Lattice constants rare earth elements

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. 

While perhaps with not as wide variety as the borides, the rare earth elements also form interesting compounds with carbon to form the rare earth carbides. The phases are particularly rich for the relatively metal-rich carbide compounds. Adachi etal. have written a detailed, 129-page long review on the rare earth carbides, while Gschneidner and Calderwood have comprehensively reviewed phase diagrams and lattice constants of binary rare earth carbides. ... 

SmSe exists, like SmS (see Rare Earth Elements C 7, 1983, p. 221), in a semiconducting form with divalent Sm and a metallic form with Sm in the intermediate valence state between two and three. Both forms have a cubic NaCl structure, but the metallic SmSe has smaller lattice constants. In contrast to SmS, the pressure-induced phase transition of SmSe is continuous. Only Singh et al. [1, 2] report the existence of hexagonal SmSe films. 

Details regarding the crystal structure of intermetallic compounds based on rare earth elements can be found in Volume 2 of the Handbook on the Physics and Chemistry of Rare Earths (landelli and Palenzona, 1979). Lattice constants and structure type of these intermetallics have furthermore been compiled in the two reviews published by Buschow (1977a, 1979). As will be shown in the following sections, many investigations of hydrogen sorption in intermetallics pertain to compounds formed between rare earth elements and 3d transition elements. 

FIGURE 29 Lattice constants a and c of RNi2B2C for various elements R versus the ionic radii of R3+ ions, measured at 300 K. For R = Ce (open symbols) both Ce3+ and Ce4+ have been considered on the abscissa. Both the radii of Ce3+ and Ce4+ do not fit the curve observed for the other rare earths (from Siegrist et al., 1994b). 

The trend of the cube root of V is reported in figs. 13.23 and 13.24. For CsCl type compounds the straight lines have slopes near V3, which corresponds to the additivity of the interatomic R-X distances, as in a hard sphere model. Lattice constants of these compounds can therefore be estimated by summing the rare earth ionic radius with an apparent radius for the partner element and multiplying this sum by 2V3 (table 13.45). 

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