Model structures and electron density distributions for the silica polymorph coesite at pressure: An assessment of OO bonded interactions

The crystal structure and the bond critical point, bcp, properties of the e lectron density distribution for the high-pressure silica polymorph coesite were generated for pressures up to similar to 17 GPa, using first-principl es calculations. The nonequivalent SiO bond lengths and the SiOSi and OSiO angles of the generated structures agree with those observed to within simi lar to1%. With compression, the SiO bond lengths and the variable SiOSi ang les of the structures both decrease while the value of the electron density , p(r(c)), the curvatures, and the Laplacian of the electron density distri bution at the bond critical points each increases slightly. As found in a r ecent modeling of the structure of low quartz, the calculated electron dens ity distributions are nearly static and change relatively little with compr ession. The bcp properties of the model structure agree with those observed at ambient conditions to within similar to 10%, on average, with several o f the properties observed to correlate with the observed SiO bond lengths, R(SIO). This agreement is comparable with that observed for several other s ilicates. As predicted, the bonded radius of the oxide anion, the curvature s of p(r(c)) paralleling the bond paths and the Laplacian of p(r(c)) each c orrelates with the observed bond lengths. However, the observed p(r(c)) val ues and the curvatures of p(r(c)) perpendicular to the paths fail to show a correlation with the observed bond lengths. The ellipticity of the SiO bon ds in both the model and the observed structures tends to decrease in value as the SiOSi angle approaches 180 degrees, indicating that the bonds becom e more circular in cross sections as the angle widens. Ridges of electron d ensity and bond critical points were found between the intertetrahedral oxi de anions at each pressure. The existence of these features appears to be c losely related to purely geometrical factors of the coesite structure rathe r than to bonded interactions. None of these features was found between the intratetrahedral oxide anions.