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