Lanthanum, hexagonal structure

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 In addition, unpublished data indicate that lanthanum and cerium form still another polymorph (Prokofev, 1980).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). 

At 31OC, lanthanum changes from a hexagonal to a face-centered cubic structure, and at 865C it again transforms into a body-centered cubic structure. 

Structures of the lanthanide nitridoborates appear as layered structures with approximate hexagonal arrangements of metal atoms, and typical coordination preferences of anions. As in many metal nitrides, the nitride ion prefers an octahedral environment such as in lanthanum nitride (LaN). As a terminal constituent of a BNx anion, the nitrogen atom prefers a six-fold environment, such as B-N Lns, where Ln atoms form a square pyramid around N. Boron is typically surrounded by a trigonal prismatic arrangement of lanthanide atoms, as in many metal borides (Fig. 8.10). All known structures of lanthanide nitridoborates compromise these coordination patterns. 

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. 

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). 

Structure Non-hydrated rare earth fluorides have two different crystal systems, a hexagonal system (lanthanum to terbium) and an orthorhombic system (dysprosium to lutetium, yttrium). In the crystal of LaFs, the central ion is nine coordinated by nine fluoride atoms. Each fluoride atom further connects with two lanthanum atoms through a [13-bridge to form an infinite polymer. 

The structure of the Rh-doped lanthanum manganite is modified when stabilised in the reduced state the XRD patterns corresponds now to the stoichiometric form LaMn03.oo (orthoihombic, ICDD n° 35-1353), which is slightly different fiom that of (hexagonal, thombohedrally... 

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