A New Material Harder Than Diamond ? Carbon ( iv )

The prediction of the existence and the structure of previously unknown solid-state compounds seems to be par­ ticularly difficult, because numerous complex, interacting factors must be taken into account. A simplification to a certain extent is possible with predominantly covalent poly­ meric solid-state compounds in which coordination numbers and geometries are unambiguously stipulated for the con­ stituent elements. Desired properties of new materials are, above all, extreme hardness, good thermal conduction, and high thermal stabil­ ity. These properties are particularly likely in compounds with highly cross-linked structures with mainly covalent chemical bonds that are very stable. The material property, hardness, can be described by the bulk modulus 2?, which, on atomic scale, is influenced fundamentally by the strength and compressibility of the chemical bonds. Thus logically, the highest bulk moduli have been found for covalent materials: diamond not only has the highest value (4.43 Mbar), but also is the solid with the greatest hardness. A simple empirical relationship for the estimation of the bulk modulus Β of solids with the zinc blende structure was presented recently by Liu and Cohen [Eq. (a)]. 1 1 This pre-