Ab Initio Study of Elastic Properties of High-Pressure Polymorphs of CO₂ Phases II and V

Abstract

Elastic properties of prototypical CO2 polymorphs under compression are essential to understanding the nature of their pressure-induced structural changes. Despite the fundamental importance in physical chemistry and condensed matter physics and geophysical implications for the nature of fluids in the Earth and planetary interiors, the elastic properties of these polymorphs are not fully understood because of intrinsic uncertainty and difficulties in experimental estimation of elasticity. Theoretical calculations of elastic properties of high-pressure CO2 polymorphs allow us to reveal the previously unknown details of elasticity of the diverse polymorphs under extreme compression. As a step toward getting insights into the deep carbon cycle, we carried out density-functional-theory calculations and investigated the elastic constants, bulk modulus, shear modulus, Poisson ratios and acoustic wave velocities of CO2 polymorphs: II (tetragonal, P42/mnm), β-cristobalite-like V (VCR, tetragonal, I4̅2d) and tridymite-like V (VTD, orthorhombic, P2 12121) up to approximately 40 GPa. Particularly, the elastic properties and bulk moduli of all the three CO2 phases except the elastic constants of CO2–II are the first calculation results, and the elastic constants and bulk modulus calculated for CO2–II are improved. The change in elastic properties with varying pressure shows distinct trends among CO2–II, CO2–VCR, and CO2–VTD. Despite these differences, the bulk moduli for CO2 of phases I, II, VCR and VTD exhibit a gradual increase with increasing density without major discontinuity. On the basis of the calculated elastic properties of CO2–II, CO2–VCR, and CO2–VTD and a comparison between these CO2 units and SiO2 materials, we suggest that these polymorphs may be classified into two groups: (1) a weakly connected group: CO2–II, cristobalite, and tridymite and (2) a strongly connected group: CO2–VCR, CO2–VTD, and stishovite. This classification does not depend on crystal symmetry. The bulk modulus of a CO2 solid is greater than that of a SiO2 solid of the same density, and the shear modulus of a CO2 solid is smaller than that of a SiO2 solid of the same density. The elasticity of CO2 polymorphs shown here may hold some promise for investigating the elasticity of diverse solids consisting of oxide molecules under extreme pressure.

Atomistic View of Mercury Cycling in Polar Snowpacks: Probing the Role of Hg2+ Adsorption Using Ab Initio Calculations

“…The on-site electron-electron interactions were determined using the generalized gradient approximation (GGA)-based Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional [57]. The long-range order dispersion interactions, such as the van der Waals interaction, significantly influence the molecular crystal structures and cleaved surfaces with hydrogen bond-like long range interactions [58][59][60][61]. Accordingly, the hybrid semi-empirical dispersion correction term based on the Tkatchenko-Scheffler method was applied to the single-point energy calculations and the structure optimization calculations [58,59].…”

Reductive Adsorption of Atmospheric Oxidized Mercury on Ice: Insights from Density Functional Calculations

“…DFT has been used in many studies to complement experimental investigations of environmentally important materials: for example, the surface adsorption behavior, − the metal intercalation tendency into clay minerals, , the electronic structure of high-pressure minerals, − and electronic transition spectroscopy (X-ray Raman scattering, X-ray absorption, optical properties, etc. ). ,,− We established detailed calculation conditions by referring to these previous studies with suitable adjustments (see also Section 2 in the Supporting Information).…”

Computational searches for crystal structures of dioxides of group 14 elements (CO2, SiO2, GeO2) under ultrahigh pressure

“…Previous theoretical studies have successfully reproduced properties of known crystal phases and proposed unidentied crystal structures of AO 2 (A ¼ C, Si, or Ge) under ultrahigh pressure. [25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44] These theoretical predictions were made by stochastic trials including genetic algorithms, or by geometry optimization or MD simulation starting from a known structure or an intuitive initial guess. Since a stable structure in an ultrahigh pressure environment might be completely different from that under normal pressure, especially in the case of CO 2 , it is not easy to comprehensively collect all polymorphs of AO 2 .…”