High-pressure Phase Transition Behavior And Properties Of Carbon Dioxide And Silicon Dioxide

Pressure as a of outermost importance parameter in extreme conditions,could fundamentally alter the physical and chemistry properties of materials.Even in simple molecular system,high pressure could perturb the intrinsic molecular configuration to form wide-ranging 3-dimensional network extended covalent solids,therefore reach the purpose of minimize the overall energy of the system,make it extremely attractive for future energy and technological needs.Typical Group ? oxide as one of the archetypal compounds in high-pressure research,has been the subject of a series of scientific investigations comprising material synthesis in supercritical conditions,planetary science,and condensed matter physics,thus play a significance role in fundamental research and practical application.Carbon dioxide as the first member of Typical Group ? oxide,which is also the only substance that exists as a gas form at ambient condition,would transform to a richness of high-pressure polymorphs ranging from typical molecular solids to 3-dimensional network extended covalent solids with structures similar to silicon dioxide.To a first approximation,high pressure can be regarded as a bridge between carbon dioxide and silicon dioxide.Both CO₂ and SiO₂ are the dominant component of nature,exhibit a richness and complexity response under various pressure and thermal conditions,hence possess a rather complex phase diagram,carrying out the high-pressure research could not only provide a method for understanding of its own structure and properties,but also play an important role in interpreting the simple molecular systems and many other oxides materials.In addition,CO₂ plays a vital role in the deep carbon cycle,oxygen fugacity and the chemical composition of the earth lower mantle;CO₂ also exists as the dominant component of the atmospheres as Mars and Venus,as well as other planets,thus the high-pressure behavior and properties of CO₂ under extreme condition play a vital role in understanding the composition of planets.Besides,silicon dioxide as a major component in nature and possesses tremendous application technological material,the high-pressure research could shed light on the densification mechanism for this fundamental material and other oxide compounds.1?The single-crystal X-ray diffraction and Brillouin scattering spectroscopy of CO₂-I were performed under high temperatures and pressure.Densities,acoustic velocities,and elastic moduli of CO₂-I were obtained along 300,400 and 580 K isotherms up to the phase-transition boundaries.CO₂-I transforms to phase ? and phase ? at room temperature(at 12.19 GPa)and 580 K(at 10.83 GPa),respectively.It was observed that high temperature suppresses pressure-induced stress in single-crystal CO₂-I.Compared with other molecular crystals,elastic anisotropy A is small and practically independent of pressure.This result will provide new physical images and guidelines for the study of carbon dioxide under extreme conditions.2?The elastic properties of CO₂-? under high pressure was investigated through the density information derived from single-crystal x-ray diffraction and the sound velocity from Brillouin scattering.Through the Raman spectra measurements of quenched CO₂-?,more than nine vibrations were found,precluding the rhombohedral R(?)c structure.Therefore,we deduced complete sets of elastic constants of CO₂-? based on the orthorhombic Pbcn and tetragonal P41212 structures,they all exhibit monotonic increases with the applied compression over the pressure range studied here.We compared the bulk moduli of several CO₂ polymorphs and found that the bulk moduli of CO₂-? are much lower than others,inclining a molecular crystal nature.3?The sound velocities and Raman vibration modes of?-quartz under high pressure was investigated through Raman scattering and Brillouin scattering.The acoustic velocity of quasi-longitudinal and quasi-transverse acoustic modes of single crystal CO₂-? as a function of azimuthal angles remain its own tendency until the highest pressure,with the same angle and crystal orientation,the sound velocity of?-quartz increasing monotonously with increasing pressure.The experiment carried out by means of different pressure-transmitting medium indicated that the 128 cm^-1 and 265cm^-1 vibrations,exhibit a splitting upon compression,and the splitting is found to be related to the initial stress conditions of the sample.The 509 cm^-1E mode pressure dependence has been first measured in this study and display red shift with applied pressure,besides all other vibrational modes exhibit increase behavior with pressure,or display weak pressure dependence with increasing pressure.