# Curium

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Curium, 96Cm
Curium
Pronunciation
Appearancesilvery metallic, glows purple in the dark
Mass number [247]
Curium 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
Gd

Cm

(Upn)
americiumcuriumberkelium
96
Group f-block groups (no number)
Period period 7
Block   f-block
Electron configuration [ Rn ] 5f7 6d1 7s2
Electrons per shell2, 8, 18, 32, 25, 9, 2
Physical properties
Phase at  solid
Melting point 1613  K (1340 °C,2444 °F)
Boiling point 3383 K(3110 °C,5630 °F)
Density (near r.t.)13.51 g/cm3
Heat of fusion 13.85  kJ/mol
Vapor pressure
P (Pa)1101001 k10 k100 k
at T (K)17881982
Atomic properties
Oxidation states +3, +4, +5, [1] +6 [2] (an  amphoteric oxide)
Electronegativity Pauling scale: 1.3
Ionization energies
• 1st: 581 kJ/mol
Atomic radius empirical:174  pm
Covalent radius 169±3 pm
Spectral lines of curium
Other properties
Natural occurrence synthetic
Crystal structure double hexagonal close-packed (dhcp)
Electrical resistivity 1.25 µΩ⋅m [3]
Magnetic ordering antiferromagnetic-paramagnetic transition at 52 K [3]
CAS Number 7440-51-9
History
Namingnamed after Marie Skłodowska-Curie and Pierre Curie
Discovery Glenn T. Seaborg, Ralph A. James, Albert Ghiorso (1944)
Isotopes of curium
Main isotopes Decay
abun­dance half-life (t1/2) mode pro­duct
242Cm syn 162.8 d α 238Pu
SF
34Si...
243Cmsyn29.1 yα 239Pu
ε 243Am
SF
244Cmsyn18.11 yα 240Pu
SF
245Cmsyn8250 yα 241Pu
SF
246Cmsyn4060 yα 242Pu
SF
247Cmsyn1.56×107 yα 243Pu
248Cmsyn3480×105 yα 244Pu
SF
250Cmsyn8300 ySF
α 246Pu
β 250Bk
Category: Curium
| references

Curium is a transuranic, radioactive chemical element with the symbol Cm and atomic number 96. This actinide element was named after eminent scientists Marie and Pierre Curie, both known for their research on radioactivity. Curium was first intentionally made by the team of Glenn T. Seaborg, Ralph A. James, and Albert Ghiorso in 1944, using the cyclotron at Berkeley. They bombarded the newly discovered element plutonium (the isotope 239Pu) with alpha particles. This was then sent to the Metallurgical Laboratory at University of Chicago where a tiny sample of curium was eventually separated and identified. The discovery was kept secret until after the end of World War II. The news was released to the public in November 1947. Most curium is produced by bombarding uranium or plutonium with neutrons in nuclear reactors – one tonne of spent nuclear fuel contains ~20 grams of curium.

## Contents

Curium is a hard, dense, silvery metal with a high melting and boiling point for an actinide. It is paramagnetic at ambient conditions, but becomes antiferromagnetic upon cooling, and other magnetic transitions are also seen in many curium compounds. In compounds, curium usually has valence +3 and sometimes +4; the +3 valence is predominant in solutions. Curium readily oxidizes, and its oxides are a dominant form of this element. It forms strongly fluorescent complexes with various organic compounds, but there is no evidence of its incorporation into bacteria and archaea. If it gets into the human body, curium accumulates in bones, lungs, and liver, where it promotes cancer.

All known isotopes of curium are radioactive and have small critical mass for a nuclear chain reaction. They mostly emit α-particles; radioisotope thermoelectric generators can use the heat from this process, but this is hindered by the rarity and high cost of curium. Curium is used in making heavier actinides and the 238Pu radionuclide for power sources in artificial cardiac pacemakers and RTGs for spacecraft. It served as the α-source in the alpha particle X-ray spectrometers of several space probes, including the Sojourner , Spirit , Opportunity , and Curiosity Mars rovers and the Philae lander on comet 67P/Churyumov–Gerasimenko, to analyze the composition and structure of the surface.

## History

Though curium had likely been produced in previous nuclear experiments as well as the natural nuclear fission reactor at Oklo, Gabon, it was first intentionally synthesized, isolated and identified in 1944, at University of California, Berkeley, by Glenn T. Seaborg, Ralph A. James, and Albert Ghiorso. In their experiments, they used a 60-inch (150 cm) cyclotron. [4]

Curium was chemically identified at the Metallurgical Laboratory (now Argonne National Laboratory), University of Chicago. It was the third transuranium element to be discovered even though it is the fourth in the series – the lighter element americium was still unknown. [5] [6]

The sample was prepared as follows: first plutonium nitrate solution was coated on a platinum foil of ~0.5 cm2 area, the solution was evaporated and the residue was converted into plutonium(IV) oxide (PuO2) by annealing. Following cyclotron irradiation of the oxide, the coating was dissolved with nitric acid and then precipitated as the hydroxide using concentrated aqueous ammonia solution. The residue was dissolved in perchloric acid, and further separation was done by ion exchange to yield a certain isotope of curium. The separation of curium and americium was so painstaking that the Berkeley group initially called those elements pandemonium (from Greek for all demons or hell) and delirium (from Latin for madness). [7] [8]

