to be kept well below 1 °C/min. Similar procedure is adopted for the monarsenide SmAs, but the synthesis temperature is higher at . Numerous crystalline binary compounds are known for samarium and one of the group 14, 15, or 16 elements X, where X is Si, Ge, Sn, Pb, Sb or Te, and metallic alloys of samarium form another large group. They are all prepared by annealing mixed powders of the corresponding elements. Many of the resulting compounds are non-stoichiometric and have nominal compositions SmaXb, where the b/a ratio varies between 0.5 and 3. Organometallic compounds Samarium forms a cyclopentadienide and its chloroderivatives and . They are prepared by reacting samarium trichloride with in tetrahydrofuran. Contrary to cyclopentadienides of most other lanthanides, in some rings bridge each other by forming ring vertexes η1 or edges η2 toward another neighboring samarium, thus creating polymeric chains. The chloroderivative has a dimer structure, which is more accurately expressed as . There, the chlorine bridges can be replaced, for instance, by iodine, hydrogen or nitrogen atoms or by CN groups. The ()− ion in samarium cyclopentadienides can be replaced by the indenide ()− or cyclooctatetraenide ()2− ring, resulting in or . The latter compound has a structure similar to uranocene. There is also a cyclopentadienide of divalent samarium, 2− a solid that sublimates at about . Contrary to ferrocene, the rings in are not parallel but are tilted by 40°. A metathesis reaction in tetrahydrofuran or ether gives alkyls and aryls of samarium: Here R is a hydrocarbon group and Me = methyl. Isotopes Naturally occurring samarium is composed of five stable isotopes: 144Sm, 149Sm, 150Sm, 152Sm and 154Sm, and two extremely long-lived radioisotopes, 147Sm (half-life t1/2 = 1.06 years) and 148Sm (7 years), with 152Sm being the most abundant (26.75%). 149Sm is listed by various sources either as stable or radioactive, but only a lower bound for its half-life is given. Some observationally stable samarium isotopes are predicted to decay to isotopes of neodymium. The long-lived isotopes 146Sm, 147Sm, and 148Sm undergo alpha decay to neodymium isotopes. Lighter unstable isotopes of samarium mainly decay by electron capture to promethium, while heavier ones beta decay to europium. Natural samarium has a radioactivity of 127 Bq/g, mostly due to 147Sm, which alpha decays to 143Nd with a half-life of 1.06 years and is used in samarium–neodymium dating. 146Sm can be used as an extinct radionuclide