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 beta-C3N4. There was a report (Niu, Lu and Lieber, Science, 261 (1993) 334-337) on the growth beta-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 alpha-C3N4 and not to beta-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 alpha-C3N4 is more stable than the beta-C3N4 phase. We discuss the recent report (Sarikaya, Qian, Martin-Gil, Martin-Gil and Jose-Yacaman, to be published) in which a new phase with the composition alpha-CxNy with x approximate to 4 and y approximate to 4 is found. We discuss the applications and future trends on these materials.