Curium-242 was made in July–August 1944 by bombarding 239Pu with α-particles to produce curium with the release of a neutron:

${\displaystyle {\ce {^{239}_{94}Pu + ^{4}_{2}He -> ^{242}_{96}Cm + ^{1}_{0}n}}}$

Curium-242 was unambiguously identified by the characteristic energy of the α-particles emitted during the decay:

${\displaystyle {\ce {^{242}_{96}Cm -> ^{238}_{94}Pu + ^{4}_{2}He}}}$

The half-life of this alpha decay was first measured as 150 days and then corrected to 162.8 days. [9]

Another isotope 240Cm was produced in a similar reaction in March 1945:

${\displaystyle {\ce {^{239}_{94}Pu + ^{4}_{2}He -> ^{240}_{96}Cm + 3^{1}_{0}n}}}$

The α-decay half-life of 240Cm was correctly determined as 26.7 days. [9]

The discovery of curium and americium in 1944 was closely related to the Manhattan Project, so the results were confidential and declassified only in 1945. Seaborg leaked the synthesis of the elements 95 and 96 on the U.S. radio show for children, the Quiz Kids, five days before the official presentation at an American Chemical Society meeting on November 11, 1945, when one listener asked if any new transuranic element beside plutonium and neptunium had been discovered during the war. [7] The discovery of curium (242Cm and 240Cm), its production, and its compounds was later patented listing only Seaborg as the inventor. [10]

The element was named after Marie Curie and her husband Pierre Curie, who are known for discovering radium and for their work in radioactivity. It followed the example of gadolinium, a lanthanide element above curium in the periodic table, which was named after the explorer of rare-earth elements Johan Gadolin: [11]

"As the name for the element of atomic number 96 we should like to propose "curium", with symbol Cm. The evidence indicates that element 96 contains seven 5f electrons and is thus analogous to the element gadolinium, with its seven 4f electrons in the regular rare earth series. On this basis element 96 is named after the Curies in a manner analogous to the naming of gadolinium, in which the chemist Gadolin was honored." [5]

The first curium samples were barely visible, and were identified by their radioactivity. Louis Werner and Isadore Perlman made the first substantial sample of 30 µg curium-242 hydroxide at University of California, Berkeley in 1947 by bombarding americium-241 with neutrons. [12] [13] [14] Macroscopic amounts of curium(III) fluoride were obtained in 1950 by W. W. T. Crane, J. C. Wallmann and B. B. Cunningham. Its magnetic susceptibility was very close to that of GdF3 providing the first experimental evidence for the +3 valence of curium in its compounds. [12] Curium metal was produced only in 1951 by reduction of CmF3 with barium. [15] [16]

## Characteristics

### Physical

A synthetic, radioactive element, curium is a hard, dense metal with a silvery-white appearance and physical and chemical properties resembling gadolinium. Its melting point of 1344 °C is significantly higher than that of the previous elements neptunium (637 °C), plutonium (639 °C) and americium (1176 °C). In comparison, gadolinium melts at 1312 °C. Curium boils at 3556 °C. With a density of 13.52 g/cm3, curium is lighter than neptunium (20.45 g/cm3) and plutonium (19.8 g/cm3), but heavier than most other metals. Of two crystalline forms of curium, α-Cm is more stable at ambient conditions. It has a hexagonal symmetry, space group P63/mmc, lattice parameters a = 365  pm and c = 1182 pm, and four formula units per unit cell. [17] The crystal consists of double-hexagonal close packing with the layer sequence ABAC and so is isotypic with α-lanthanum. At pressure >23  GPa, at room temperature, α-Cm becomes β-Cm, which has face-centered cubic symmetry, space group Fm3m and lattice constant a = 493 pm. [17] On further compression to 43 GPa, curium becomes an orthorhombic γ-Cm structure similar to α-uranium, with no further transitions observed up to 52 GPa. These three curium phases are also called Cm I, II and III. [18] [19]

Curium has peculiar magnetic properties. Its neighbor element americium shows no deviation from Curie-Weiss paramagnetism in the entire temperature range, but α-Cm transforms to an antiferromagnetic state upon cooling to 65–52 K, [20] [21] and β-Cm exhibits a ferrimagnetic transition at ~205 K. Curium pnictides show ferromagnetic transitions upon cooling: 244CmN and 244CmAs at 109 K, 248CmP at 73 K and 248CmSb at 162 K. The lanthanide analog of curium, gadolinium, and its pnictides, also show magnetic transitions upon cooling, but the transition character is somewhat different: Gd and GdN become ferromagnetic, and GdP, GdAs and GdSb show antiferromagnetic ordering. [22]

