Cerium

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Not to be confused with caesium (also spelled "cesium") or curium.

Cerium, 58Ce
Cerium
Pronunciation/ˈsɪəriəm/ ​(SEER-ee-əm)
Appearancesilvery white
Standard atomic weight Ar°(Ce)
Cerium in the periodic table
Hydrogen Helium
Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon
Sodium Magnesium Aluminium Silicon Phosphorus Sulfur Chlorine Argon
Potassium Calcium Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Zinc Gallium Germanium Arsenic Selenium Bromine Krypton
Rubidium Strontium Yttrium Zirconium Niobium Molybdenum Technetium Ruthenium Rhodium Palladium Silver Cadmium Indium Tin Antimony Tellurium Iodine Xenon
Caesium Barium Lanthanum Cerium Praseodymium Neodymium Promethium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium Hafnium Tantalum Tungsten Rhenium Osmium Iridium Platinum Gold Mercury (element) Thallium Lead Bismuth Polonium Astatine Radon
Francium Radium Actinium Thorium Protactinium Uranium Neptunium Plutonium Americium Curium Berkelium Californium Einsteinium Fermium Mendelevium Nobelium Lawrencium Rutherfordium Dubnium Seaborgium Bohrium Hassium Meitnerium Darmstadtium Roentgenium Copernicium Nihonium Flerovium Moscovium Livermorium Tennessine Oganesson
–
↑
Ce
↓
Th
lanthanum ← cerium → praseodymium
kJ/mol
Heat of vaporization398 kJ/mol
Molar heat capacity26.94 J/(mol·K)
Vapor pressure
P (Pa) 1 10 100 1 k 10 k 100 k
at T (K) 1992 2194 2442 2754 3159 3705
Atomic properties
Jöns Jakob Berzelius, Wilhelm Hisinger (1803)
First isolationCarl Gustaf Mosander (1838)
Isotopes of cerium
Main isotopes[7] Decay
abun­dance half-life (t1/2) mode pro­duct
134Ce synth 3.16 d ε
134La
136Ce 0.186%
stable
138Ce 0.251% stable
139Ce synth 137.640 d ε
139La
140Ce 88.4% stable
141Ce synth 32.501 d
β−
141Pr
142Ce 11.1% stable
143Ce synth 33.039 h β−
143Pr
144Ce synth 284.893 d β−
144Pr
 Category: Cerium
| references

Cerium is a

ductile, and silvery-white metal that tarnishes when exposed to air. Cerium is the second element in the lanthanide series, and while it often shows the oxidation state of +3 characteristic of the series, it also has a stable +4 state that does not oxidize water. It is also considered one of the rare-earth elements
. Cerium has no known biological role in humans but is not particularly toxic, except with intense or continued exposure.

Despite always occurring in combination with the other rare-earth elements in minerals such as those of the

ppm
.

Cerium was the first of the lanthanides to be discovered, in

pyrophoric properties. Cerium-doped YAG phosphor is used in conjunction with blue light-emitting diodes
to produce white light in most commercial white LED light sources.

Characteristics

Physical

Cerium is the second element of the

ductile metal with a hardness similar to that of silver.[8] Its 58 electrons are arranged in the configuration [Xe]4f15d16s2, of which the four outer electrons are valence electrons.[9] The 4f, 5d, and 6s energy levels are very close to each other, and the transfer of one electron to the 5d shell is due to strong interelectronic repulsion in the compact 4f shell. This effect is overwhelmed when the atom is positively ionised; thus Ce2+ on its own has instead the regular configuration [Xe]4f2, although in some solid solutions it may be [Xe]4f15d1.[10] Most lanthanides can use only three electrons as valence electrons, as afterwards the remaining 4f electrons are too strongly bound: cerium is an exception because of the stability of the empty f-shell in Ce4+ and the fact that it comes very early in the lanthanide series, where the nuclear charge is still low enough until neodymium to allow the removal of the fourth valence electron by chemical means.[11]

Cerium has a variable

electronic structure. The energy of the 4f electron is nearly the same as that of the outer 5d and 6s electrons that are delocalized in the metallic state, and only a small amount of energy is required to change the relative occupancy of these electronic levels. This gives rise to dual valence states. For example, a volume change of about 10% occurs when cerium is subjected to high pressures or low temperatures. It appears that the valence changes from about 3 to 4 when it is cooled or compressed.[12]

Chemical properties of the element

Like the other lanthanides, cerium metal is a good

pyrophoric:[14]

