Crystal structures of the rare-earth metals

The 17 rare-earth metals are known to adopt five crystalline forms. At room temperature, nine exist in the hexagonal closest packed structure, four in the double c-axis hep (dhep) structure, two in the cubic closest packed structure and one in each of the body-centered cubic packed and rhombic (Sm-type) structures, as listed in Table 18.1.1. This distribution changes with temperature and pressure as many of the elements go through a number of structural phase transitions. All of the crystal structures, with the exception of bep, are closest packed, which can be defined by the stacking sequence of the layers of close-packed atoms, and are labeled in Fig. 18.1.1. 

and Yb are excluded, then the remaining 14 rare-earth metals can be divided into two major subgroups  

Within each group, the chemical properties of the elements are very similar, so that they invariably occur together in mineral deposits. 

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In summary, the surprises in the crystal structures of the rare earth metals are ... 

Crystal structure of the rare earth metals and related properties at 24°C... 

Table 2 Crystal structures of the rare earth metal dihalides, stmcture types (ST), coordination numbers (CN), and shortest R-R distances (wherever known)...

Finally, the crystal structure of the rare-earth trihalides and the coordination of the rare-earth ion in the crystal lattice represents substantial interest and it will be addressed, since it has been argued (Papatheodorou 1982) that the coordination of the metal atom in a series of transition and rare-earth metal halide vapor complexes is most probably preserved on going from the solid halide to the vapor complex molecule. In each group... 

The crystal structures of the borides of the rare earth metals (M g) are describedand phase equilibria in ternary and higher order systems containing rare earths and B, including information on structures, magnetic and electrical properties as well as low-T phase equilibria, are available. Phase equilibria and crystal structure in binary and ternary systems containing an actinide metal and B are... 

The monoxides are known for all of the rare earth metals. They are all crystallizing at low temperatures isostructural to the NaCl-type. EU3O4 crystallizes orthorhombic [45, 46] with space group Pnam. This mixed valent compound contains Eu + and Eu + in the cation sublattice with a distorted octahedral coordination around Eu + and a coordination number of eight around Eu +. Fig. 3-19 shows the projection of the structure down [010]. The octahedral framework around Eu + can be related by a twinning operation (glide reflection) to the structure of ramsdellite (hep). 

Recently, Hoffmann et al. (1989) reported a series of the ErgRhjCij-type compounds present in the R-Rh-C system. These compounds RgRhjCjj (R=Y, Gd-Tm) are thermodynamically stable at 900°C and have a monoclinic structure, space group C2/m, with Z — 2 formula units per unit cell. The lattice parameters of these compounds have been measured by single-crystal X-ray diffraction. The structure contains a finite chain-like centrosymmetric polyanion [Rh5Ci2] with two Rh-Rh bonds (2.708 A) and six pairs of carbon atoms. The shortest distances between adjacent Rh5Ci2 clusters are the Rh-C distances of 2.94 A and the Rh-Rh distances of 3.27 A, and thus the Rh5Ci2 units may be treated as isolated from each other. This structure is characterized by three different kinds of C2 pairs. The C-C bond distances of 1.27, 1.32 and 1.33 A are between those of a triple bond (1.20 A) and a double bond (1.34 A) in hydrocarbons, and are the shortest found so far in ternary carbides of the rare earth metals with transition metals. 

Although the crystal structures of about half of the rare earth metals were well established by the mid- to late 1930 s the work by Klemm and Bommer (1937) stands as a landmark paper because they determined the crystal structures and the lattice parameters of all the lanthanides, except holmium, samarium and radioactive promethium. In so doing they... 

Summary of the most important crystal structures at the six most common compositions, and the corresponding M element which form the given structure with at least some of the rare earth metals. 

The first crystal of the rare earth metals grown from the vapor for a property determination (its crystal structure) was a Sm sample grown by Daane, et al. (1953). Schieber (1967) reported crystals of Sm up to 25 mm long grown by vapor... 

PREPARATION AND BASIC PROPERTIES OF THE RARE EARTH METALS 215 6. Crystal structure... 

There have been several papers which have appeared in the last five years concerning unusual lattice parameters and crystal structures in thin films of the rare earth metals. It is believed that these results are for highly contaminated metal which results from the high reactivity of the metallic rare earth thin films with residual gaseous impurities in the high vacuums [ 10" Torr (10 Pa)], Boulesteix et al. (1970b) and Gasgnier et al. (1974) have shown many of these structures to be impurity induced. 

Fig. 8.1. A bar graph summary of room temperature diamond pyramid hardness values of the rare earth metals including melting temperatures (K) and room temperature crystal structures. Solid portions of the bars indicate the lowest known value while the cross-hatched portions represent the range of values reported.

Figure 3 Crystal structure (tetragonal) of the rare earth metal diiodides Lal2, Cel2, Prl2-1, and HP-Ndl2 projection approximately down [100]...

FIGURE 4 Crystal structures exhibited by the rare earth elements, (a) Hexagonal dose packed (hep), (b) cubic dose packed (cep), (c) double hexagonal close packed (dhep), and (d) the complex structure of Sm. [From Gschneider, K. A., Jr. (1961). Crystallography of the rare-earth metals. In The Rare Earths (F. H. Spedding and A. H. Daane,... 

FIGURE 7 Magnetic structures of the rare earth elements Tm, Er, Ho, Dy, Tb, and Gd. The oval shape represents a plane normal to the unique direction of the crystal. This is the plane defined by seven, atoms of a hexagonal face in Fig. 2. The arrows represent the direction of the magnetic moments with respect to this plane. [From Koehler, W. C. (1972). Magnetic structures of rare earth metais and alloys. In Magnetic Properties of Rare Earth Metals (R. J. Elliot, ed.), p. 88, Pienum Press, New York.]... 

Another feature of the systems of this subgroup is the large extent of the limited solid solutions. Differences in the crystal chemistry characteristics of the rare-earth metals in the systems considered are insignificant, but they influence composition, structure and the number of ternary compounds. Ternary compounds with the composition Sco.3Ro.7Ge2 (R=Y, Dy) were found to exist for the Sc and Ge ternaries with Y and Dy on the digermanide section. They have no equivalent in other... 

The preceding chapter (173) of this volume (Salamakha et al. 1999) is a review of ternary R-M-Ge systems which have been studied over the entire concentration region and which have been only partly investigated. The character of interaction in these systems varies, and major differences are seen in the number of ternary germanides observed within the investigated concentration range. This chapter is devoted to the description of the structure types and the crystal chemistry of the ternary (quaternary) germanides of the rare-earth metals. 

This paper is dedicated to the memory of Professor Frank H. Spedding, Iowa State University, who died on December 15, 1984 at the age of 82. It is especially appropriate to the subject of this paper because a fair fraction of his many scientific endeavors was devoted to the study of the rare earth metals—especially their preparation, properties and alloying behavior of the intra rare earth binary alloys. His contributions (along with his many collaborators) in this area include the first to prepare metallic Sm metal the determination of its unique crystal structure the discovery of the Sm-type structure in an intermediate alloy phase formed by a light and a heavy lanthanide the first to ascertain that the crystal structure of the high temperature phase was bcc and the first to show that there is no detectable separation between liquidus and solidus (< 1°C) in binary alloy systems of the lanthanide metals that have atomic numbers within four of each other. 

The crystal structures, lattice parameters, metallic radii for a coordination number of 12, atomic volumes and the densities of the rare earth metals at 24°C or below are... 

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