In accordance with magnetic data, electrical resistivity of curium increases with temperature – about twice between 4 and 60 K – and then is nearly constant up to room temperature. There is a significant increase in resistivity over time (~10 µΩ·cm/h) due to self-damage of the crystal lattice by alpha decay. This makes uncertain the true resistivity of curium (~125 µΩ·cm). Curium's resistivity is similar to that of gadolinium, and the actinides plutonium and neptunium, but significantly higher than that of americium, uranium, polonium and thorium. [3] [23]

Under ultraviolet illumination, curium(III) ions show strong and stable yellow-orange fluorescence with a maximum in the range of 590–640 nm depending on their environment. [24] The fluorescence originates from the transitions from the first excited state 6D7/2 and the ground state 8S7/2. Analysis of this fluorescence allows monitoring interactions between Cm(III) ions in organic and inorganic complexes. [25]

### Chemical

Curium ion in solution almost always has a +3 oxidation state, the most stable oxidation state for curium. [26] A +4 oxidation state is seen mainly in a few solid phases, such as CmO2 and CmF4. [27] [28] Aqueous curium(IV) is only known in the presence of strong oxidizers such as potassium persulfate, and is easily reduced to curium(III) by radiolysis and even by water itself. [29] Chemical behavior of curium is different from the actinides thorium and uranium, and is similar to americium and many lanthanides. In aqueous solution, the Cm3+ ion is colorless to pale green; [30] Cm4+ ion is pale yellow. [31] The optical absorption of Cm3+ ion contains three sharp peaks at 375.4, 381.2 and 396.5 nm and their strength can be directly converted into the concentration of the ions. [32] The +6 oxidation state has only been reported once in solution in 1978, as the curyl ion (CmO2+
2
): this was prepared from beta decay of americium-242 in the americium(V) ion 242
AmO+
2
. [2] Failure to get Cm(VI) from oxidation of Cm(III) and Cm(IV) may be due to the high Cm4+/Cm3+ ionization potential and the instability of Cm(V). [29]

Curium ions are hard Lewis acids and thus form most stable complexes with hard bases. [33] The bonding is mostly ionic, with a small covalent component. [34] Curium in its complexes commonly exhibits a 9-fold coordination environment, with a tricapped trigonal prismatic molecular geometry. [35]

### Isotopes

About 19 radioisotopes and 7 nuclear isomers, 233Cm to 251Cm, are known; none are stable. The longest half-lives are 15.6 million years (247Cm) and 348,000 years (248Cm). Other long-lived ones are 245Cm (8500 years), 250Cm (8300 years) and 246Cm (4760 years). Curium-250 is unusual: it mostly (~86%) decays by spontaneous fission. The most commonly used isotopes are 242Cm and 244Cm with the half-lives 162.8 days and 18.1 years, respectively. [9]

Thermal neutron cross sections (barns) [36]
242Cm243Cm244Cm245Cm246Cm247Cm
Fission56171.0421450.1481.90
Capture1613015.203691.2257
C/F ratio3.200.2114.620.178.710.70
LEU spent nuclear fuel 20 years after 53 MWd/kg burnup [37]
3 common isotopes513700390
Fast-neutron reactor MOX fuel (avg 5 samples, burnup 66–120 GWd/t) [38]
Total curium 3.09×10−3%27.64%70.16%2.166%0.0376%0.000928%
 Isotope 242Cm 243Cm 244Cm 245Cm 246Cm 247Cm 248Cm 250Cm Critical mass, kg 25 7.5 33 6.8 39 7 40.4 23.5

All isotopes 242Cm-248Cm, and 250Cm, undergo a self-sustaining nuclear chain reaction and thus in principle can be a nuclear fuel in a reactor. As in most transuranic elements, nuclear fission cross section is especially high for the odd-mass curium isotopes 243Cm, 245Cm and 247Cm. These can be used in thermal-neutron reactors, whereas a mixture of curium isotopes is only suitable for fast breeder reactors since the even-mass isotopes are not fissile in a thermal reactor and accumulate as burn-up increases. [39] The mixed-oxide (MOX) fuel, which is to be used in power reactors, should contain little or no curium because neutron activation of 248Cm will create californium. Californium is a strong neutron emitter, and would pollute the back end of the fuel cycle and increase the dose to reactor personnel. Hence, if minor actinides are to be used as fuel in a thermal neutron reactor, the curium should be excluded from the fuel or placed in special fuel rods where it is the only actinide present. [40]

The adjacent table lists the critical masses for curium isotopes for a sphere, without moderator or reflector. With a metal reflector (30 cm of steel), the critical masses of the odd isotopes are about 3–4 kg. When using water (thickness ~20–30 cm) as the reflector, the critical mass can be as small as 59 gram for 245Cm, 155 gram for 243Cm and 1550 gram for 247Cm. There is significant uncertainty in these critical mass values. While it is usually on the order of 20%, the values for 242Cm and 246Cm were listed as large as 371 kg and 70.1 kg, respectively, by some research groups. [39] [42]

Curium is not currently used as nuclear fuel due to its low availability and high price. [43] 245Cm and 247Cm have very small critical mass and so could be used in tactical nuclear weapons, but none are known to have been made. Curium-243 is not suitable for such, due to its short half-life and strong α emission, which would cause excessive heat. [44] Curium-247 would be highly suitable due to its long half-life, which is 647 times longer than plutonium-239 (used in many existing nuclear weapons).

