Crystal structure hexaboride

Hexaborides of a CaBg type are formed by K, the alkaline earths, Y and the larger lanthanides, as well as Th and some actinides ". The crystal structure of these compounds with cubic symmetry (Pm3m, O, ) (see Fig. 1) is characterized by a three-dimensional skeleton of Bg boron octahedra, the interstices of which are filled by metal atoms. The connection between two octahedra is by a B—B bond of length 1.66 X 10 pm, whereas the B—B bond lengths in one octahedron are 1.76 X 10 pm. ... 

Figure 10.7 illustrates the prototype hexaboride crystal structure, that of lanthanum hexaboride. It consists of a simple cubic array of boron octahedra surrounding a metal atom at the body center of each cube. The octahedra are linked by B-B bonds connecting their comers. This makes the overall structure relatively hard with approximately the hardness of boron itself since plastic shear must break B-B bonds. The open volumes surrounded by boron octahedra are occupied by the relatively large lanthanum atoms as the figure shows schematically. 

The hexaboride crystal structure is related to the CsCl structure so by analogy the glide planes are (100) and the glide directions are (100). At the cores of glide disloca-tions the structure becomes quasi-hexagonal. 

Figure 10.7 Crystal structure of Lanthanum Hexaboride (prototypre hexaboride). The black circles represent boron octahedra. They form a simple cubic arrangement surrounding the central metal atom.

An alternative version of the lanthanum hexaboride crystal structure has the boron octahedra occupying the body centered positions of the cubic array of lanthanum atoms (Figure 10.8). This version makes it clear that in order to plastically shear the structure, the boron octahedra must be sheared. Note that the octahedra are linked together both internally and externally by B-B bonds. 

Figure 10.8 Alternative drawing of the crystal structure of Lanthanum Hexaboride with the metal atoms occupying the cube corners.

Figure 11.10 Crystal structure of oxygen hexaboride showing one layer of the hexagonal structure.

Figure 4.10. The crystal structure of cubic metal hexaborides The boron network consists of interlinked octahedra.

With regard to synthesis, it appears that it is harder to grow single crystals for the rare earth diborides compared to other systems such as the tetraborides and hexaborides. Therefore, there have been relatively fewer investigations into its physical properties despite its simple crystal structure and composition. 

The rare earth hexaborides RBg crystallize in the cubic (CaBe type) structure which possesses a CsCl type arrangement of R atoms and Be octahedra. Figure 7 shows the crystal structure of RBe and its simple cubic Brillouin zone. LaBe is a reference non-f compound. CeBg is a typical Kondo-lattice compound undergoing two magnetic ordering... 

No attempt however has been made to include papers concerning numerous technical alloys (i.e., much has been said about physical properties of R hexaborides), unless they revealed valuable information on phase equilibria, solid solubilities, lattice parameters or crystal structure data. 

The hexaborides of the rare earth elements show interesting properties. RB crystallizes in the CaB structure, where the cations and the B6-octahedrons form a lattice of CsCl-type (fig. 133). In such a lattice two Debye temperatures are observed a low-temperature one (0i 150 K) connected with the soft metal sublattice and a high-temperature one (0 600 K) related to the hard boron sublattice (Paderno... 

A continuous solid solution (Y, La)B6 with CaB -type of structure, Pm3m, was found from specimens b), but a restricted solid solution appears from alloys a), due to the different melting behavior of YBj (incongruent) and LaBg (congruent), which results in a nearly complete fractional crystallization of the two hexaborides. At variance with the results obtained by Bondarenko et al. (1966) on samples prepared by borothermal reduction, lattice parameters of specimens b) closely obey Vegard s rule and are shown in fig. 36 including superconductivity data. 

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