Structure lanthanide oxides

A predominant feature of the atomic structure of the lanthanide group is the sequential addition of 14 electrons to the 4f subshell (Table 1). The /"electrons do not participate in bond formation and in ordinary aqueous solutions all of the lanthanides exhibit a principal (III) state. The common (III) state confers a similarity in chemical properties to all lanthanide elements. Some of the lanthanides can also exist in the (II) state (Nd, Sm, Eu, Tm, Yh) or in the (IV) state (Ce, Pr, Nd, Tb, Dy). Except for Ce(IV), Eu(II), and Yb(II), these unusual lanthanide oxidation states can only be prepared under drastic redox pressure and temperature conditions, and they are not stable in aqueous solutions. Cerium (IV) is a strong oxidizing agent... 

The lanthanide oxides provide an interesting system for the study of pol5unor-phism. The structures of each of the three polymorphic forms are built up either from a single t57pe of coordination polyhedron MO7 in the hexagonal A-form, MOe in the cubic C-form or from a mixture of MOe and MO7 polyhedra as in the monocHnic B-form. 

The unique properties of lanthanide-based materials, e.g., lanthanide-silicates and lanthanide-doped silicas, can be related to the special properties of the 4f" orbitals. Among lanthanide oxides, only Ce, Pr and Tb form dioxides, which crystallize in one simple structure with M4+ ions showing octahedral coordination [17]. For instance, cerium dioxide exhibits an 8 4 catiomanion coordination [18]. Its characteristic feature is the ability to undergo oxidation-reduction cycles in a reversible way [19], It was shown that the presence of Ce and La additives in mesoporous silicas, e.g., MCM-41 [10,11] and MSU-X [12], improves their thermal and hydrothermal stability. 

Hydrated salts are readily prepared by reaction of the lanthanide oxide or carbonate with the acid. Salts of non coordinating anions most often crystallise as salts [Ln(OH2)9]X3 (X e g. bromate, triflate, ethylsulphate, tosylate). The tricapped trigonal prismatic structure was first established by X-ray diffraction in 1939 for the [Nd(H20)9] + ion in [Nd(H20)9] (6103)3 this was an important structure historically, as it was the first clear indication that lanthanides conld have high coordination numbers. It has since been verified for many other salts (Figure 10). 

Relatively little is known about the chemistry of promethium, although the established facts highlight the resemblance of promethium to the lanthanides that flank it. The halides are well defined, and have been shown to have the expected structures (Table 2). Thus in a recent study, starting with some 100 J,g of Pm203, the halides were synthesized microchemically, using the reaction with fluorine for PmFs and the reaction with HX (X = Cl, Br) for the chloride and bromide the latter two reacted with HI to give the iodide. Pm203 is one of the few lanthanide oxides to crystallize in three modifications. 

Cerium (Ce) is a second element of lanthanides in periodic table. Cerium oxide (Ce02) has a cubic fluorite-type structure with a lattice constant (a) of 0.5411 nm. Ce02 thin films are highly attractive for electronic and electrochemical device applications as insulating buffer layers, ion-conducting layers, or ion-storage layers. Recently, a lot of interest has been generated in nano-structured cerium oxide for various electro-catalytic applications due to its... 

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