Ce + O2 → CeO2

Being highly electropositive, cerium reacts with water. The reaction is slow with cold water but speeds up with increasing temperature, producing cerium(III) hydroxide and hydrogen gas:[15]

2 Ce + 6 H2O → 2 Ce(OH)3 + 3 H2

Allotropes

Cerium phase diagram
Phase diagram of cerium

Four allotropic forms of cerium are known to exist at standard pressure and are given the common labels of α to δ:[16]

  • The high-temperature form, δ-cerium, has a bcc (
    body-centered cubic
    ) crystal structure and exists above 726 °C.
  • The stable form below 726 °C to approximately room temperature is γ-cerium, with an fcc (
    face-centered cubic
    ) crystal structure.
  • The DHCP (double
    hexagonal close-packed
    ) form β-cerium is the equilibrium structure approximately from room temperature to −150 °C.
  • The fcc form α-cerium is stable below about −150 °C; it has a density of 8.16 g/cm3.
  • Other solid phases occurring only at high pressures are shown on the phase diagram.
  • Both γ and β forms are quite stable at room temperature, although the equilibrium transformation temperature is estimated at 75 °C.[16]

At lower temperatures the behavior of cerium is complicated by the slow rates of transformation. Transformation temperatures are subject to substantial hysteresis and values quoted here are approximate. Upon cooling below −15 °C, γ-cerium starts to change to β-cerium, but the transformation involves a volume increase and, as more β forms, the internal stresses build up and suppress further transformation.[16] Cooling below approximately −160 °C will start formation of α-cerium but this is only from remaining γ-cerium. β-cerium does not significantly transform to α-cerium except in the presence of stress or deformation.[16] At atmospheric pressure, liquid cerium is more dense than its solid form at the melting point.[8][17][18]

Isotopes

Main article: Isotopes of cerium

Naturally occurring cerium is made up of four isotopes: 136Ce (0.19%), 138Ce (0.25%), 140Ce (88.4%), and 142Ce (11.1%). All four are

stable isotope. None of these decay modes have yet been observed, though the double beta decay of 136Ce, 138Ce, and 142Ce have been experimentally searched for. The current experimental limits for their half-lives are:[19]

136Ce: >3.8×1016 y
138Ce: >5.7×1016 y
142Ce: >5.0×1016 y

All other cerium isotopes are

inverse beta decay or electron capture to isotopes of lanthanum, while that of the heavier isotopes is beta decay to isotopes of praseodymium.[19] Some isotopes of neodymium can alpha decay or are predicted to decay to isotopes of cerium.[20]

The rarity of the proton-rich 136Ce and 138Ce is explained by the fact that they cannot be made in the most common processes of

primordial radionuclides 138La and 144Nd, respectively.[19]

Compounds

Cerium exists in two main oxidation states, Ce(III) and Ce(IV). This pair of adjacent oxidation states dominates several aspects of the chemistry of this element. Cerium(IV) aqueous solutions may be prepared by reacting cerium(III) solutions with the strong oxidizing agents

peroxodisulfate or bismuthate. The value of E⦵(Ce4+/Ce3+) varies widely depending on conditions due to the relative ease of complexation and hydrolysis with various anions, although +1.72 V is representative. Cerium is the only lanthanide which has important aqueous and coordination chemistry in the +4 oxidation state.[13]

Halides

Cerium forms all four trihalides CeX3 (X = F, Cl, Br, I) usually by reaction of the oxides with the hydrogen halides. The anhydrous halides are pale-colored, paramagnetic, hygroscopic solids. Upon hydration, the trihalides convert to complexes containing aquo complexes [Ce(H2O)8-9]3+. Unlike most lanthanides, Ce forms a tetrafluoride, a white solid. It also forms a bronze-colored diiodide, which has metallic properties.[22] Aside from the binary halide phases, a number of anionic halide complexes are known. The fluoride gives the Ce(IV) derivatives CeF4−8 and CeF2−6. The chloride gives the orange CeCl2−6.[13]

Oxides and chalcogenides

nonstoichiometric chalcogenides are also known, along with the trivalent Ce2Z3 (Z = S, Se, Te). The monochalcogenides CeZ conduct electricity and would better be formulated as Ce3+Z2−e−. While CeZ2 are known, they are polychalcogenides with cerium(III): cerium(IV) derivatives of S, Se, and Te are unknown.[23]

Cerium(IV) complexes

Ceric ammonium nitrate
Ceric ammonium nitrate

The compound

volumetric analysis in cerimetric titrations.[27]

A white LED in operation
A white LED in operation: the diode produces monochromatic blue light but the Ce:YAG phosphor converts some of it into yellow light; the combination is perceived as white by the human eye.

