information:classification
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/cm, curium is lighter than neptunium (20.45 g/cm) and plutonium (19.8 g/cm), 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 P6 /mmc, lattice parameters a = 365 pm and c = 1182 pm, and four formula units per unit cell. 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 Fm 3 m and lattice constant a = 493 pm. 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.

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, and β-Cm exhibits a ferrimagnetic transition at ~205 K. Curium pnictides show ferromagnetic transitions upon cooling: CmN and CmAs at 109 K, CmP at 73 K and CmSb 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.

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.

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. The fluorescence originates from the transitions from the first excited state D and the ground state S. Analysis of this fluorescence allows monitoring interactions between Cm(III) ions in organic and inorganic complexes.