Clusters boride

The structures of boron-rich borides (e.g. MB4, MBfi, MBio, MB12, MBe6) are even more effectively dominated by inter-B bonding, and the structures comprise three-dimensional networks of B atoms and clusters in which the metal atoms occupy specific voids or otherwise vacant sites. The structures are often exceedingly complicated (for the reasons given in Section 6.2.2) for example, the cubic unit cell of YB e has ao 2344 pm and contains 1584 B and 24 Y atoms the basic structural unit is the 13-icosahedron unit of 156 B atoms found in -rhombohedral B (p. 142) there are 8 such units (1248 B) in the unit cell and the remaining 336 B atoms are statistically distributed in channels formed by the packing of the 13-icosahedron units. 

Formation of tetrahedral My4 clusters is the structural unit common to all known MRgMT4B4-type phases with strong M.,—B, but weak B-M g interaction in accord to the characterization of borides. Involving two remote Mre atoms, the B... 

The electrical conductivity of these alloys is metallic i.e., inverse proportional to the temperature between 4 to 300 K and is about 7 x 106 (Qm) 1 at room temperature. The crystal structure determination [7] shows the B atomic arrangement to consist of coplanar 4B clusters (isolated from one another) with three B atoms at the vertices and one B at the center of a triangle as shown in Fig. 5. This isolated 4B atomic arrangement has no precedent either in metal-borides [14] or boron hydrides [15]. 

Up to the early 1990s, the only rare earth boride known with RB (n > 12) was RB66. These compounds have been primarily studied for their interesting structure and structurally derived features like the amorphous behavior of thermal properties. However, in a recent development, new higher borides have been discovered like the RB25 and RB50 compounds. Furthermore, with addition of small amounts of 3rd elements like C, N, Si, the boron cluster framework was found to arrange... 

Incidentally, the B6 octahedron (Longuet and Roberts, 1955) and B12 cuboocta-hedron (Lipscomb and Britton, 1960) are also found to be two-electron deficient. This electron deficiency of the clusters leads to dramatically different physical properties for trivalent and divalent metal borides as illustrated for example in the next section on the dodecaborides. More on the bonding requirements in the higher borides will be discussed in later sections. 

Finally, the framework formed by the boron clusters is relatively rigid and therefore, there are constraints on the size of the metal atoms/ions which can occupy the voids. That is why there are different boundaries regarding which rare earth atoms can form each particular higher boride (as will be seen in the following sections). A rare earth existence diagram for all the higher borides will be presented at the end of this review, see Section 13. 

As will also be seen in later sections on other higher borides, the experimental results indicated that the B12 icosahedra clusters are functioning to mediate the... 

Finally, we would like to stress again that it was the addition of small amounts of C and N in the synthesis of higher borides which caused these new structures to form. As a result, hitherto unknown configurations of the rare earth atoms confined in the boron cluster network were observed to appear, leading to these interesting properties. 

First of all, a rare earth existence diagram is given in Figure 45 for all the higher boride compounds discussed in this review. As noted before, size constraints on the voids which are created among the boron cluster networks result in different ranges of possible rare earth elements for the different compounds. 

Most of the higher borides described in this work were discovered within the space of 10 years, yet what other new attractive cluster compounds are waiting to be discovered along with their exciting and unexpected magnetic, electrical, and thermal properties ... 

Au2(PR3)2BH (observed for R = Ph) presumably leads to a true boride cluster at present only spectroscopic data are available (4). 

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