Due to ligand-to-metal charge transfer, aqueous cerium(IV) ions are orange-yellow.[28] Aqueous cerium(IV) is metastable in water[29] and is a strong oxidizing agent that oxidizes hydrochloric acid to give chlorine gas.[13] In the Belousov–Zhabotinsky reaction, cerium oscillates between the +4 and +3 oxidation states to catalyze the reaction.[30]

Organocerium compounds

Organocerium chemistry is similar to that of the other lanthanides, often involving complexes of cyclopentadienyl and cyclooctatetraenyl ligands. Cerocene (Ce(C8H8)2) adopts the uranocene molecular structure.[31] The 4f electron in cerocene, Ce(C
8H
8)
2, is poised ambiguously between being localized and delocalized and this compound is considered intermediate-valent.[32]

organolithium or Grignard reagents, and are more nucleophilic but less basic than their precursors.[33][34]

History

The dwarf planet Ceres.
The dwarf planet and asteroid Ceres, after which cerium is named
Jöns Jakob Berzelius
Jöns Jakob Berzelius, one of the discoverers of cerium

Cerium was discovered in

Jöns Jakob Berzelius and Wilhelm Hisinger, and independently in Germany by Martin Heinrich Klaproth, both in 1803.[35] Cerium was named by Berzelius after the asteroid Ceres, discovered two years earlier.[35][36] The asteroid is itself named after the Roman goddess Ceres, goddess of agriculture, grain crops, fertility and motherly relationships.[35]

Cerium was originally isolated in the form of its oxide, which was named ceria, a term that is still used. The metal itself was too electropositive to be isolated by then-current smelting technology, a characteristic of rare-earth metals in general. After the development of electrochemistry by Humphry Davy five years later, the earths soon yielded the metals they contained. Ceria, as isolated in 1803, contained all of the lanthanides present in the cerite ore from Bastnäs, Sweden, and thus only contained about 45% of what is now known to be pure ceria. It was not until Carl Gustaf Mosander succeeded in removing lanthana and "didymia" in the late 1830s that ceria was obtained pure. Wilhelm Hisinger was a wealthy mine-owner and amateur scientist, and sponsor of Berzelius. He owned and controlled the mine at Bastnäs, and had been trying for years to find out the composition of the abundant heavy gangue rock (the "Tungsten of Bastnäs", which despite its name contained no tungsten), now known as cerite, that he had in his mine.[36] Mosander and his family lived for many years in the same house as Berzelius, and Mosander was undoubtedly persuaded by Berzelius to investigate ceria further.[37][38][39][40]

The element played a role in the

Ames Laboratory).[42] Production of extremely pure cerium in Ames commenced in mid-1944 and continued until August 1945.[42]

Occurrence and production

Cerium is the most abundant of all the lanthanides, making up 66 

cerianite-(Ce),[50][46][44] (Ce,Th)O
2.[51][52][53]

Crystal structure of basntäsite-(Ce).
Crystal structure of bastnäsite-(Ce). Color code: carbon, C, blue-gray; fluorine, F, green; cerium, Ce, white; oxygen, O, red.

Bastnäsite, LnIIICO3F, is usually lacking in thorium and the heavy lanthanides beyond samarium and europium, and hence the extraction of cerium from it is quite direct. First, the bastnäsite is purified, using dilute hydrochloric acid to remove calcium carbonate impurities. The ore is then roasted in the air to oxidize it to the lanthanide oxides: while most of the lanthanides will be oxidized to the sesquioxides Ln2O3, cerium will be oxidized to the dioxide CeO2. This is insoluble in water and can be leached out with 0.5 M hydrochloric acid, leaving the other lanthanides behind.[48]

The procedure for

228Ra, the daughter of 232Th, which is a strong gamma emitter.[48]

Applications

Carl Auer von Welsbach
Carl Auer von Welsbach, who discovered many applications of cerium

Cerium has two main applications, both of which use CeO2. The industrial application of ceria is for polishing, especially chemical-mechanical planarization (CMP). In its other main application, CeO2 is used to decolorize glass. It functions by converting green-tinted ferrous impurities to nearly colorless ferric oxides.[54] Ceria has also been used as a substitute for its radioactive congener thoria, for example in the manufacture of electrodes used in gas tungsten arc welding, where ceria as an alloying element improves arc stability and ease of starting while decreasing burn-off.[55]

