Alpha-C3N4 (28 atoms per Unit Cell).
The optimized structure has NC3 in a piramidal configuration and is more stable than the beta phase.

Introduction

The ab initio methods have progressed to the point where new materials with specific properties can be predicted on the computer (Liu and Cohen, 1989). An important difference between GDS/DFT and other ab initio methods is that it can be used for any elements of the periodic table with or without the pseudo potential approximation. Thus, applications to materials with first row or transition metal elements are now possible. C3N4 has been found to have hardness potentially rival that of diamond (Liu and Cohen, 1989, Niu, Lu and Lieber, 1993), with possible perculiar properties such as negative Poisson ratio (Guo and Goddard, 1995). Predictions of the thermal properties and ground state of C3N4 is however still inconclusive. Although plane-wave pseudopotential calculations (Liu and Cohen, 1989) assume C3N4 to be the same as Si3N4 (planar, beta phase), reasonings based on chemical bonding and preliminary calculations suggest the most stable phase is likely to be alpha phase with NC3 in a piramidal configuration (Guo and Goddard 1995, Chen, Guo and Goddard, to be published).