### Occurrence

The longest-lived isotope, 247Cm, has half-life 15.6 million years; so any primordial curium, that is, present on Earth when it formed, should have decayed by now. Its past presence as an extinct radionuclide is detectable as an excess of its primordial, long-lived daughter 235U. [45] Traces of curium may occur naturally in uranium minerals due to neutron capture and beta decay, though this has not been confirmed. [46] [47] Traces of 247Cm are also probably brought to Earth in cosmic rays, but again this has not been confirmed. [48]

Curium is made artificially in small amounts for research purposes. It also occurs as one of the waste products in spent nuclear fuel. [49] [50] Curium is present in nature in some areas used for nuclear weapons testing. [51] Analysis of the debris at the test site of the United States' first thermonuclear weapon, Ivy Mike, (1 November 1952, Enewetak Atoll), besides einsteinium, fermium, plutonium and americium also revealed isotopes of berkelium, californium and curium, in particular 245Cm, 246Cm and smaller quantities of 247Cm, 248Cm and 249Cm. [52]

Atmospheric curium compounds are poorly soluble in common solvents and mostly adhere to soil particles. Soil analysis revealed about 4,000 times higher concentration of curium at the sandy soil particles than in water present in the soil pores. An even higher ratio of about 18,000 was measured in loam soils. [53]

The transuranium elements from americium to fermium, including curium, occurred naturally in the natural nuclear fission reactor at Oklo, but no longer do so. [54]

Curium, and other non-primordial actinides, have also been detected in the spectrum of Przybylski's Star. [55]

## Synthesis

### Isotope preparation

Curium is made in small amounts in nuclear reactors, and by now only kilograms of 242Cm and 244Cm have been accumulated, and grams or even milligrams for heavier isotopes. Hence the high price of curium, which has been quoted at 160–185 USD per milligram, [12] with a more recent estimate at US$2,000/g for 242Cm and US$170/g for 244Cm. [56] In nuclear reactors, curium is formed from 238U in a series of nuclear reactions. In the first chain, 238U captures a neutron and converts into 239U, which via β decay transforms into 239Np and 239Pu.

${\displaystyle {\ce {^{238}_{92}U->[{\ce {(n,\gamma )}}]{^{239}_{92}U}->[\beta ^{-}][23.5\ {\ce {min}}]_{93}^{239}Np->[\beta ^{-}][2.3565\ {\ce {d}}]_{94}^{239}Pu}}}$(the times are half-lives).

(1)

Further neutron capture followed by β-decay gives americium (241Am) which further becomes 242Cm:

${\displaystyle {\ce {^{239}_{94}Pu->[{\ce {2(n,\gamma )}}]_{94}^{241}Pu->[\beta ^{-}][14.35\ {\ce {yr}}]{^{241}_{95}Am}->[{\ce {(n,\gamma )}}]_{95}^{242}Am->[\beta ^{-}][16.02{\ce {h}}]_{96}^{242}Cm}}}$.

(2)

For research purposes, curium is obtained by irradiating not uranium but plutonium, which is available in large amounts from spent nuclear fuel. A much higher neutron flux is used for the irradiation that results in a different reaction chain and formation of 244Cm: [6]

${\displaystyle {\ce {^{239}_{94}Pu->[{\ce {4(n,\gamma )}}]_{94}^{243}Pu->[\beta ^{-}][4.956\ {\ce {h}}]_{95}^{243}Am->[({\ce {n}},\gamma )]_{95}^{244}Am->[\beta ^{-}][10.1{\ce {h}}]_{96}^{244}Cm->[\alpha ][18.11\ {\ce {yr}}]_{94}^{240}Pu}}}$

(3)

Curium-244 alpha decays to 240Pu, but it also absorbs neutrons, hence a small amount of heavier curium isotopes. Of those, 247Cm and 248Cm are popular in scientific research due to their long half-lives. But the production rate of 247Cm in thermal neutron reactors is low because it is prone to fission due to thermal neutrons. [57] Synthesis of 250Cm by neutron capture is unlikely due to the short half-life of the intermediate 249Cm (64 min), which β decays to the berkelium isotope 249Bk. [57]

${\displaystyle {\ce {^{\mathit {A}}_{96}Cm{}+_{0}^{1}n->_{96}^{{\mathit {A}}+1}Cm{}+\gamma }}\ ({\text{for }}244\leq A\leq 248)}$

(4)

The above cascade of (n,γ) reactions gives a mix of different curium isotopes. Their post-synthesis separation is cumbersome, so a selective synthesis is desired. Curium-248 is favored for research purposes due to its long half-life. The most efficient way to prepare this isotope is by α-decay of the californium isotope 252Cf, which is available in relatively large amounts due to its long half-life (2.65 years). About 35–50 mg of 248Cm is produced thus, per year. The associated reaction produces 248Cm with isotopic purity of 97%. [57]

${\displaystyle {\begin{matrix}{}\\{\ce {^{252}_{98}Cf ->[\alpha][2.645\ {\ce {yr}}] ^{248}_{96}Cm}}\\{}\end{matrix}}}$

(5)

Another isotope, 245Cm, can be obtained for research, from α-decay of 249Cf; the latter isotope is produced in small amounts from β-decay of 249 Bk.