Gas mantles and pyrophoric alloys

The first use of cerium was in

thorium oxide produced a much better, though blue, light, and that mixing it with cerium dioxide resulted in a bright white light.[57] Cerium dioxide also acts as a catalyst for the combustion of thorium oxide.[citation needed
]

This resulted in commercial success for von Welsbach and his invention, and created great demand for thorium. Its production resulted in a large amount of lanthanides being simultaneously extracted as by-products.[58] Applications were soon found for them, especially in the pyrophoric alloy known as "mischmetal" composed of 50% cerium, 25% lanthanum, and the remainder being the other lanthanides, that is used widely for lighter flints.[58] Usually iron is added to form the alloy ferrocerium, also invented by von Welsbach.[59] Due to the chemical similarities of the lanthanides, chemical separation is not usually required for their applications, such as the addition of mischmetal to steel as an inclusion modifier to improve mechanical properties, or as catalysts for the cracking of petroleum.[48] This property of cerium saved the life of writer Primo Levi at the Auschwitz concentration camp, when he found a supply of ferrocerium alloy and bartered it for food.[60]

Pigments and phosphors

The photostability of pigments can be enhanced by the addition of cerium, as it provides pigments with lightfastness and prevents clear polymers from darkening in sunlight.[61] An example of a cerium compound used on its own as an inorganic pigment is the vivid red cerium(III) sulfide (cerium sulfide red), which stays chemically inert up to very high temperatures. The pigment is a safer alternative to lightfast but toxic cadmium selenide-based pigments.[36] The addition of cerium oxide to older cathode-ray tube television glass plates was beneficial, as it suppresses the darkening effect from the creation of F-center defects due to the continuous electron bombardment during operation. Cerium is also an essential component as a dopant for phosphors used in CRT TV screens, fluorescent lamps, and later white light-emitting diodes.[62][63] The most commonly used example is cerium(III)-doped yttrium aluminium garnet (Ce:YAG) which emits green to yellow-green light (550–530 nm) and also behaves as a scintillator.[64]

Other uses

Cerium salts, such as the sulfides

aluminium alloys with 6–16 wt.% Ce, to which other elements such as Mg, Ni, Fe and Mn can be added. These Al-Ce alloys have excellent high temperature strength and are suitable for automotive applications e.g. in cylinder heads.[65] Other alloys of cerium include Pu-Ce and Pu-Ce-Co plutonium alloys, which have been used as nuclear fuel.[66]

Other

automotive applications for the lower sesquioxide are as a catalytic converter for the oxidation of CO and NOx emissions in the exhaust gases from motor vehicles.[67][68]

Biological role and precautions

Cerium
Hazards
GHS labelling:[69]
GHS02: Flammable
Danger
H228
P210
NFPA 704 (fire diamond)
NFPA 704 four-colored diamondHealth 2: Intense or continued but not chronic exposure could cause temporary incapacitation or possible residual injury. E.g. chloroformFlammability 0: Will not burn. E.g. waterInstability 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g. liquid nitrogenSpecial hazards (white): no code
2
0
0

The early lanthanides have been found to be essential to some

methanotrophic bacteria living in volcanic mudpots, such as Methylacidiphilum fumariolicum: lanthanum, cerium, praseodymium, and neodymium are about equally effective.[70][71] Cerium is otherwise not known to have biological role in any other organisms, but is not very toxic either; it does not accumulate in the food chain to any appreciable extent.[72][73][74] Because it often occurs together with calcium in phosphate minerals, and bones are primarily calcium phosphate, cerium can accumulate in bones in small amounts that are not considered dangerous.[75]

methanotrophic bacterium Methylacidiphilum fumariolicum SolV, for which lanthanum, cerium, praseodymium, and neodymium alone are about equally effective.[78]

Like all rare-earth metals, cerium is of low to moderate toxicity. A strong reducing agent, it ignites spontaneously in air at 65 to 80 °C. Fumes from cerium fires are toxic. Water should not be used to stop cerium fires, as cerium reacts with water to produce hydrogen gas. Workers exposed to cerium have experienced itching, sensitivity to heat, and skin lesions. Cerium is not toxic when eaten, but animals injected with large doses of cerium have died due to cardiovascular collapse.[36] Cerium is more dangerous to aquatic organisms because it damages cell membranes; this is an important risk because it is not very soluble in water, thus causing contamination of the environment.[36]

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Bibliography
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