Predicting Useful Materials

Researchers have synthesized a compound, beta-Csub 3Nsub 4, whose properties correspond to those predicted from theory. The synthesis of this compound, which is carbon nitride in the beta-Sisub 3Nsub 4 structure, shows that theory can point the way toward the creation of materials with hardness comparable with or larger than that of diamond and that such a material has been correctly predicted. The theoretical approach is based upon the modern standard model of solids. One of the cornerstones of this approach is the ability to predict hardness from microscopic information. A semiempirical approach uses the formula B = Nsub c/4 times (1972 - 220I)/dsup 3.5, where B = bulk modulus, Nsub c = coordination number, I = empirical ionicity parameter, and d = bond length in angstroms. This equation yields values of B that are in good agreement with experiment and rival costly first-principles calculations for accuracy. In synthesizing beta-Csub 3Nsub 4, the researchers examined the binding energy for C and N and determined that the C-N covalent bond is strong and similar to the C-C bond in diamond. These studies demonstrate that it is possible to use theory to design materials with predictable properties.