Elements, rare earth physical characteristics

As the first element in the third series of the transition elements, hafnium s atomic number ( jHf) follows the lanthanide series of rare-earths. The lanthanide series is separated out of the normal position of sequenced atomic numbers and is placed below the third series on the periodic table ( La to 7,Li). This rearrangement of the table allowed the positioning of elements of the third series within groups more related to similar chemical and physical characteristics—for example, the triads of Ti, Zr, and Hf V, Nb, andTa and Cu, Ag, and Au. 

The lanthanide series of metals includes the 15 elements with atomic numbers 57-71, plus yttrium (atomic number 39). The lanthanides occur in the earth s crust at concentrations exceeding some commonly used industrial elements making the term rare earths something of a misnomer. For example, yttrium, cerium, lanthanum, and neodymium are present in the earth s crust at higher concentrations than lead. Of the 15 lanthanides, only promethium does not occur in nature - it is a man-made element. All of the lanthanides have similar physical and chemical properties. Because of similarities in their chemistry and toxicity, the characteristics of the lanthanides are often described as a group. Within the lanthanide group, however, there are differences between the toxicity of the individual lanthanide elements and their compounds. 

In view of the importance of high-temperature superconductivity in the layered cuprates and the role played by the rare earths, it seemed appropriate to prepare these volumes of the Handbook on the Physics and Chemistry of Rare Earths on High-Temperature Superconductivity in Layered Cuprates . We believe that researchers already working in this field, as well as those intending to enter this field, will find valuable information in the review articles contained in these volumes. Since many of the cuprate superconductors do not contain rare-earth or actinide elements, yet have characteristics and properties similar to those that do, the range of materials considered in these volumes has been broadened to a limited extent to include all high-temperature cuprate superconductors, irrespective of whether they contain rare-earth or actinide elements. 

The physical influences of rare-earth elements on rates and direction of growth and structural characteristics of oxide scales mainly result fi om the incorporation of the rare-earth compounds in the scale. Other influences relate to their capacity to nucleate more uniform fine-grained oxide, inhibit the formation of transitional oxides and promote, for instance, Cr203 formation where it would not normally be expected, as exemplified in studies by Stringer et al. (1972), Whittle et al. (1977), Rhys-Jones et al. (1987) and Rhys-Jones and Grabke (1988). 

The rare earths as used here include the fifteen lanthanide elements and scandium and yttrium. This large group of elements with measured differences provides one of the best sources available for studying developing chemical and physical characteristics. They do not possess the full range of solid state properties but they will be appreciated as one of the best of model systems since they provide subtle variations in properties which may be utilized to test hypotheses or theories. 

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