532 °F), a low vapor pressure, and excellent thermal emission properties, making it the material of choice for electron guns in electron microscopes.

The metallic compound PrNi₅ also has a small market but made history by setting a world record. It shares the same crystal structure as LaNi₅ and remains non-magnetic down to the microkelvin range (0.000001 K, equivalent to -273.14999999 °C or -459.669998 °F). This makes it an excellent candidate for cooling via nuclear adiabatic demagnetization.

PrNi₅ was used as the first stage, together with copper as the second stage, to achieve an operational temperature of 0.000027 K (-273.149973 °C or -459.669951 °F). At this temperature, experimental measurements could be performed for the first time on materials other than the magnetic refrigerant itself.

For this reason, PrNi₅ is widely used as a refrigerant in many low-temperature laboratories around the world.

All magnetically ordered materials exposed to an applied magnetic field expand or contract depending on the orientation of the sample relative to the magnetic field direction. This phenomenon is called magnetostriction. For most materials, the effect is quite small, but in 1971 it was discovered that TbFe₂ exhibits a very large magnetostriction—about 1,000 times greater than normal magnetic substances. Today, one of the best commercial magnetostrictive materials is Tb₀.₃Dy₀.₇Fe₁.₉, known as Terfenol D, which is used in devices such as sonar systems, micro-positioners, and fluid control valves.

Magnetostriction can also manifest as audible humming or clicking noises.

Magnetic materials that undergo a magnetic transition typically heat up when exposed to an increasing magnetic field (although a few substances can also cool down). When the field is removed, the opposite occurs. This phenomenon is called the magnetocaloric effect (MCE). In 1997, American materials scientists Vitalij K. Pecharsky and Karl A. Gschneidner Jr. discovered that Gd₅(Si₂Ge₂) exhibits an exceptionally large MCE, referred to as the giant magnetocaloric effect (GMCE). The GMCE arises from a simultaneous crystallographic and magnetic transition when Gd₅(Si₂Ge₂) becomes magnetically ordered, which can be controlled by varying the magnetic field. This discovery gave a major impetus to the potential use of GMCE for magnetic refrigeration. Since then, about six more GMCE materials have been discovered, with one of the most promising being another lanthanide compound, La(FeₓSiₓ)₁₃.

Magnetic refrigeration has not yet been commercialized, but many test devices and prototype cooling machines have been built. Should magnetic refrigeration become economically viable in the future, it could reduce energy consumption and cooling costs by about 20 percent. This technology is also more environmentally friendly because it eliminates harmful, ozone-depleting (greenhouse) gases still used in today’s gas-compression refrigeration technology.

The rare earth elements react with many organic molecules to form complexes. Many of these were used in the 1950s and 1960s as aids to help separate the rare earth elements through ion exchange or solvent extraction processes. Since then, they have increasingly been studied as standalone materials and for other applications such as phosphors, lasers, and nuclear magnetic resonance. Magnetic resonance imaging (MRI) is an important medical technology used to examine patients. The key materials for enhancing MRI images are gadolinium-based complexes, such as Gd(dtpa)-1, where dtpa stands for diethylenetriamine-N,N,N′,N′,N′,N″-pentaacetate. Millions of doses are administered annually worldwide as contrast agents. Each vial contains 1.57 grams (0.06 ounces) of gadolinium.

As a group of elements, the rare earths possess a large number of isotopes. Scandium, for example, has 27 known isotopes (1 of which is stable), cerium has 42 (4 stable), and on average, there are about 35 isotopes per rare earth element, not counting nuclear isomers. Elements with odd atomic numbers typically have only one or at most two stable (or very long-lived) isotopes, while those with even atomic numbers have between four and seven stable isotopes. Promethium has no stable isotopes at all; Promethium-145 has the longest half-life, at 17.7 years. Some of the unstable isotopes are weakly radioactive and have extremely long half-lives.

The unstable radioactive isotopes are produced in various ways — for example, through fission, neutron bombardment, radioactive decay of neighboring elements, and bombardment of adjacent elements with charged particles. The lanthanide isotopes are of particular interest to nuclear scientists because they provide a rich field for exploring nuclear theory, especially since many of these nuclei are non-spherical — a characteristic that significantly affects nuclear stability. When either the protons or the neutrons complete a nuclear shell (i.e., reach a specific fixed value), the nucleus becomes exceptionally stable. The number of protons or neutrons required to complete a shell is known as a magic number. These values are 2, 8, 20, 28, 50, 82, and 126. In the lanthanide series, the magic number for neutrons is 82.

Some lanthanide elements have large capture cross sections for thermal neutrons, meaning they absorb a high number of neutrons per unit area. The cross section values for naturally occurring samarium, europium, gadolinium, and dysprosium are 5,600b, 4,300b, 49,000b, and 1,100b respectively (barns, a unit of measurement in nuclear engineering: 1b = 10⁻²⁴ cm² = 10⁻²⁸ m²). Some of these elements are therefore used in control rods that regulate or shut down nuclear reactors. Europium and dysprosium are used to control reactor operation, while gadolinium is used for emergency shutdowns. Naturally occurring europium absorbs 4.0 neutrons per atom, dysprosium 2.4, samarium 0.4, and gadolinium 0.3 before becoming ineffective as neutron absorbers. This is why europium and dysprosium are preferred for control rods, rather than samarium or gadolinium.

In addition, lanthanides can be used as burnable neutron absorbers to help maintain reactor reactivity at a nearly constant level. When uranium undergoes fission, it produces some fission products that absorb neutrons and tend to slow down the nuclear reaction. When the appropriate amounts of lanthanides are present, they burn up at roughly the same rate as the other absorbers. Most other rare earth elements are relatively transparent to thermal neutrons, with cross sections ranging from 0.7b for cerium to 170b for erbium.

Some of the most important radionuclides include yttrium-90 (cancer therapy), cerium-144 and promethium-147 (industrial measuring instruments and power sources), gadolinium-153 (industrial X-ray fluorescence), and ytterbium-169 (portable X-ray source).

Rare earth elements have low toxicities and can be handled safely with normal precautions. Solutions injected into the peritoneum or its organs can cause hyperglycemia (an excess of sugar in the blood), a drop in blood pressure, spleen degeneration, and fatty liver. When injected into muscle tissue, about 75 percent of the rare earth content remains at the injection site, while the rest is distributed to the liver and skeleton. When taken orally, only a small percentage of a rare earth element is absorbed into the body. Organically complexed ions are somewhat more toxic than solids or inorganic solutions. As with most chemicals, dust and vapors should not be inhaled or ingested, splashes in the eyes should be rinsed out, and metal fragments should be removed.