(Sm), chemical element, rare-earth metal of the lanthanoid series of the periodic table. Samarium is silvery white in colour and is relatively stable in air. It was isolated as an impure oxide and spectroscopically identified as a new element (1879) by P.-E. Lecoq de Boisbaudran. Samarium occurs in many other rare-earth minerals but is almost exclusively obtained from monazite; it is also found in the products of nuclear fission. Ion-exchange techniques are used for its commercial separation and purification. The metal is conveniently prepared by the thermoreduction of its oxide, Sm2O3, with lanthanum metal, followed by distillation of the samarium metal, which is one of the most volatile rare-earth elements. Several allotropes (structural forms) of samarium exist; at room temperature its structure is rhombohedral
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