Lanthanum , crystal structure

Fig. 8.12 Crystal structure (a), band structure of La3(B2N4) (b), and orbital interactions along [B2N4] stacks (c) (interactions with lanthanum orbitals are omitted for clarity).

The difference in catalytic activity between the La- and the Ba-based hexa-aluminates results from the following reasons the first difference is the valence of cation in the mirror pleuie between tri-valent lanthanum ion and di-valent barium ion. The second is the crystal structure between magnetoplumbite and P-alumina, which are different in the coordination of ions and concentration of Frenkel-type defect in mirror plane. The redox cycle of transition metal in hexa-aluminate lattice, which closely related with catalytic activity, is affected sensitively with these two factors. 

Figure 10.7 illustrates the prototype hexaboride crystal structure, that of lanthanum hexaboride. It consists of a simple cubic array of boron octahedra surrounding a metal atom at the body center of each cube. The octahedra are linked by B-B bonds connecting their comers. This makes the overall structure relatively hard with approximately the hardness of boron itself since plastic shear must break B-B bonds. The open volumes surrounded by boron octahedra are occupied by the relatively large lanthanum atoms as the figure shows schematically. 

Figure 10.7 Crystal structure of Lanthanum Hexaboride (prototypre hexaboride). The black circles represent boron octahedra. They form a simple cubic arrangement surrounding the central metal atom.

An alternative version of the lanthanum hexaboride crystal structure has the boron octahedra occupying the body centered positions of the cubic array of lanthanum atoms (Figure 10.8). This version makes it clear that in order to plastically shear the structure, the boron octahedra must be sheared. Note that the octahedra are linked together both internally and externally by B-B bonds. 

Figure 10.8 Alternative drawing of the crystal structure of Lanthanum Hexaboride with the metal atoms occupying the cube corners.

Perovskites, 27 358 band structure, 38 131-132 crystal structure, 38 123-125 Perovskite-type oxides see also specific lanthanum-based catalysts actinide storage in radioactive waste, 36 315-316... 

The trivalent rare-earth crystal structure sequence from hep - Sm type -> La type -> fee, which is observed for both decreasing atomic number and increasing pressure, is also determined by the d-band occupancy. Figure 8.11(a) shows the self-consistent LDA energy bands of fee lanthanum as a function of the normalized atomic volume fi/Q0, where Q0 is the equilibrium atomic volume. We see that the bottom of the NFE sp band moves up rapidly in energy in the vicinity of the equilibrium atomic volume as the free electrons are compressed into the ion core region from where they are repelled by orthogonality constraints (cf eqn (7.29)). At the same time the d band widens, so that the number of d electrons increases under pressure... 

Bkouche-Waksman, I. Guilhem, J. Pascard, C. Alpha, B. Deschenaux, R. Lehn, J.-M. 110. Crystal structures of the lanthanum(III), europium(III), and terbium(III) cryptates of tris(bipyridine) macrobicyclic ligands. Helv. Chim. Acta 1991, 74,1163-1170. 

All the determined crystal structures exhibited hexagonal arrays of metal-metal interactions with diagonal interactions in the layer. All the lanthanum salts and the europium one were isostructural. 

Ciampolini, M., Dapporto, P., and Nardi, N. (1979) Structure and properties of some lanthanoid(III) perchlorates with the cryptand 4,7,13,16,21,24-hexaoxa-l,10-diazabicyclo[8.8.8]hexacosane, Dalton Trans, 974-977 Hart, F. A., Hursthouse, M. B., Abdul Malik, K. M., and Moorhouse, S. (1978) X-ray crystal structure of a cryptate complex of lanthanum nitrate, Chem. Commun. 549-50. 

To evaluate the factors affecting the structural stability of some crystalline materials that are potential hosts for radioactive wastes, the crystal structures of a series of 3+p5 xv5+o compounds, where A is lanthanum or a member of the rare-earth series, were determined. The end-member phosphates (APO4) have the monoclinic Monazite structure (P2 /n) for A La, Ce-Gd, and the tetragonal Zircon structure (l4]/amd) for A Tb - Lu. The corresponding vanadates have the Monazite structure only for LaVO, and the Zircon structure for A = Ce - Lu. When the end members are iso-structural, e.g., LaPO /LaVO, Monazite, YbPC /YbVOA,... 

The lanthanum or europium atoms had no interaction with the gold and silver centers, and the determined crystal structures exhibited hexagonal arrays of metal-metal interactions with diagonal interactions in the layer and the lanthanoids in the middle of the hexagonal prisms. In both examples, the lanthanum and the europium salts were isostructural (Fig. 5). 

One property of a transition metal ion that is particularly sensitive to crystal field interactions is the ionic radius and its influence on interatomic distances in a crystal structure. Within a row of elements in the periodic table in which cations possess completely filled or efficiently screened inner orbitals, there should be a decrease of interatomic distances with increasing atomic number for cations possessing the same valence. The ionic radii of trivalent cations of the lanthanide series for example, plotted in fig. 6.1, show a relatively smooth contraction from lanthanum to lutecium. Such a trend is determined by the... 

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. 

The crystal structures of actinium compounds, where they have been studied, for example, in AcH3, AcF3 AczS3i and AcOCl, are the same as those of the analogous lanthanum compounds. 

Figure 1.29 The coordination status of the central ion and the crystal structure of the REPO4 (RE = lanthanum to gadolinium) compounds [25]. (Reprinted from Inorganica Chimica Acta, 109, no. 2, D.F. Mullica, D.A. Grossie, and L.A. Boatner, Coordination geometry and structural determinations of SmP04, EUPO4 and GdP04, 105-110, 1985, with permission from Elsevier.)...

The materials derived from YBa2Cu307 by replacing yttrium with other rare earth elements (lutetium, ytterbium, thulium, erbium, hohnium, dysprosium, gadolinium, europium, samarium, neodymium, lanthanum) are also superconductors, with r, s of 88 to 96 K. The crystal structures of RBa2Cu307 are almost the same as those of YBa2Cu307. The lattice constant is slightly different for the different ionic radii of the rare earth elements, and yet their chemical and physical properties are almost the same as those of YBa2Cu307. 

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