${\displaystyle {\ce {^{249}_{97}Bk ->[\beta^-][330\ {\ce {d}}] ^{249}_{98}Cf ->[\alpha][351\ {\ce {yr}}] ^{245}_{96}Cm}}}$

(6)

### Metal preparation

Most synthesis routines yield a mix of actinide isotopes as oxides, from which a given isotope of curium needs to be separated. An example procedure could be to dissolve spent reactor fuel (e.g. MOX fuel) in nitric acid, and remove the bulk of the uranium and plutonium using a PUREX (Plutonium – URanium EXtraction) type extraction with tributyl phosphate in a hydrocarbon. The lanthanides and the remaining actinides are then separated from the aqueous residue (raffinate) by a diamide-based extraction to give, after stripping, a mixture of trivalent actinides and lanthanides. A curium compound is then selectively extracted using multi-step chromatographic and centrifugation techniques with an appropriate reagent. [58] Bis-triazinyl bipyridine complex has been recently proposed as such reagent which is highly selective to curium. [59] Separation of curium from the very chemically similar americium can also be done by treating a slurry of their hydroxides in aqueous sodium bicarbonate with ozone at elevated temperature. Both americium and curium are present in solutions mostly in the +3 valence state; americium oxidizes to soluble Am(IV) complexes, but curium stays unchanged and so can be isolated by repeated centrifugation. [60]

Metallic curium is obtained by reduction of its compounds. Initially, curium(III) fluoride was used for this purpose. The reaction was done in an environment free of water and oxygen, in an apparatus made of tantalum and tungsten, using elemental barium or lithium as reducing agents. [6] [15] [61] [62] [63]

${\displaystyle \mathrm {CmF_{3}\ +\ 3\ Li\ \longrightarrow \ Cm\ +\ 3\ LiF} }$

Another possibility is reduction of curium(IV) oxide using a magnesium-zinc alloy in a melt of magnesium chloride and magnesium fluoride. [64]

## Compounds and reactions

### Oxides

Curium readily reacts with oxygen forming mostly Cm2O3 and CmO2 oxides, [51] but the divalent oxide CmO is also known. [65] Black CmO2 can be obtained by burning curium oxalate (Cm
2
(C
2
O
4
)
3
), nitrate (Cm(NO
3
)
3
), or hydroxide in pure oxygen. [28] [66] Upon heating to 600–650 °C in vacuum (about 0.01 Pa), it transforms into the whitish Cm2O3: [28] [67]

${\displaystyle {\ce {4CmO2 ->[\Delta T] 2Cm2O3 + O2}}}$.

Or, Cm2O3 can be obtained by reducing CmO2 with molecular hydrogen: [68]

${\displaystyle {\ce {2CmO2 + H2 -> Cm2O3 + H2O}}}$

Also, a number of ternary oxides of the type M(II)CmO3 are known, where M stands for a divalent metal, such as barium. [69]

Thermal oxidation of trace quantities of curium hydride (CmH2–3) has been reported to give a volatile form of CmO2 and the volatile trioxide CmO3, one of two known examples of the very rare +6 state for curium. [2] Another observed species was reported to behave similar to a supposed plutonium tetroxide and was tentatively characterized as CmO4, with curium in the extremely rare +8 state; [70] but new experiments seem to indicate that CmO4 does not exist, and have cast doubt on the existence of PuO4 as well. [71]

### Halides

The colorless curium(III) fluoride (CmF3) can be made by adding fluoride ions into curium(III)-containing solutions. The brown tetravalent curium(IV) fluoride (CmF4) on the other hand is only obtained by reacting curium(III) fluoride with molecular fluorine: [6]

${\displaystyle \mathrm {2\ CmF_{3}\ +\ F_{2}\ \longrightarrow \ 2\ CmF_{4}} }$

A series of ternary fluorides are known of the form A7Cm6F31 (A = alkali metal). [72]

The colorless curium(III) chloride (CmCl3) is made by reacting curium hydroxide (Cm(OH)3) with anhydrous hydrogen chloride gas. It can be further turned into other halides such as curium(III) bromide (colorless to light green) and curium(III) iodide (colorless), by reacting it with the ammonia salt of the corresponding halide at temperatures of ~400–450°C: [73]

${\displaystyle \mathrm {CmCl_{3}\ +\ 3\ NH_{4}I\ \longrightarrow \ CmI_{3}\ +\ 3\ NH_{4}Cl} }$

Or, one can heat curium oxide to ~600°C with the corresponding acid (such as hydrobromic for curium bromide). [74] [75] Vapor phase hydrolysis of curium(III) chloride gives curium oxychloride: [76]

