Curium

Curium is a synthetic element of the periodic table whose symbol is Cm and its atomic number is 96. It is produced by bombarding plutonium with alpha particles. It is an actinide. Curium does not exist in the terrestrial environment, but it can be produced artificially. Its chemical properties are so similar to those of typical rare earths that, were it not for its radioactivity, it could easily be mistaken for one of these elements. Known isotopes of curium include those with mass numbers 238 to 250. The isotope ₂₄₄Cm is of particular interest because of its potential use as a compact source of thermoelectric force, using the heat generated by nuclear decay to generate electrical force.

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

Metal curium can be produced by reduction of curium trifluoride with barium vapor. The metal has a silver luster, which is lost on contact with air, and a relative density of 13.5. The melting point is 1340 (+/-) 40 °C (2444 +/- 72 °F). The metal dissolves easily in common mineral acids, with the formation of a tripositive ion.

Several solid compounds of curium have been prepared and their structures determined by X-ray diffraction. These include CmF₄, CmF₃, CmCl₃ , CmBr₃, CmI₃, Cm₂O₃, CmO₂. In the lanthanides there are isostructural analogues of curium compounds.

History

The chemical element received the last name of the marriage Curie, pioneers in the field of radioactivity.

Curium was first synthesized at the University of California, Berkeley, also by Glenn T. Seaborg, Ralph A. James, and Albert Ghiorso in 1944. The name curium was chosen in honor of Marie Curie and her husband Pierre, famous for discovering radium and for other important work on radioactivity.

Features

Physics

Compact double hexagonal network with the ABAC sequence layer in the α-curio crystalline structure (A: green, B: blue, C: red)

Orange fluorescence of Cm ions³⁺in a solution of the complex tris(hydrotris)pirazolilborato-Cm(III), excited with 396.6 nm.

Curium is a synthetic radioactive element, a hard, dense metal with a silvery-white appearance and physical and chemical properties resembling those of gadolinium. Its melting point is 1344 °C which is significantly higher than that of the previous transuranium elements neptunium (637 °C), plutonium (639 °C) and americium (1173 °C). For comparison, gadolinium melts at 1312 °C. The boiling point of curium is 3556 °C. With a density of 13.52 g/cm³, curium is significantly lighter than neptunium (20.45 g/cm³) and plutonium (19.8 g/cm³), but it is heavier than many of the other metals. Among the two crystalline forms of curium, α-Cm is more stable under ambient conditions. It has hexagonal symmetry, space group P6₃/mmc, lattice parameters a = 365 pm and c = 1182 pm, and four formula units in each unit cell. The crystal consists of a close-packed double hexagonal lattice with the ABAC shell sequence and is therefore isotypic with α-lanthanum. At pressures above 23 GPa, at room temperature, α-Cm transforms into β-Cm, which has face-centered cubic symmetry, space group Fm3m and lattice constant a = 493 pm. If compressed to a pressure of 43 GPa, curium assumes a γ-Cm orthorhombic structure similar to that of α-uranium, without that other transitions are observed up to 52 GPa. These three phases of the curium are also called Cm I, II and III.

Curium has peculiar magnetic properties. While its neighbor element americium exhibits no Curie-Weiss paramagnetism deviation over the entire temperature range, α-Cm transforms to an antiferromagnetic state on cooling to 65–52 K, and possesses a ferrimagnetic β-Cm transition. at about 205 K. In contrast, curium pnictogens exhibit ferromagnetic transitions when cooled: ²⁴⁴CmN and ²⁴⁴CmAs at 109 K, ²⁴⁸CmP at 73 K and ²⁴⁸CmSb at 162 K. The lanthanide analogue of curium, gadolinium, as well as its pnictogens, also exhibit magnetic transitions on cooling, but the nature of the transition is somewhat different: Gd and the GoN become ferromagnetic, while the GoP, GdAs and GdSb present antiferromagnetic arrangements.

According to magnetic records, the electrical resistivity of curium increases with temperature – roughly doubling between 4 and 60 K – and then remains nearly constant up to room temperature. There is a significant increase in resistivity over time (about 10 µΩ·cm/h) due to self-inflicted damage to the crystal lattice by alpha radiation. This makes the absolute value of the absolute resistivity of curium uncertain (about 125 µΩ cm. The resistivity of curium is similar to that of gadolinium and to that of the actinides plutonium and neptunium, but significantly higher than that of americium, uranium, polonium and thorium.

Under ultraviolet light, curium(III) ions exhibit intense and stable yellow-orange fluorescence with a maximum in the range of 590 to 640 nm depending on the environment. The fluorescence results from transitions between the primer success state ⁶D_7/2 and base state ⁸S_7/2. The analysis of this fluorescence allows to monitor the interactions between Cm(III) ions in organic and inorganic complexes.

