The characterization of carbon nitride films with stoichiometry C3N4 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 ;→; 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 ;C11N4. 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 CNx compounds with different stoichiometries. For the sake of simplicity we have here compared the C3N4 and C11N4 systems, which are isoelectronic to each other. For this purpose two C11N4 phases, namely, α; and β,; are presented and investigated with density-functional-theory methods within the local density approximation. These phases contain less than ∼30%; of nitrogen than the well-known C3N4 and are formally derived from the so-called pseudocubic C3N4. 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.< Réduire