Curium

Double-hexagonal close packing with the layer sequence ABAC in the crystal structure of α-curium (A: green, B: blue, C: red)

Orange fluorescence of Cm³⁺ ions in a solution of tris(hydrotris)pyrazolylborato-Cm(III) complex, excited at 396.6 nm.

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 is 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 ~ 590–640 nm depending on their environment.[24] The fluorescence originates from the transitions from the first excited state ⁶D_7/2 and the ground state ⁸S_7/2. Analysis of this fluorescence allows monitoring interactions between Cm(III) ions in organic and inorganic complexes.[25]

A solution of curium

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] +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, within a tricapped trigonal prismatic geometry.[35]

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]

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]

Transmutation flow between ²³⁸Pu and ²⁴⁴Cm in LWR.[41]
Fission percentage is 100 minus shown percentages.
Total rate of transmutation varies greatly by nuclide.
²⁴⁵Cm–²⁴⁸Cm are long-lived with negligible decay.

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).

Several isotopes of curium were detected in the fallout from the Ivy Mike nuclear test.

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]

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 ²⁴⁹Bk.

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

(6)

Chromatographic elution curves revealing the similarity between Tb, Gd, Eu lanthanides and corresponding Bk, Cm, Am actinides.

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]

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
₂O
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 (CmH_2–3) 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]

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]

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

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]

Predicted curocene structure

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
₃H
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]

The radiation from curium is so strong that the metal glows purple in the dark.

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
₉₆Cm
+ ⁴
₂He
→ ²⁴⁵
₉₈Cf
+ ¹
₀n

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.

Alpha-particle X-ray spectrometer of a Mars exploration rover

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]