Crystal structure rare earth elements

So far, the bonding and surface structure aspects of electrocatalysis have been presented in a somewhat abstract sort of way. In order to make electrocatalysis a little more real, it is helpful to go through an example—that of the catalysis of the evolution of oxygen from alkaline solutions onto substances called perovskites. Such materials are given by the general formula RT03, where R is a rare earth element such as lanthanum, and T is a transition metal such as nickel. In the electron catalysis studied, the lattice of the perovskite crystal was replicated with various transition metals, i.e., Ni, Co, Fe, Mn, and Cr, the R remaining always La. 

The present chapter reviews the structural chemistry (Section 2) and properties and applications (Section 3) of polyoxometalates that incorporate one or more rare-earth elements. In most cases these are discrete anionic entities within the crystal and in solution, but there are also extended lattices in which POM groups are linked by rare-earth cations. Solids which can best be described as mixed oxides, or which appear to be salts of common polyoxometalate architectures such as the... 

The rare earth elements are different from other elements because the optical transitions between levels of the fn configuration are inherently very sharp-lined and have well-resolved structure characteristic of the local crystal fields around the ion. In minerals, this characteristic provides an excellent probe of the local structure at the atomic level. Examples will be shown from our work of how site selective laser spectroscopy can be used to determine the thermal history of a sample, the point defect equilibria that are important, the presence of coupled ion substitution, the determination of multiple phases, and stoichiometry of the phase. The paper will also emphasize the fact that the usefulness and the interpretation of the rare earth luminescence is complicated by the presence of quenching and disorder in mineral samples. One in fact needs to know a great deal about a sample before the wealth of information contained in the site selective luminescence spectrum can be understood. 

Many oxohalides of rare-earth elements have been characterized. The crystal of y-LaOF has a tetragonal unit cell with a = 409.1 pm and c = 583.6 pm, space group PAInmm. In this structure, each La3+ is coordinated by four O2- and four F anions, forming a distorted cube, as shown in Fig. 18.2.4. The distance of La-O is 261.3 pm and La-F 242.3 pm. 

Figure 2(C) is the result of a calculation that illustrates which cations might fit into sixfold coordination position in the calcite group structures. It is an interesting insight as both light and heavy rare earth elements are possible substitutes for calcium in the calcium carbonate structure, i.e., they plot within 15% of the calcium ionic size. However, some of the end-members incorporate elements into this crystal structure and are outside this deviation but within 30%, an expression of the potential physical expansion for this layered crystal structure. These are ionic charge differences important in whether a stable crystalline structure can be produced. Trace amounts of all these ions can be incorporated in calcite and may dictate the morphology of the crystallites. Therefore, the presence and amount of any ions in the environment in which carbonate crystallization occurs may possibly be recorded. However, in spite of the predominance of sodium and potassium in the solutions where...

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. 

The crystal structure of LuSBr has been determined.262 The structure comprises planes of Lu4S tetrahedra, with Lu—S bond distance 2.66 A, separated by layers of halogen atoms. The oxysulphides of the rare-earth elements have been prepared263 by the action of sulphur vapour, diluted with argon, on the oxides at temperatures between 1050 and 1120°C. 

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. 

---