Chemicals

A curio solution.

Curium ions in solution take on an oxidation state of +3, which is the most stable oxidation state of curium. The +4 oxidation state is mainly observed in a few solid phases, such as CmO₂ and CmF₄. Aqueous curium(IV) has only been observed in the presence of strong oxidants such as potassium persulfate, and is easily reducible to curium(III) by radiolysis and even by water itself. The chemical behavior of curium is different from the actinides thorium and uranium, and is similar to that of americium and numerous lanthanides. In aqueous solution, the Cm³⁺ ion is colorless to light green, and the Cm⁴⁺ ion is light yellow. Optical absorption of Cm3+ contains three well-defined peaks at 375.4, 381.2, and 396.5 nanometers and its magnitude is directly dependent on the ion concentration. The +6 oxidation state has only been identified once in solution in 1978, as the ion (CmO2+
2): this time it was obtained from beta decay of americium-242 in the americium(V) ion 242
AmO+
2. Sometimes it is not possible to obtain Cm(VI) from the oxidation of Cm(III) and Cm(IV) can due to the high oxidation potential of Cm⁴⁺/Cm³⁺ and the instability of Cm(V). Curium ions are strong Lewis acids and therefore form very stable complexes with strong bases. The binding is mainly ionic, with a small covalent component. The curium in its complexes usually presents a 9-fold coordination environment, within a trigonal prismatic geometry.

Uses

Radionuclides

Curium is one of the most radioactive isolable elements. Its two most common isotopes ²⁴²Cm and ²⁴⁴Cm are alpha emitters as strong as 6 MeV); which have relatively short half-lives of 162.8 days and 18.1 years, and produce 120 W/g and 3 W/g of thermal power, respectively. Curium can therefore be used in its common oxide form in radioactive thermoelectric generators which are often used in spacecraft. This use has been developed for the isotope ²⁴⁴Cm, while ²⁴²Cm was discarded due to its high cost of around 2000 USD/g. The ²⁴³Cm with a half-life of about ~30 years and a good level of energy output of ~1.6 W/g which makes it acceptable as a fuel, but produces a significant amount of gamma radiation and dangerous beta from their radioactive decay products. Although as an α emitter, ²⁴⁴Cm requires a thin radiation shielding, it has a high spontaneous fission rate, and therefore neutron and gamma radiation rates are relatively high.. Compared to a competing thermoelectric generator using the ²³⁸Pu isotope, the ²⁴⁴Cm emits 500 times more neutrons, and its higher gamma emission requires shielding that is 20 times thicker about 5 cm of lead for a 1 kW source, compared to 2.5 mm for the ²³⁸Pu case. Therefore, this use of curium is currently not considered practical.

A more promising use of ²⁴²Cm is to produce ²³⁸Pu, a radioisotope more suitable for thermoelectric generators such as those used in cardiac pacemakers. Alternative routes to ²³⁸Pu use the (n,γ) reaction of ²³⁷Np, or bombardment by uranium deuteron bombardment, which always yields ²³⁶ Pu as an unwanted by-product since the latter decays at ²³²U with a strong gamma emission. Curium is also a material used to produce transuranic elements and superheavy elements superiors. Therefore, bombardment of ²⁴⁸Cm with neon (²²Ne), magnesium (²⁶Mg), or calcium (⁴⁸ Ca) produce certain isotopes of seaborgium (²⁶⁵Sg), hasium (²⁶⁹Hs and ²⁷⁰Hs), and livermorium (²⁹²Lv, ²⁹³Lv, and possibly ²⁹⁴Lv). Californium was discovered when a microgram-sized target of curium-242 was irradiated with 35 MeV alpha particles using Berkeley's 60-inch (152.4 cm) cyclotron:

242
96Cm + 4
2He → 245
98Cf + n

Only 5,000 californium atoms were produced in this experiment.

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

X-ray spectrometer by alpha particles of a Mars exploration robot.

X-ray spectrometer

The most practical use of the ²⁴⁴Cm—although limited in total volume—is as a source of α particles in the Alpha Particle X-ray Spectrometer (APXS). These instruments were installed on the Sojourner, Mars, Mars 96, Mars Exploration Rovers, and Philae space probes, and also at the Mars Science Laboratory to analyze the composition and structure of rocks on the surface of the planet Mars. The APXS also it was used on the Lunar Surveyor 5–7 probes but with a source of ²⁴²Cm.

A scheme beyond APXS is equipped with a sensor head containing six curium sources with a radioactive decay activity of several tens of millicuries (approximately one gigabecquerel). The sources are collimated in the sample, and the energy spectrum of alpha particles and protons scattered from the sample are analyzed (proton analysis is implemented only in some spectrometers). These spectra contain quantitative information for all the major elements in the samples except hydrogen, helium, and lithium.