  To understand the crystal structure of n-diamond, a hydrogen-doped (H-doped)
diamond model has been investigated using first principles calculations. In
particular, hydrogen concentration dependent elastic constants and lattice
parameters for the H-doped diamond have been analyzed. Our results indicate
that when the hydrogen concentration is less than 19 at.%, the H-doped diamond
is mechanically stable. When the hydrogen concentration is about 4 at.%, the
optimized lattice parameter, simulated XRD pattern and electronic properties
for the H-doped diamond all agree well with the corresponding experimental
values of n-diamond. The results imply that the n-diamond is likely to be an
H-doped diamond.
