Crystal structure of CO2, SiO2 and GeO2 clathrate. Carbon, silicon, and germanium were classified into three sites (24k, 16i and 6c); meanwhile, oxygen was classified into four sites (48l, 24k, 12f, and 8e).
The energy as function of CO2, SiO2 and GeO2 volume. The “f.u.” in the y-axis stands for formula unit. I42d stands for . Fd3m stands for .
The enthalpy of CO2, SiO2 and GeO2 polymorphs as function of pressure.
Comparison of GGA and meta-GGA calculation of several SiO2 polymorphs. Inset: the enthalpy as function of pressure from GGA and meta-GGA was shown within a range of 1.0 GPa.
The “temporal” evolution of enthalpy for classical molecular dynamics (steepest descent) of SiO2 guest-free clathrate at 0 GPa and 0 K (a). Structure at the initial stage has cubic symmetry Pm3n space group (State I). Low symmetry structure with a space group was State II. The angle distribution function of Si-O-Si of these two configurations (States I and II) were shown in (b) and (d) in comparison with the angle distribution of Si-O-Si at 300 K (c).
ELF isosurfaces for CO2, SiO2 and GeO2 clathrates. Blue, yellow, green, and red spheres represent carbon, silicon, germanium, and oxygen atoms, respectively. The ELF isosurface value of 0.506 (a,c,e) and ELF = 0.894 (b,d,f).
Comparison of the total and projected electronic density of states of SiO2 (a), GeO2 (b), and CO2 clathrates (c).
The bulk modulus of some CO2, SiO2 and GeO2 solid phase derived from fit energy curve to Birch-Murnaghan equation of states.
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