Density-functional theory investigation of hardness, stability, and electron-energy-loss spectra of carbon nitrides with (formula presented) stoichiometry

Abstract

The characterization of carbon nitride films with stoichiometry (formula presented) is heavily restricted by the problem of getting pure crystalline samples with the right C/N ratio. However, thin films with lower nitrogen concentration (5-25 %) have been found relatively easier to deposit, for example, with reactive magnetron sputtering. It is also in this range of nitrogen content that the recently discovered “graphiticlike (formula presented) fullerenelike” phase transition has been suggested to take place. Therefore, in order to add more information to the above experimental evidence, it is important to use theoretical methods to obtain further characterization of carbon nitride models with a high C/N ratio such as that of (formula presented) It is relevant to propose a cross checking on the role played by the nitrogen concentration in determining the stability, hardness, and electronic properties of (formula presented) compounds with different stoichiometries. For the sake of simplicity we have here compared the (formula presented) and (formula presented) systems, which are isoelectronic to each other. For this purpose two (formula presented) phases, namely, (formula presented) and (formula presented) are presented and investigated with density-functional-theory methods within the local density approximation. These phases contain less than (formula presented) of nitrogen than the well-known (formula presented) and are formally derived from the so-called pseudocubic (formula presented) Cohesive properties, heats of formation, bulk and elastic moduli have been calculated and a full detailed analysis of the density-of-states and energy-loss-near-edge-structure spectra is presented. We propose that the lowering of the nitrogen concentration does not prevent the finding of new ultrahard materials and indeed brings a significant increase in the cohesive energy of carbon nitrides. However, the computed enthalpies of formation have shown values that are positive and generally larger than the analog carbon-deficient phases. © 2002 The American Physical Society.