3. RESULTS AND DISCUSSIONS
3.1 Structures
All first-principles calculations are carried out by the CASTEP program
under the Material Studio 7.0 version.45 The energy
cutoff is 400 eV. The ultrasoft form pseudopotentials are utilized in
the calculations. The convergence criteria of energy, maximum force,
maximum stress, and maximum displacement were set as
5.0×10-6eV/atom, 0.01 eV Å -1, 0.02
Gpa, and 5.0×10-4 Å, respectively, for optimization.
In our previous work, generalized gradient approximation (GGA) with the
revised Perdew-Burke-Ernerhorf (revPBE) and the
Perdew-Burke-Ernerhorf (PBE) functionals have been used to calculate
the equilibrium lattice parameters of methane hydrates. Comparing the
results with previous ones and the experimental value, the prediction
from revPBE is found closer to the experimental measurement and other
theoretical works35. Therefore, revPBE functional is
adopted in the following calculations in this work. Here, we calculated
three type sI clathrate hydrates, i.e., NH3 hydrates and
N2O hydrates and NO hydrates, all the crystalline
structures are plotted in Figure 1 . The lattice parameters by
the equations of state (EOS) fitting,46 and the
fitting curve of Energy vs. Volume for NH3,
N2O and NO hydrates are shown in Figure 2 . The
predicted lattice parameters for NH3, NO and
N2O hydrates are 12.095, 12.126 and 12.36 Å,
respectively, higher than the value for the CH4hydrate(11.955 Å).47 The lattice parameters of the
N2O hydrate can be found to be larger than those of NO
and NH3 mainly because N2O has a larger
diameter. It is interesting to further study the bulk modulus in terms
of the attractive and repulsive interatomic interactions and the type of
occupied cage.