STRUCTURAL-PROPERTIES AND ELECTRONIC-STRUCTURE OF LOW-COMPRESSIBILITY MATERIALS - BETA-SI3N4 AND HYPOTHETICAL BETA-C3N4

We present a first-principles pseudopotential study of the structural and electronic properties of -Si3N4 and the hypothetical compound -C3N4. -C3N4, which is C3N4 in the -Si3N4 structure, with C substituted for Si, is used as a prototype for investigating the properties of possible covalent C-N solids. The calculated lattice constant, bulk modulus, and electronic band structure of -Si3N4 are in good agreement with experimental results. This gives support for the predicted properties of -C3N4. The bulk modulus of -C3N4 is found to be comparable to diamond, and its moderately large cohesive energy suggests that the prototype structure may be metastable. Although the crystal structure and the valencies of the constituent atoms are similar in -Si3N4 and -C3N4, the electronic bonding properties in these two solids are found to differ. The large core size and repulsive p pseudopotential of the second-row element, Si, results in a more ionic Si-N bond compared with a covalent C-N bond. © 1990 The American Physical Society.