Lanthanoids occurrence

Key words rare earths, lanthanoids, occurrence, uses, corrosion inhibitors, rare earth chemistry, rare earth carboxylates, synthesis, stmctures, modelling surface films. 

Compounds with Sc, Y, lanthanoids and actinoids are of three types. Those with composition ME have the (6-coordinated) NaCl structure, whereas M3E4 (and sometimes M4E3) adopt the body-centred thorium phosphide structure (Th3P4) with 8-coordinated M, and ME2 are like ThAsi in which each Th has 9 As neighbours. Most of these compounds are metallic and those of uranium are magnetically ordered. Full details of the structures and properties of the several hundred other transition metal-Group 15 element compounds fall outside the scope of this treatment, but three particularly important structure types should be mentioned because of their widespread occurrence and relation to other structure types, namely C0AS3,... 

Lanthanum, La, is commonly classified with the lantha-noids (see Figure 1.13) even though lanthanoid means like lanthanum and La is strictly a group 3 metal. Because of the chemical similarity of La to the elements Ce—Lu, we consider them together in Chapter 24 our only mention of La in this chapter is its occurrence. 

The lanthanoids resemble each other much more closely than do the members of a row of t/-block metals. The chemistry of the actinoids is more complicated, and in addition, only Th and U have naturally occurring isotopes. Studies of the transuranium elements (those with Z > 92) require specialized techniques. The occurrence of artihcial isotopes among the /-block elements can be seen from Appendix 5 all the actinoids are unstable with respect to radioactive decay (see Section 24.9), although the half-lives of the most abundant isotopes of thorium and uranium ( Th and t = 1.4 x lO and 4.5 x 10 yr respectively) are... 

Because of the inner nature of the 4f orbitals, the dififerenees of eleetron configuration between the lanthanoid elements are associated to eleetrons relatively well screened from the chemical surroundings by the outer (5s p ) shell. This implies weak crystal fields splitting effects [28], and a relatively small eovalent contribution to the bonding, particularly in the sesquioxides. Accordingly, the ionic model plays an important role in determining their chemistry [21]. Also related to these chemical characteristics, the lanthanoid compounds exhibit a rich variety of stmctures, often reflected in the occurrence of polymorphism phenomena. 

The divalent state of Sm, Eu, Tm and Yb should become unstable under high pressure. This can be foreseen from the fact that they have smaller ionic radii in their trivalent state and hence high pressure could favor the higher valence state. Recent high pressure experiments on a series of lanthanoid compounds, in particular the monochalcogenides Sm, Eu, Yb, Tm, have demonstrated the occurrence of such transformations. 

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