New carbon-nitrogen materials. A likely alternative to diamond

In a recent work Liu and Cohen (Phys. Rev. B., 41 (1990) 10 727-10 734) reported the theoretical prediction of a C-N phase whose hardness was competing with that of diamond. This work provoked extensive experimental search for the theoretical material with a composition β-C₃N₄. There was a report (Niu, Lu and Lieber, Science, 261 (1993) 334-337) on the growth β-C₃N₄ films. However, this has not been reproduced by others workers. Recently, Guo and Goddard (Chem. Phys. Lett., 237 (1995) 72) reported that the synthesized material, in most cases, corresponds to α-C₃N₄ and not to β-C₃N₄. We review the evidence from a number of papers and conclude that there is not enough evidence that C₃N₄ can be produced as large crystals. It is clear, however, that the β-C₃N₄ is more stable than the β-C₃N₄ phase. We discuss the recent report (Sarikaya, Qian, Martin-Gil, Martin-Gil and José-Yacamán, to be published) in which a new phase with the composition α-C_xN_y with x ≈ 4 and y ≈ 4 is found. We discuss the applications and future trends on these materials.