${\displaystyle \mathrm {CmCl_{3}\ +\ \ H_{2}O\ \longrightarrow \ CmOCl\ +\ 2\ HCl} }$

### Chalcogenides and pnictides

Sulfides, selenides and tellurides of curium have been obtained by treating curium with gaseous sulfur, selenium or tellurium in vacuum at elevated temperature. [77] [78] Curium pnictides of the type CmX are known for nitrogen, phosphorus, arsenic and antimony. [6] They can be prepared by reacting either curium(III) hydride (CmH3) or metallic curium with these elements at elevated temperature. [79]

### Organocurium compounds and biological aspects

Organometallic complexes analogous to uranocene are known also for other actinides, such as thorium, protactinium, neptunium, plutonium and americium. Molecular orbital theory predicts a stable "curocene" complex (η8-C8H8)2Cm, but it has not been reported experimentally yet. [80] [81]

Formation of the complexes of the type Cm(n-C
3
H
7
-BTP)
3
(BTP = 2,6-di(1,2,4-triazin-3-yl)pyridine), in solutions containing n-C3H7-BTP and Cm3+ ions has been confirmed by EXAFS. Some of these BTP-type complexes selectively interact with curium and thus are useful for separating it from lanthanides and another actinides. [24] [82] Dissolved Cm3+ ions bind with many organic compounds, such as hydroxamic acid, [83] urea, [84] fluorescein [85] and adenosine triphosphate. [86] Many of these compounds are related to biological activity of various microorganisms. The resulting complexes show strong yellow-orange emission under UV light excitation, which is convenient not only for their detection, but also for studying interactions between the Cm3+ ion and the ligands via changes in the half-life (of the order ~0.1 ms) and spectrum of the fluorescence. [25] [83] [84] [85] [86]

Curium has no biological significance. [87] There are a few reports on biosorption of Cm3+ by bacteria and archaea, but no evidence for incorporation of curium into them. [88] [89]

## Applications

### Radionuclides

Curium is one of the most radioactive isolable elements. Its two most common isotopes 242Cm and 244Cm are strong alpha emitters (energy 6 MeV); they have fairly short half-lives, 162.8 days and 18.1 years, and give as much as 120 W/g and 3 W/g of heat, respectively. [12] [90] [91] Therefore, curium can be used in its common oxide form in radioisotope thermoelectric generators like those in spacecraft. This application has been studied for the 244Cm isotope, while 242Cm was abandoned due to its prohibitive price, around 2000 USD/g. 243Cm with a ~30-year half-life and good energy yield of ~1.6 W/g could be a suitable fuel, but it gives significant amounts of harmful gamma and beta rays from radioactive decay products. As an α-emitter, 244Cm needs much less radiation shielding, but it has a high spontaneous fission rate, and thus a lot of neutron and gamma radiation. Compared to a competing thermoelectric generator isotope such as 238Pu, 244Cm emits 500 times more neutrons, and its higher gamma emission requires a shield that is 20 times thicker—2 inches (51 mm) of lead for a 1 kW source, compared to 0.1 inches (2.5 mm) for 238Pu. Therefore, this use of curium is currently considered impractical. [56]

A more promising use of 242Cm is for making 238Pu, a better radioisotope for thermoelectric generators such as in heart pacemakers. The alternate routes to 238Pu use the (n,γ) reaction of 237Np, or deuteron bombardment of uranium, though both reactions always produce 236Pu as an undesired by-product since the latter decays to 232U with strong gamma emission. [92] Curium is a common starting material for making higher transuranic and superheavy elements. Thus, bombarding 248Cm with neon (22Ne), magnesium (26Mg), or calcium (48Ca) yields isotopes of seaborgium (265Sg), hassium (269Hs and 270Hs), and livermorium (292Lv, 293Lv, and possibly 294Lv). [93] Californium was discovered when a microgram-sized target of curium-242 was irradiated with 35 MeV alpha particles using the 60-inch (150 cm) cyclotron at Berkeley:

242
96
Cm
+ 4
2
He
245
98
Cf
+ 1
0
n

Only about 5,000 atoms of californium were produced in this experiment. [94]

The odd-mass curium isotopes 243Cm, 245Cm, and 247Cm are all highly fissile and can release additional energy in a thermal spectrum nuclear reactor. All curium isotopes are fissionable in fast-neutron reactors. This is one of the motives for minor actinide separation and transmutation in the nuclear fuel cycle, helping to reduce the long-term radiotoxicity of used, or spent nuclear fuel.

