We present a study of a highly symmetric crystal made of exclusively s
p(2) bonded atoms. Calculations of the structural and electronic prope
rties are performed within the pseudopotential-density-functional appr
oach for two different compositions made of (i) pure carbon and (ii) c
arbon and nitrogen compound. In both solids, one of the carbon-carbon
bond lengths is found to be 1.35 Angstrom, which is considerably small
er than any carbon-carbon bond length found in other carbon solids. Th
e bulls: moduli are calculated to be 241 and 286 GPa for the pure carb
on and the carbon-nitride compounds, respectively. We demonstrate that
the relatively low bulk moduli, considering the short bond lengths fo
und in the structure, is due to the disruption of the carbon pi bondin
g states. This is probably unavoidable when trying to form a three-dim
ensional structure out of a planar configuration like the sp2 bonds. T
he calculated density of states and band structures show that the pure
carbon form is metallic whereas the carbon nitride is semiconducting.
When carbon atoms are added to the interstitial regions, the carbon s
olid becomes insulating and the bulk modulus increases to 282 GPa. [S0
163-1829(98)01726-3].