Study On Spectral And Phase Transition Of Solid Carbon Dioxide Under High Pressures

Carbon dioxide(CO₂)is an important chemical species in the Earth's atmosphere,and its solid state form-dry ice has plenty of chemical applications.Despite its abundance in Earth and other planets in its solid forms,major gaps in understanding the solid-phase behaviors of CO₂ at high pressures still remain.For the past decades,many experimental and theoretical studies have been performed to determine the structures and stabilities of various phases of CO₂.For phase VII,only one experimental work has been reported to show its crystal structure,but not further confirmed yet,which makes its structural and stability exploration warranted.The extreme conditions limit the structural explorations in laboratory,but open a window for the theoretical study.In recent decades,great advances have been made in electronic structure methods for predicting molecular structure and crystal energy.Density functional theory(DFT)has been the standard method for handling solids and liquids with its efficiency.However,its accuracy depends on the choice of density functionals,and there is currently no general procedure to systematically improve existing density functionals.To improve the calculation accuracy,the development in fragment-based electronic structure methods greatly extend the power of correlated wave function methods(such as the second-order M?ller-Plesset perturbation(MP2)theory,and the coupled cluster singles,doubles,and perturbative triples(CCSD(T))theory),into pharmaceutical and other chemically relevant molecular crystals.This article studies the structure of solid carbon dioxide phase VII and the phase transition line of I-VII.First,1038 structures were obtained through crystal structure prediction,and then 30 energy-lowest structures were obtained by energy sorting and selection.These 30 structures belong to 13 different space groups.Based on the second-order M?ller-Plesset perturbation(MP2)theory,the crystal structure of solid carbon dioxide was optimized.In addition,4 structures were screened out by analyzing the Raman spectrum and comparing with experimental data.The four structures and the structure reconstructed from the experimental data were further calculated.Here,we applied the MP2 algorithm,the simplest and effective many-body theory,along with the electrostatically embedded generalized molecular fractionation(EE-GMF)-based quantum mechanical(QM)method to calculate the structures and Gibbs free energies of CO2phases at various temperatures and high pressures.By comparing the Gibbs free energy,the phase transition lines of each structure and phase I were obtained.By comparison with the experimental data,it is found that the structure reconstructed from the experimental data is most consistent with the phase diagram.Therefore,we verified the correctness of the experimental structure.At the same time,through high-precision calculations,we obtained the phase transition lines of phases I-VII and the solid carbon dioxide phase diagram.Specifically,the carbon dioxide phase I occupies a relatively low temperature and low pressure region,and the phase transition between phase I and phase VII occurs in a high temperature region of 11-12 GPa.The phase transition temperature of phase I-VII is very sensitive to pressure.On the carbon dioxide phase diagram,the phase transition line of phase I-VII is a vertical line respect to the pressure axis.