Solid state metal borides structures

Fehlner, T.P. Molecular models of solid state metal boride structure. J. Solid State Chem. 154, 110-113 (2000)... 

The second example comes from solid-state chemistry, and the coimection with cluster chemistry is less obvious. Despite this, it is an excellent example of how E/M variation can lead to systematic variation in structure and, consequently, to properties. Although the example is taken from solid-state metal borides, the silicides see Silicon Inorganic Chemistry) and phosphides (see Phosphides Solid-state Chemistry) could have been used. 

Table 12.3 Classification of the structures of solid state metal borides. 

Borides Sohd-state Chemistry Carbides Transition Metal Solid-state Chemistry Electronic Structure of Sohds Quasicrystals Structure Property Maps for Inorganic Solids Superconductivity Zintl Compounds. 

The metal borides are one of the five major classes of boron compounds (1). In the following we review the geometric and electronic structural data with an emphasis on the transition metal borides. Because the structures of transition metals and elemental boron provide end points, we begin by reviewing the solid state structures of these elements. A brief survey of the range of metal boride structures in general is followed by some more detailed consideration of the problems of electronic structure raised by the geometries of the transition metal borides. 

Finally, the structural modifications of elemental boron exhibit complex extended lattices of cages in the solid state, whereas those of metals possess much simpler close-packed atomic lattices. These differences are a direct reflection of atomic properties and result in the respective nonmetallic and metallic behavior. However, boron combines with most other elements including metals. There are a wide range of metal borides known with stoichiometric as well as nonstoi-chiometric atomic ratios. The amazingly varied interpenetration of the two characteristic structural motifs and the subtly balanced competition between the two modes of solid state bonding found in the metal borides constitutes further justification of our theme. This is discussed in some detail in Section II,C. 

We have already pointed out that the most stable forms of the solid state bonding of elemental boron and metals differ in an essential aspect. Hence, in the solidification of a melt containing a random mixture of metal and boron atoms the observed structure will be determined by a balance between the tendencies for boron to form a covalently bound network and the metal to form a close-packed lattice. Among other things, this competition will depend on relative metal and boron concentrations and one expects in proceeding from the metal-rich to the boron-rich borides that the B-B bonded network will become more extensive and dominant. 

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