Samarium is a chemical element with the symbol Sm, atomic number
62 and atomic weight 150.36. It is a moderately hard silvery metal
which readily oxidizes in air. Being a typical member of the
lanthanide series, samarium usually assumes the oxidation state +3;
however, compounds of samarium(II) are also known, most notably
monoxide SmO, monochalcogenides SmS, SmSe and SmTe, as well as
samarium(II) iodide. The last compound is a common reducing agent
in chemical synthesis. Samarium has no significant biological role
and is only slightly toxic.
Samarium was discovered in 1879 by the French chemist Paul Émile
Lecoq de Boisbaudran and named after the mineral samarskite where
it was isolated from. The mineral itself was earlier named after
the Russian military engineer Vasili Samarsky-Bykhovets who thereby
became the first person to have a chemical element named after him,
albeit indirectly. Although classified as a rare earth element,
samarium is the 40th most abundant element in the Earth's crust and
is more common than such metals as tin. Samarium occurs with
concentration up to 2.8% in several minerals including cerite,
gadolinite, samarskite, monazite and bastnäsite, the last two being
the most common commercial sources of the element. These minerals
are mostly found in China, the USA, Brazil, India, Sri Lanka and
Australia; China is by far the world leader in samarium mining and
production.
The major commercial application of samarium is in
samarium-cobalt magnets which have permanent magnetization second
only to neodymium magnets; however, samarium compounds can
withstand significantly higher temperatures, above 700 °C, without
losing their magnetic properties. Radioactive isotope samarium-153
is the major component of the drug samarium (153Sm) lexidronam
(Quadramet) which kills cancer cells in the treatment of lung
cancer, prostate cancer, breast cancer and osteosarcoma. Another
isotope, samarium-149, is a strong neutron absorber and is
therefore added to the control rods of nuclear reactors. It is also
formed as a decay product during the reactor operation and is one
of the important factors considered in the reactor design and
operation. Other applications of samarium include catalysis of
chemical reactions, radioactive dating and an X-ray laser.
Samarium is a rare earth metal having the hardness and density
similar to those of zinc. With the boiling point of 1794 °C,
samarium is the third most volatile lanthanide after ytterbium and
europium; this property facilitates separation of samarium from the
mineral ore. At ambient conditions, samarium normally assumes a
rhombohedral structure (a form). Upon heating to 731 °C, its
crystal symmetry changes into hexagonal close-packed (hcp), however
the transition temperature depends on the metal purity. Further
heating to 922 °C transforms the metal into a body-centered cubic
(bcc) phase. Heating to 300 °C combined with compression to 40 kbar
results in a double-hexagonal close-packed structure (dhcp).
Applying higher pressure of the order hundreds or thousands
kilobars induces a series of phase transformations, in particular
with a tetragonal phase appearing at about 900 kbar. In one study,
the dhcp phase could be produced without compression, using a
nonequilibrium annealing regime with a rapid temperature change
between about 400 and 700 °C, confirming the transient character of
this samarium phase. Also, thin films of samarium obtained by vapor
deposition may contain the hcp or dhcp phases at ambient
conditions.
Samarium (and its sesquioxide) are paramagnetic at room
temperature. Their corresponding effective magnetic moments, below
2 µB, are the 3rd lowest among the lanthanides (and
their oxides) after lanthanum and lutetium. The metal transforms to
an antiferromagnetic state upon cooling to 14.8 K. Individual
samarium atoms can be isolated by encapsulating them into fullerene
molecules. They can also be doped between the C60
molecules in the fullerene solid, rendering it superconductive at
temperatures below 8 K. Samarium doping of iron-based
superconductors – the most recent class of high-temperature
superconductors – allows to enhance their transition
temperature to 56 K, which is the highest value achieved so far in
this series.