Comparison of thermodynamic stabilities and mechanical properties of CO2, SiO2, and GeO2 polymorphs by first-principles calculations

The recent discovery that molecular CO2 transforms under compression into carbon four-coordinated, 3-dimensional network solid phases has generated considerable interests on possible new phases in the fourth-main-group elemental oxides. Based on density-functional theory calculations, we have investigated the thermodynamic stability, mechanical properties and electronic structure of proposed guest-free clathrates, quartz and cristobalite phases for CO2, SiO2, and GeO2, and the dry ice phase for CO2. It was predicted that a GeO2 clathrate, likely a semiconductor, could be synthesized presumably with some suitable guest molecules. The hypothetical CO2 guest-free clathrate phase was found hardly to be formed due to the large energy difference with respect to the other polymorphs. This phase is unstable at all pressures, which is also implied by its different electronic structure in comparison with SiO2 and GeO2. Finally, the SiO2 clathrate presents a uniquely high bulk modulus, which is higher than that of quartz and three times of the experimental data, might not be a weak point of ab-initio calculations such as pseudopotentials, correlation functional etc., instead it can be readily understood by the constraint as imposed by the high symmetry. Either temperature or an exhausted relaxation (without any symmetry constraint) can remedy this problem. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4735077]