Structure types lanthanum

Structure type LasCuSiSy (Collin and Laruelle, 1971) (Figure 91, Table 41). SGP63, Z = 2, a = 1.0310, c = 0.5794 run. The sulfur atoms form bicapped trigonal prismatic arrangement aroimd the lanthanum atom, and the shortest La-S distance is 0.2869 nm. The copper atom is located on the... 

Structure type La4Ag2ln4Si3 (Gulay et ah, 2008d) (Figure 106, Table 56). SG Pham, Z = A, a = 2.0523, h = 2.5118, c = 0.40241 nm. All four independent lanthanum atoms are located in bicapped trigonal prisms made of sulfur, and the shortest La-S distance is 0.2933 nm. The silver atoms... 

Double chromates of lanthanum, promethium and neodymium of the type RbLn-(Cr04)2 have been found to be anhydrous. The samarium compound crystallized with 0.5 molecules of water whilst those from europium to lutetium had the composition RbLn(Cr04)2,H20. Y-Ray and i.r. studies showed the existence of three structural types as the rare earth element series was transversed. 

Figure 9 Crystal structure of NiAs-type lanthanum monoiodide, Lai...

The crystal structure of the rare earth oxyorthosUicates has been determined from single crystals prepared using Bi203 as a flux material (Buisson and Michel, 1968). The oxyorthosilicates from praseodymium to terbium are isomorphic and crystallize in the monocUnic space group P2 /c with Z = 4. According to Ananeva et al. (1981) dysprosium can also have this structure type (A) too, but more often has another structure type (B). It is not clear whether or not lanthanum has a structure of its own. The unit cell dimensions of lanthanide oxyorthosilicates, except those of cerium and promethium compounds, are presented in table 9. 

Curium metal exists in two modifications, a double hexagonal close-packed (dhcp) structure (a-lanthanum type) and a high-temperature cubic close-packed (fee) structure. Using Cm, the dhcp form was found to have lattice constants a = 3.496(3)andc = 11.331(5) A, giving a calculated density of 13.5 gem and a metallic radius of 1.74 A [27,97]. In a recent pressure study, slightly less accurate lattice constants were obtained (yielding a calculated density of 13.8 g cm ) but did establish that the dhcp phase was stable at least to 6.5 GPa [162]. 

The RbR(S04)2 compounds can be prepared either by isothermal evaporation of an Rb2S04-R2(S04)3 aqueous solution at 70°C or by a solid-state synthesis. The former method can be applied only for the lighter rare earths (La Eu) for with the heavier rare earths the monohydrate forms under these conditions (Sarukhanyan et al., 1982, 1984d). The recent studies by Prokofev et al. (1979) and Sarukhanyan et al. (1983, 1984b,d) have established the existence of three main structure types (a) monoclinic (La - Tb), (b) orthorhombic (Pr Er), and (c) monoclinic (Dy--Lu). In addition, unpublished data indicate that lanthanum and cerium form still another polymorph (Prokofev, 1980). Owing to its smaller ionic size, scandium probably forms a sulfate with a different structure and indeed a hexagonal structure has been proposed on the basis of powder diffraction patterns (Ivanov-Emin et al., 1966 Erametsa and Haukka, 1968). 

Of the rare earth orthovanadates only LaV04 crystallizes with monazite structure type, isomorphic with monoclinic LaP04. The structure contains approximately tetrahedral VO4 units and a very irregular coordination polyhedron around lanthanum (fig. 111). In a preliminary structure determination of LaV04... 

Table 4. Structural parameters of Fe-doped apatite-type lanthanum silicate prepared via MA of Si02-La203-Fe203 followed by annealing at 1200°C.

Table 6. Structural parameters of LaiOs, La(OH)3 and apatite-type lanthanum silicate.

The structure of the perovskite-type lithium ion conductor Li0 29La0 57Ti03 is represented in Fig. 6. The small gray circles depict the lithium ions, the big gray circles the lanthanum ions. These are randomly distributed over the A sites 14 per-... 

Figure 6. Structure of the perovskite-type lithium-ion conductor Li 2yLa057TiO3. The lithium ions (small, gray) and the lanthanum ions (large, gray) are randomly distributed over the A sites, of which 14 percent are vacancies, enabling the lithium ions to be mobile. Titanium forms TiOh octahedra, as shown in yellow. The unit cell is indicated.

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. 

Lanthanum chromite is a p-type conductor so divalent ions, which act as electron acceptors on the trivalent (La3+ or Cr3+) sites, are used to increase the conductivity. As discussed above, the most common dopants are calcium and strontium on the lanthanum site. Although there is considerable scatter in the conductivities reported by different researchers due to differences in microstrucure and morpohology, the increase in conductivity with calcium doping is typically higher than that with strontium doping [4], The increase in conductivity at 700°C in air with calcium additions is shown in Figure 4.1 [1, 2, 28-44], One of the advantages of the perovskite structure is that it... 

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... 

A similar structure for La(III) in La2(S04)3 9H2O has been established (186). Here the twelve sulphate oxygens constitute a slightly distorted icosahedron (La—0 = 2.60—2.74 A) around type I lanthanum ions. The distortion is probably due to the packing of the crystals. 

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... 

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