AB-INITIO HARTREE-FOCK STUDY OF STRUCTURAL AND ELECTRONIC-PROPERTIES OF BETA-SI3N4 AND BETA-C3N4 COMPOUNDS

Structural and electronic properties of beta-Si3N4 and beta-C3N4 compo unds are studied using the ab initio Hartree-Fock method. The crystall ine orbital program CRYSTAL-92 has been used to evaluate the equilibri um binding energy, lattice parameters, bulk modulus, band structure, d ensity of states, and charge-density data of these materials. The calc ulated structural properties show good agreement with previous density functional calculations and with available experimental information. As expected, the Hartree-Fock cohesive energy is underestimated with r espect to the values obtained with other methods that include electron correlation. On the other hand, the band energy gaps are overestimate d. The calculated value of the bulk modulus for beta-C3N4 Of 4.50 Mbar is slightly smaller than the value of 4.66 Mbar obtained for diamond with the same method. The charge-density analysis shows a small amount of charge transference between carbon and nitrogen indicating the pre valence of a covalent bond in beta-C3N4 A mixture of ionic and covalen t behavior was found from the charge distribution Of beta-Si3N4 The ch arge-density maps and a Mulliken population analysis emphasize the exi stence of two nitrogen sites with different bonding properties, for th e two compounds studied.