Ab initio search of carbon nitrides, isoelectronic with diamond, likely tolead to new ultra hard materials

Covalently bonded networks of carbon nitride materials are expected to poss ess outstanding physical properties such as high bulk moduli, wide band gap and high thermal conductivity. As a consequence they are nowadays widely i nvestigated both experimentally and theorectically as potential candidates for new ultra-hard materials likely to replace expensive diamond in its dif ferent applications. We here give an overview of recent research works usin g up-to-date, first principals computational methods based on the local den sity approximation (LDA) of the density functional theory (DFT), in order t o closely examine ground state as well as excited states properties of carb on nitrides. Our purpose is two-fold; 1) to achieve a quantitative predicti on of the hardness for different phases of the C3N4 system with a descripti on of their chemical bonding properties; and 2) to extract modelled spectro scopic quantities such as 1s core ionisation energies to help experimentali sts assessing the properties obtained from X-ray absorption spectra of the synthesised carbon nitrides. (C) 2001 Academie des sciences / Editions scie ntifiques et medicales Elsevier SAS.