### X-ray spectrometer

The most practical application of 244Cm—though rather limited in total volume—is as α-particle source in alpha particle X-ray spectrometers (APXS). These instruments were installed on the Sojourner, Mars, Mars 96, Mars Exploration Rovers and Philae comet lander, [95] as well as the Mars Science Laboratory to analyze the composition and structure of the rocks on the surface of planet Mars. [96] APXS was also used in the Surveyor 5–7 moon probes but with a 242Cm source. [53] [97] [98]

An elaborate APXS setup has a sensor head containing six curium sources with a total decay rate of several tens of millicuries (roughly one gigabecquerel). The sources are collimated on a sample, and the energy spectra of the alpha particles and protons scattered from the sample are analyzed (proton analysis is done only in some spectrometers). These spectra contain quantitative information on all major elements in the sample except for hydrogen, helium and lithium. [99]

## Safety

Due to its radioactivity, curium and its compounds must be handled in appropriate labs under special arrangements. While curium itself mostly emits α-particles which are absorbed by thin layers of common materials, some of its decay products emit significant fractions of beta and gamma rays, which require a more elaborate protection. [51] If consumed, curium is excreted within a few days and only 0.05% is absorbed in the blood. From there, ~45% goes to the liver, 45% to the bones, and the remaining 10% is excreted. In bone, curium accumulates on the inside of the interfaces to the bone marrow and does not significantly redistribute with time; its radiation destroys bone marrow and thus stops red blood cell creation. The biological half-life of curium is about 20 years in the liver and 50 years in the bones. [51] [53] Curium is absorbed in the body much more strongly via inhalation, and the allowed total dose of 244Cm in soluble form is 0.3 μCi. [12] Intravenous injection of 242Cm- and 244Cm-containing solutions to rats increased the incidence of bone tumor, and inhalation promoted lung and liver cancer. [51]

Curium isotopes are inevitably present in spent nuclear fuel (about 20 g/tonne). [100] The isotopes 245Cm–248Cm have decay times of thousands of years and must be removed to neutralize the fuel for disposal. [101] Such a procedure involves several steps, where curium is first separated and then converted by neutron bombardment in special reactors to short-lived nuclides. This procedure, nuclear transmutation, while well documented for other elements, is still being developed for curium. [24]

## Related Research Articles

Americium is a synthetic radioactive chemical element with the symbol Am and atomic number 95. It is a transuranic member of the actinide series, in the periodic table located under the lanthanide element europium, and thus by analogy was named after the Americas.

The actinide or actinoid series encompasses the 15 metallic chemical elements with atomic numbers from 89 to 103, actinium through lawrencium. The actinide series derives its name from the first element in the series, actinium. The informal chemical symbol An is used in general discussions of actinide chemistry to refer to any actinide.

Berkelium is a transuranic radioactive chemical element with the symbol Bk and atomic number 97. It is a member of the actinide and transuranium element series. It is named after the city of Berkeley, California, the location of the Lawrence Berkeley National Laboratory where it was discovered in December 1949. Berkelium was the fifth transuranium element discovered after neptunium, plutonium, curium and americium.

Californium is a radioactive chemical element with the symbol Cf and atomic number 98. The element was first synthesized in 1950 at Lawrence Berkeley National Laboratory, by bombarding curium with alpha particles. It is an actinide element, the sixth transuranium element to be synthesized, and has the second-highest atomic mass of all elements that have been produced in amounts large enough to see with the naked eye. The element was named after the university and the U.S. state of California.

Einsteinium is a synthetic element with the symbol Es and atomic number 99. Einsteinium is a member of the actinide series and it is the seventh transuranium element. It was named in honor of Albert Einstein.

Fermium is a synthetic element with the symbol Fm and atomic number 100. It is an actinide and the heaviest element that can be formed by neutron bombardment of lighter elements, and hence the last element that can be prepared in macroscopic quantities, although pure fermium metal has not yet been prepared. A total of 19 isotopes are known, with 257Fm being the longest-lived with a half-life of 100.5 days.

Neptunium is a chemical element with the symbol Np and atomic number 93. A radioactive actinide metal, neptunium is the first transuranic element. Its position in the periodic table just after uranium, named after the planet Uranus, led to it being named after Neptune, the next planet beyond Uranus. A neptunium atom has 93 protons and 93 electrons, of which seven are valence electrons. Neptunium metal is silvery and tarnishes when exposed to air. The element occurs in three allotropic forms and it normally exhibits five oxidation states, ranging from +3 to +7. It is radioactive, poisonous, pyrophoric, and capable of accumulating in bones, which makes the handling of neptunium dangerous.

Mixed oxide fuel, commonly referred to as MOX fuel, is nuclear fuel that contains more than one oxide of fissile material, usually consisting of plutonium blended with natural uranium, reprocessed uranium, or depleted uranium. MOX fuel is an alternative to the low-enriched uranium (LEU) fuel used in the light-water reactors that predominate nuclear power generation.

Fertile material is a material that, although not fissile itself, can be converted into a fissile material by neutron absorption.

Plutonium (94Pu) is an artificial element, except for trace quantities resulting from neutron capture by uranium, and thus a standard atomic weight cannot be given. Like all artificial elements, it has no stable isotopes. It was synthesized long before being found in nature, the first isotope synthesized being 238Pu in 1940. Twenty plutonium radioisotopes have been characterized. The most stable are plutonium-244 with a half-life of 80.8 million years, plutonium-242 with a half-life of 373,300 years, and plutonium-239 with a half-life of 24,110 years. All of the remaining radioactive isotopes have half-lives that are less than 7,000 years. This element also has eight meta states; all have half-lives of less than one second.

