Cerium


The element cerium, in the form of an oxide, was "discovered" in 1930 simultaneously and independently by Klaproth and by Berzelius and Hsinger. The name ceria, which was proposed in honor of the newly-sighted asteroid Ceres, was given by Berzelius and Hisinger. This name was accepted by the scientific community of the time. Thirty-six years later, however, Monsander showed that the oxide isolated by these researchers was composed of at least two oxides, for one of which he retained the name ceria and the second he called lanthana, which subsequently was shown to consist of not only lanthana, but also praseodymia and neodymia.

Metallic cerium was first isolated by Mosander in 1825 by reducing cerous chloride with sodium. It should be noted that the term "cerium" is used quite loosely, since Mosander's material contained other rare earths which had not yet been isolated and identified. In the succeeding following years several other investigators prepared metallic cerium by a metallothermic technique, but it was not until 1875, that Hillebrand and Norton prepared cerium by electrolysis of fused chlorides.

Today, these are the two most important methods used to prepare metallic cerium. Generally speaking, the metallothermic technique is used to prepare high-purity metal, while the electrolytic method is used to prepare many of the commercial cerium-based alloys.

The discovery of the electronic transformation in metallic cerium wherein at least a greater portion of the 4f electron is transferred to the valence level, is contemporary history. A transformation in cerium, in which a large volume (about 10%) occurs, was observed by Bridgman (1948) at high pressure and by Trombe and Föex (1943) at low temperature. Further studies showed that, in spite of this volume change, cerium keeps its structure. Zachariasen (1949) and Pauling (1950), working independently, suggested that the valence of metallic cerium changes from about 3 to about 4 when either cooled or compressed.

Presently there is a great deal of research, both experimental and theoretical, being carried out on metallic cerium to better characterize its electronic behavior in the various polymorphic modifications.