page_title: Curium
section_title: Physical
hierarchical_section_title: Curium / Characteristics / Physical
caption_attribution_description: Deutsch: Laserinduzierte Fluoreszenz von Cm(III) in Lösung English: laser-induced Cm(III) fluoresence in solution
context_page_description: Curium is a transuranic radioactive chemical element with the symbol Cm and atomic number 96. This element of the actinide series was named after Marie and Pierre Curie – both were known for their research on radioactivity. Curium was first intentionally produced and identified in July 1944 by the group of Glenn T. Seaborg at the University of California, Berkeley. The discovery was kept secret and only 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 about 20 grams of curium.
  Curium is a hard, dense, silvery metal with a relatively high melting point and boiling point for an actinide. Whereas it is paramagnetic at ambient conditions, it becomes antiferromagnetic upon cooling, and other magnetic transitions are also observed for many curium compounds. In compounds, curium usually exhibits valence +3 and sometimes +4, and the +3 valence is predominant in solutions. Curium readily oxidizes, and its oxides are a dominant form of this element.
context_section_description: A synthetic, radioactive element, curium is a hard, dense metal with a silvery-white appearance and physical and chemical properties resembling those of gadolinium. Its melting point of 1344 °C is significantly higher than that of the previous transuranic elements neptunium (637 °C), plutonium (639 °C) and americium (1173 °C). In 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 is heavier than most other metals. Between two crystalline forms of curium, the 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. The crystal consists of a double-hexagonal close packing with the layer sequence ABAC and so is isotypic with α-lanthanum. At pressures above 23 GPa, at room temperature, α-Cm transforms into β-Cm, which has a face-centered cubic symmetry, space group Fm3m and the lattice constant a = 493 pm. Upon further compression to 43 GPa, curium transforms to an orthorhombic γ-Cm structure similar to that of α-uranium, with no further transitions observed up to 52 GPa. These three curium phases are also referred to as Cm I, II and III.
  Curium has peculiar magnetic properties. Whereas its neighbor element americium shows no deviation from Curie-Weiss paramagnetism in the entire temperature range, α-Cm transforms to an antiferromagnetic state upon cooling to 65–52 K, and β-Cm exhibits a ferrimagnetic transition at about 205 K. Meanwhile, curium pnictides show ferromagnetic transitions upon cooling: 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 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.
  In accordance with magnetic data, electrical resistivity of curium increases with temperature – about twice 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-damage of the crystal lattice by alpha radiation. This makes uncertain the absolute resistivity value for curium (about 125 µΩ·cm). The resistivity of curium is similar to that of gadolinium and of the actinides plutonium and neptunium, but is significantly higher than that of americium, uranium, polonium and thorium.
  Under ultraviolet illumination, curium(III) ions exhibit strong and stable yellow-orange fluorescence with a maximum in the range about 590–640 nm depending on their environment. 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.
caption: Orange fluorescence of Cm3+ ions in a solution of tris(hydrotris)pyrazolylborato-Cm(III) complex, excited at 396.6 nm.
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