Americium (95Am) is an artificial element, and thus a standard atomic weight cannot be given. Like all artificial elements, it has no known stable isotopes. The first isotope to be synthesized was 241Am in 1944. The artificial element decays by ejecting alpha particles. Americium has an atomic number of 95. Despite 243
Am
being an order of magnitude longer lived than 241
Am
, the former is harder to obtain than the latter as more of it is present in spent nuclear fuel.

The minor actinides are the actinide elements in used nuclear fuel other than uranium and plutonium, which are termed the major actinides. The minor actinides include neptunium, americium, curium, berkelium, californium, einsteinium, and fermium. The most important isotopes of these elements in spent nuclear fuel are neptunium-237, americium-241, americium-243, curium-242 through -248, and californium-249 through -252.

Curium(III) oxide is a compound composed of curium and oxygen with the chemical formula Cm2O3. It is a crystalline solid with a unit cell that contains two curium atoms and three oxygen atoms. The simplest synthesis equation involves the reaction of curium(III) metal with O2−: 2 Cm3+ + 3 O2− ---> Cm2O3. Curium trioxide can exist as five polymorphic forms. Two of the forms exist at extremely high temperatures, making it difficult for experimental studies to be done on the formation of their structures. The three other possible forms which curium sesquioxide can take are the body-centered cubic form, the monoclinic form, and the hexagonal form. Curium(III) oxide is either white or light tan in color and, while insoluble in water, is soluble in inorganic and mineral acids. Its synthesis was first recognized in 1955.

Environmental radioactivity is not limited to actinides; non-actinides such as radon and radium are of note. While all actinides are radioactive, there are a lot of actinides or actinide-relating minerals in the Earth's crust such as uranium and thorium. These minerals are helpful in many ways, such as carbon-dating, most detectors, X-rays, and more.

Plutonium is a radioactive chemical element with the symbol Pu and atomic number 94. It is an actinide metal of silvery-gray appearance that tarnishes when exposed to air, and forms a dull coating when oxidized. The element normally exhibits six allotropes and four oxidation states. It reacts with carbon, halogens, nitrogen, silicon, and hydrogen. When exposed to moist air, it forms oxides and hydrides that can expand the sample up to 70% in volume, which in turn flake off as a powder that is pyrophoric. It is radioactive and can accumulate in bones, which makes the handling of plutonium dangerous.

Plutonium-241 is an isotope of plutonium formed when plutonium-240 captures a neutron. Like some other plutonium isotopes, 241Pu is fissile, with a neutron absorption cross section about one-third greater than that of 239Pu, and a similar probability of fissioning on neutron absorption, around 73%. In the non-fission case, neutron capture produces plutonium-242. In general, isotopes with an odd number of neutrons are both more likely to absorb a neutron, and more likely to undergo fission on neutron absorption, than isotopes with an even number of neutrons.

Plutonium-242 is one of the isotopes of plutonium, the second longest-lived, with a half-life of 375,000 years. The half-life of 242Pu is about 15 times that of 239Pu; so it is one-fifteenth as radioactive, and not one of the larger contributors to nuclear waste radioactivity. 242Pu's gamma ray emissions are also weaker than those of the other isotopes.

Long-lived fission products (LLFPs) are radioactive materials with a long half-life produced by nuclear fission of uranium and plutonium. Because of their persistent radiotoxicity it is necessary to isolate them from man and biosphere and to confine them in nuclear waste repositories for geological periods of time.

Nuclear transmutation is the conversion of one chemical element or an isotope into another chemical element. Nuclear transmutation occurs in any process where the number of protons or neutrons in the nucleus of an atom is changed.

Americium-241 is an isotope of americium. Like all isotopes of americium, it is radioactive, with a half-life of 432.2 years. 241
Am
is the most common isotope of americium as well as the most prevalent isotope of americium in nuclear waste. It is commonly found in ionization type smoke detectors and is a potential fuel for long-lifetime radioisotope thermoelectric generators (RTGs). Its common parent nuclides are β from 241
Pu
, EC from 241
Cm
, and α from 245
Bk
. 241
Am
is fissile and the critical mass of a bare sphere is 57.6–75.6 kilograms (127.0–166.7 lb) and a sphere diameter of 19–21 centimetres (7.5–8.3 in). Americium-241 has a specific activity of 3.43 Ci/g (126.91 GBq/g). It is commonly found in the form of americium-241 dioxide. This isotope also has one meta state, 241m
Am
, with an excitation energy of 2.2 MeV (0.35 pJ) and a half-life of 1.23 μs. The presence of americium-241 in plutonium is determined by the original concentration of plutonium-241 and the sample age. Because of the low penetration of alpha radiation, americium-241 only poses a health risk when ingested or inhaled. Older samples of plutonium containing 241
Pu
contain a buildup of 241
Am
. A chemical removal of americium-241 from reworked plutonium may be required in some cases.

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