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 wa s competing with that of diamond. This work provoked extensive experim ental 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, corre sponds 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 tha t C3N4 can be produced as large crystals. It is clear, however, that t he 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-CxN y with x approximate to 4 and y approximate to 4 is found. We discuss the applications and future trends on these materials.