Light elements-based materials are of relevant interest because of their specific physicochemical properties mainly dependent on the involved strong chemical bonds. Among them, diamond and c-BN are still today the most representative materials especially concerning super hardness property. Nevertheless, since the prediction by Liu and Cohen (on the basis of ab initio calculation) of theoretical materials with C 3 N 4 composition exhibiting low compressibility, many efforts have been devoted to the synthesis of carbon nitrides. The first part of this review deals with the brief description of the different predicted C 3 N 4 polymorphs exhibiting either a dense network or a graphitic one. In addition, predicted carbon nitrides with different stoichiometry are also described. The second part presents the analysis of the processing routes described in the literature for the synthesis of carbon nitrides as bulk materials. If many attempts have been devoted to high-pressure processes (direct synthesis, direct conversion from the graphitic form, flux assisted conversion or solvothermal processes), some experiments performed in moderate experimental conditions have also been investigated and appear as really promising. A comparison with the different processes developed for diamond and c-BN is associated. As a conclusion, a discussion about the promising routes related to bulk carbon nitrides synthesis is given.

An hydrogenated nitrogen-rich graphitic carbon nitride, structurally related to the theoretical graphitic phase of C 3 N 4 , has been synthesized in a bulk wellcrystallized form. This new material was prepared by thermal decomposition of thiosemicarbazide up to 600°C at ambient pressure under nitrogen flow. Its composition was determined by elemental combustion analysis. Powder X-ray diffraction, infrared spectroscopy, thermogravimetric analysis, X-ray photoelectron spectroscopy and C 13MAS NMR characterizations were performed. This material can be schematically described with a two-dimensional framework and a composition close to C 3 N 4.17 H 1.12 . In this nitrogen-rich material, C 3 N 3 voids are fully occupied by water molecules which are strongly trapped into the material. A loss of crystallinity associated with a modification of the thermal behaviour is observed when the amount of trapped molecules decreases in the graphitic material, order being damaged both between and in the graphitic planes.

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