Structure Phase Transition Of Carbon Dioxide Under High Pressure

Carbon dioxide is a common compound in the air, which is essential and close toour lifes. Of course, carbon cycle is one of the most important components in theearth’s carbon cycle, sucn as the global carbon cycle, sea-air flux, karst carboncycle.The excessive emissions of carbon dioxide lead to the greenhouse effect,because of what, the world become pay attentions to the problem of absorption andmanagement. Domestic and international research work can be divided into thefollowing categories: carbon dioxide source control, carbon dioxide sequestration,absorption and utilization of carbon dioxide in air. Due to absorption, dealing with theproblem of carbon dioxide under high pressure is injected into the deep coal seams.Therefore, high-pressure carbon dioxide structural phase transition which under theconditions has become a key research and hotspots.There exists eight discovered molecular carbon dioxide phase states in recentstudies. From carbon dioxide （CO₂）molecular phaseⅠto molecular （CO₂） phase VII,and a-carbonia. The discovered phases are common in the role of temperature andpressure and widely recognized. Now researchers are also trying to find a newmolecular phase, while because of experimental conditions, that is a daunting work.In this study, I studied the phase tansition with5crystal structures under highpressures which studied in Professor Wei Dongqing research group of ShanghaiUniversity:Firstly, I used first-principles plane-wave pseudopotential method to calculatemore than ten crystal stuctures and decided to use five crystal structures aftercomparions: Pa3, Pbcn,P4₁2₁2, P4m-2, I42dc. After texting these stuctures inthe cut-off energy, method（LDA）, Pseudopotential and K point, and make sure that,the cut-off energy is420eV, method is LDA, pseudopotential is ultrasoftpseudopotential.Secondly, testing the K point of those stuctures（Pa3, Pbcn,P4₁2₁2, P4m-2,I42dc）. For Pa3, the K point is6×6×6,for Pbcn is6×4×6, forP4₁2₁2is 7×7×5, for P4m-2is8×8×6, for I42dc is5×5×6。Finally, I calculated final enthalpy of those five stuctures, and then calculatedenthalpy of each average atomic. As those values are big and very close, so Icalculated the relative enthalpy in this paper. Drawing the graph by the Origin8.0, Iobtained five carbon crystal structural phase transition point which is approximately:5Gpa,7.8Gpa,16Gpa,72.3Gp.