Cluster compounds, boron

There are a number of papers in the open literature explicitly reporting on the properties of boron cluster compounds for potential neutron capture applications.1 Such applications make full use of the 10B isotope and its relatively high thermal neutron capture cross section of 3.840 X 10 28 m2 (barns). Composites of natural rubber incorporating 10B-enriched boron carbide filler have been investigated by Gwaily et al. as thermal neutron radiation shields.29 Their studies show that thermal neutron attenuation properties increased with boron carbide content to a critical concentration, after which there was no further change. 

Another very useful rule for classifying the structures of polyboranes and hetero-boranes as well as many metal boron cluster compounds and their derivatives has been developed by Rudolph, Williams, Mingos and Wade (see Chapter 1.1.2) [4]. Today these are generally termed the Wade rules. They can be derived from the structures and electronic requirements of closed polyhedral boranes, such as an octahedron or an icosahedron, which are present in the anions B6H62 and B,2 H, 22. Since there are only exopolyhedral B-H bonds the number of electron... 

Elementary boron exists in the solid state in several allotropic forms. Those which have been properly characterized by X-ray diffraction contain icosahedral B12 units, which are also the basis of many boron cluster compounds. 

There is obviously a particular need to develop materials which can function at high temperatures. Due to their strong covalent bonding, boron cluster compounds generally possess attractive mechanical properties as materials, e.g. stability under high temperature due to their high melting points (typically >2300 K), chemical stability, resistance to acidic conditions, and small compressibility. Furthermore, importantly, the B12 icosahedra compounds have also been found to have intrinsic low thermal conductivity, as will be discussed in detail in later sections, and which is desirable for thermoelectric applications. 

Indeed, focusing on this low thermal conductivity, Slack et al. have investigated boron cluster compounds like beta boron, YB66 among others, as possible embodiments of the "electron crystal phonon glass" systems that they have proposed (Slack et al., 1971 Cahill et al., 1989). 

Boron cluster compounds have many electron deficient sites in them and they possess polyhedral skeleton. The procedure by which the boron cluster compounds are built from small polyhedral skeletons to a big polyhedral skeleton is in wide use for the synthesis of boron neutron capture therapy. The reactivity and reaction mechanism for the formation of the boron cluster compounds from a small cluster to a big cluster have been studied. 

H. A. Schroeder, Polymer chemistry of boron cluster compounds, Inorg. Macromol. Revs. 1,... 

Plesek, J. 1992. Potential applications of the boron cluster compounds. Chem. Rev. 92 269-278. 

Besides the interest in the basic aspects of the chemistry of polyhedral boron compounds, considerable effort has been put in the development of high-energy fuels for aircraft and rocket propulsion [6]. Other potential applications of the boron cluster compounds are presented in the review by J. PleSek [8]. 

All the polyhedral boron hydrides deseribed above are available from commereial sources. It should be noted, however, that the polyhedron boron hydrides, especially those produced from decaborane, are veiy expensive and find practical applications only in some exclusive areas where no alternative exists. On the other hand, a revival in industrial production of boron cluster compounds and their price reducing can be expected only after the development of some important applications for these species. 

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