Crystal structures tetravalent metals

The anions [XMoi2042]8 (Figure 19), form weak complexes in aqueous solution with divalent Mn, Fe, Co, Ni, Zn, Cu, Cd, trivalent Y, Er, Yb and tetravalent Th.91 Two crystal structures have been determined and contain the anions [(UMoi2042) Er(OH2)5 2]2 and [(UMoi2042)Th(OH2)3]4l. 1,2 In both cases the external metal atoms are attached to opposite sides of the heteropolyanion via three terminal oxygen atoms, one from each of three different Mo207 units. 

Some interesting effects associated to the presence of well-defined structural units appear on a broad class of binary alloys formed by mixing an alkali metal (Li, Na, K, Rb, Cs) with a tetravalent metal like Sn or Pb. Due to the large difference in electronegativities it is normally assumed that one electron is transferred from the alkali to the tetravalent atom. As the Sn- or Pb-anions are isoelectronic with the P and As atoms, which in the gas phase form tetrahedral molecules P4 and AS4, in the same way the anions group in the crystal compounds forming (Sn4)4- and (Pb4)4- tetrahedra, separated by the alkali cations. This building principle was developed by Zintl in the early thirties [1], and the presence of such tetrahedra has been detected in the equiatomic solid compounds of Pb and Sn with Na, K, Rb and Cs, but not with Li [2, 3, 4]. In this paper we focus on alkali-lead alloys. 

Americium was isolated first from plutonium, then from lanthanum and other impurities, by a combination of precipitation, solvent extraction, and ion exchange processes. Parallel with the separation, a vigorous program of research began. Beginning in 1950, a series of publications (1-24) on americium put into the world literature much of the classic chemistry of americium, including discussion of the hexavalent state, the soluble tetravalent state, oxidation potentials, disproportionation, the crystal structure(s) of the metal, and many compounds of americium. In particular, use of peroxydisulfate or ozone to oxidize americium to the (V) or (VI) states still provides the basis for americium removal from other elements. Irradiation of americium, first at Chalk River (Ontario, Canada) and later at the Materials Testing Reactor (Idaho), yielded curium for study. Indeed, the oxidation of americium and its separation from curium provided the clue utilized by others in a patented process for separation of americium from the rare earths. 

Tetravalent metals may also be bonded by XO4 and/or to R—XO3 groups so as to build up a layered structure different from the a-structure. These compounds are usually called y-layered M(IV) phosphates. Unfortunately, single crystals large enough for X-ray determination have not been obtained and the structure of y-compounds is still unknown. Recent... 

In order to fully understand the crystal chemistry of the anhydrous LnXs and their solvates ([LnXj(solv) ]), the Ln atomic properties of these species must be considered. The predominant oxidation state for LnX species is the +3 state however, for a number of these cations, tiie +2 (see The Divalent State in Solid Rare Earth Metal Halides) and +4 (see Tetravalent Chemistry Inorganic) states are available. Since the bonding in these compounds is mainly ionic, the cation size and sterics of the binding solvent play a significant role in determining the final crystal structures isolated. The ionic nature of the LnX complexes makes... 

Protactinium oxides can be stabilized in the tetravalent and pentavalent state. The most stable oxide phase obtained by the burning of metal or protactinium compoimds is the white pentoxide, Pa20s. The structme of the pentoxide is related to fluorite and has cubic symmetry. Pa02 is a black solid that crystallizes in the cubic fluorite structure. 

Cerium oxide, ceria, has a fluorite structure and shows oxide anion conducting behavior differ from other rare earth oxides. However, the O ionic conductivity of pure ceria is low because of a lack of oxide anion vacancies. For ion conduction, especially for anion, it is important to have such an enough vacancy in the crystal lattice for ion conduction. Therefore, the substitution of tetravalent Ce" by a lower valent cation is applied in order to introduce the anion vacancies. For the dopant cation, divalent alkaline earth metal ions and some rare earth ions which stably hold trivalent state are usually selected. Figure 9-28 shows the dopant ionic radius dependencies of the oxide ionic conductivity for the doped ceria at 800°C. In the case of rare earth doped Ce02, the highest O ion conductivity was obtained for... 

Fig. 12. Demonstration of typical chemical shifts in lanthanide Lm absorption. The energy calibration of the spectra recorded from the Pr absorption in the compounds is accurate within 0.2 eV with respect to o fixed at the intersection point of the high-energy absorption with the absorption line in (dhcp) Pr metal (dashed-dotted line). The intersection point shifts to higher energies with decreasing metallic character of the compounds. The maxima of the prominent main lines, however, remain unshifted, just as the onsets of the lines. PrCu crystallizes in orthorhombic FeB structure (a = 7.343 A, 6 = 4.584 A, c = 5.604 A). The semi-metals PrSb, PrBi have fee (NaCl) structure a = 6.366 and 6.463 A, respectively). Pr Oi] is a nonstoichiometric modification of nominally tetravalent Pr02 (fluorite type, Cap2). The bar diagram indicates ligand-field-split absorption lines for both, the tri- and tetravalent valence states in Pr Oij (cf. section 14).

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