Model Structures and Electron Density Distributions for the Silica Polymorph Coesite at Pressure:  An Assessment of OO Bonded Interactions

Abstract

The crystal structure and the bond critical point, bcp, properties of the electron density distribution for the high-pressure silica polymorph coesite were generated for pressures up to ∼17 GPa, using first-principles calculations. The nonequivalent SiO bond lengths and the SiOSi and OSiO angles of the generated structures agree with those observed to within ∼1%. With compression, the SiO bond lengths and the variable SiOSi angles of the structures both decrease while the value of the electron density, ρ(r c), the curvatures, and the Laplacian of the electron density distribution at the bond critical points each increases slightly. As found in a recent modeling of the structure of low quartz, the calculated electron density distributions are nearly static and change relatively little with compression. The bcp properties of the model structure agree with those observed at ambient conditions to within ∼10%, on average, with several of the properties observed to correlate with the observed SiO bond lengths, R(SiO). This agreement is comparable with that observed for several other silicates. As predicted, the bonded radius of the oxide anion, the curvatures of ρ(r c) paralleling the bond paths and the Laplacian of ρ(r c) each correlates with the observed bond lengths. However, the observed ρ(r c) values and the curvatures of ρ(r c) perpendicular to the paths fail to show a correlation with the observed bond lengths. The ellipticity of the SiO bonds in both the model and the observed structures tends to decrease in value as the SiOSi angle approaches 180°, indicating that the bonds become more circular in cross sections as the angle widens. Ridges of electron density and bond critical points were found between the intertetrahedral oxide anions at each pressure. The existence of these features appears to be closely related to purely geometrical factors of the coesite structure rather than to bonded interactions. None of these features was found between the intratetrahedral oxide anions.

High-pressure crystal chemistry of coesite-I and its transition to coesite-II

“…The rate of compression of the four other Si-O-Si angles ( Figure 7 ), with the smaller angles being more compressible, is in good agreement with the experimental studies at lower pressures [6,7] . The ab initio calculations [8] suggest compressional rates similar to those obtained in all of experiments, although the calculated angles appear to be smaller than those measured. Decrease of the smallest Si1-O5-Si2 angle from 137.44 ° (ambient, Angel et al [7] ) to 125.54 ° at ∼ 20 GPa contributes to the shortening along the b -axis.…”

“…Due to this arrangement there are five distinct Si-O-Si angles in the structure. The high-pressure structural behavior of coesite was previously studied using in-house based single crystal X-ray diffraction up to 5.2 GPa [6] and up to 8.7 GPa [7] in ethanol-methanol pressure medium, as well as by means of ab initio calculations up to 17 GPa [8] . These studies concluded that the symmetry of the unit cell appears less hexagonal with increasing pressure, compressing in the a direction more than twice as much than in the c direction.…”

A Bond Path and an Attractive Ehrenfest Force Do Not Necessarily Indicate Bonding Interactions: Case Study on M2X2(M=Li, Na, K; X=H, OH, F, Cl)

“…[2a,d, 12k,o] Braga et al [17] also presented convincing evidence on the repulsive AA and CC interactions in crystalline KHC 2 O 4 . Gibbs et al [18] questioned the existence of bonding interactions between the tetrahedral oxide anions in silica polymorph coesite. In their opinion, the occurrence of BPs and (3,-1) CPs (bond critical points (BCPs)) between these anions is most likely not due to the bonding interactions between the…”

Bonded Radii and the Contraction of the Electron Density of the Oxygen Atom by Bonded Interactions

“…The main objective of this study will be to examine the bonded radii for bonded pairs of atoms comprising a number of oxide crystals − and to explore the connection between the radii, the S&P ionic and crystal radii, and the experimental M–O interatomic distances. It will be of particular interest to learn the extent for which the ED distribution of the oxide atom is polarized and its radius contracted by the bonded atoms.…”