Rare earths structure

The structure of La203 [Section 5.5.10, 3 5/3POPPO(h), Figure 5.64) is known as the A-type M203 or A-type rare-earth structure. It is rather complex. The unit cell is hexagonal, but oxide ions are in an A, B, C sequence. La3+ ions fill two O layers with the next O layer vacant. This is a layer structure because the sequence is POP POP, but the first and sixth P layers are identical thus, five layers POP PO repeat. La has CN 7 the seventh oxide ion is from the next POP sandwich. 

Three principal types of structure are noted for the rare earth oxides designated as the A, B, and C rare earth structures. The A rare earth structure is the stable room temperature structure for the lighter rare earths from La to Nd while the C rare earth structure is the stable polymorphs for Sm through Lu. The B form is seen for several oxides at intermediate temperatures and/or high pressures. All polymorphic forms have been observed for several of the rare earths. Other... 

Ternary rare earth structure types relationships... 

The main relations that can be observed in the different rare earth structures, and how they are connected with one another are shown in table 11. 

The concept of structure slab shift has been used before to find geometrical relations between a number of different binary rare earth structures (Parthe and Moreau, 1977). The concept can also be applied to the structures of ternary compounds (see for example Figs. 10 and 73). 

Simple metals like alkalis, or ones with only s and p valence electrons, can often be described by a free electron gas model, whereas transition metals and rare earth metals which have d and f valence electrons camiot. Transition metal and rare earth metals do not have energy band structures which resemble free electron models. The fonned bonds from d and f states often have some strong covalent character. This character strongly modulates the free-electron-like bands. 

Reference has been made already to the existence of a set of inner transition elements, following lanthanum, in which the quantum level being filled is neither the outer quantum level nor the penultimate level, but the next inner. These elements, together with yttrium (a transition metal), were called the rare earths , since they occurred in uncommon mixtures of what were believed to be earths or oxides. With the recognition of their special structure, the elements from lanthanum to lutetium were re-named the lanthanons or lanthanides. They resemble one another very closely, so much so that their separation presented a major problem, since all their compounds are very much alike. They exhibit oxidation state -i-3 and show in this state predominantly ionic characteristics—the ions. 

The metal has a bright silvery metallic luster. Neodymium is one of the more reactive rare-earth metals and quickly tarnishes in air, forming an oxide that spalls off and exposes metal to oxidation. The metal, therefore, should be kept under light mineral oil or sealed in a plastic material. Neodymium exists in two allotropic forms, with a transformation from a double hexagonal to a body-centered cubic structure taking place at 863oC. 

As with other related rare-earth metals, gadolinium is silvery white, has a metallic luster, and is malleable and ductile. At room temperature, gadolinium crystallizes in the hexagonal, close-packed alpha form. Upon heating to 1235oG, alpha gadolinium transforms into the beta form, which has a body-centered cubic structure. 

Figure 7.6. A filled. skutterudite antimonide crystal structure. A transition niclal atom (Fc or Co) at the centre of each octahedron is bonded to antimony atoms at each corner. The rare earth atoms (small spheres) are located in cages made by eight octahedra. The large thermal motion of rattling of the rare earth atoms in their cages is believed be responsible for the strikingly low thermal conductivity of these materials (Sales 1997).

Structure and morphology. Most of the rare-earth elements were encapsulated in multilayered graphitic cages, being in the form of single-domain carbides. The carbides encapsulated were in the phase of RC2 (R stands for rare-earth elements) except for Sc, for which Sc3C4(20] was encapsulated[21]. 

The discovery of hafnium was one of chemistry s more controversial episodes. In 1911 G. Urbain, the French chemist and authority on rare earths , claimed to have isolated the element of atomic number 72 from a sample of rare-earth residues, and named it celtium. With hindsight, and more especially with an understanding of the consequences of H. G. J. Moseley s and N. Bohr s work on atomic structure, it now seems very unlikely that element 72 could have been found in the necessary concentrations along with rare earths. But this knowledge was lacking in the early part of the century and, indeed, in 1922 Urbain and A. Dauvillier claimed to have X-ray evidence to support the discovery. However, by that time Niels Bohr had developed his atomic theory and so was confident that element 72 would be a... 

Harrowfield et al. [37-39] have described the structures of several dimethyl sulfoxide adducts of homo bimetallic complexes of rare earth metal cations with p-/e rt-butyl calix[8]arene and i /i-ferrocene derivatives of bridged calix[4]arenes. Ludwing et al. [40] described the solvent extraction behavior of three calixarene-type cyclophanes toward trivalent lanthanides La (Ln = La, Nd, Eu, Er, and Yb). By using p-tert-huty ca-lix[6Jarene hexacarboxylic acid, the lanthanides were extracted from the aqueous phase at pH 2-3.5. The ex-tractability is Nb, Eu > La > Er > Yb. 

A freshly manufactured zeolite has a relatively high UCS in the range of 24.50°A to 24.75°A. The thermal and hydrothermal environment of the regenerator extracts alumina from the zeolite structure and, therefore, reduces its UCS. The final UCS level depends on the rare earth and sodium level of the zeolite. The lower the sodium and rare earth content of the fresh zeolite, the lower UCS of the equilibrium catalyst (E-cat). 

Rare Earth Level. Rare earth (RE) elements serve as a bridge to stabilize aluminum atoms in the zeolite structure. They prevent the... 

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