PNNL

Abstract

The crystal structure and the bond critical point (bcp) properties of the electron density distribution of coesite were modeled at pressures up to ~17 GPa, using first principles calculations. The nonequivalent SiO bond lengths and the SiOSi and OsiO angles of the model structures agree with those observed to within a few percent. With compression, the SiO bond lengths and the variable SiOSi angles both decrease. Also, the value of the electron density, p(rc), the curvatures and the Laplacian of the electron density distribution at the bond critical points each increase slightly. As found in a recent modeling of the low quartz structure, the distribution is nearly static and changes relatively little with compression. The bcp properties of the model structure agree with those observed for coesite at ambient conditions to within 5 and 15%, on average, and several of the properties correlate with the observed SiO bond lengths. Secondary bond critical points were found between the oxide anions of adjacent silicate tetrahedra. The value of the electron density at these points increases with decreasing separation between the anions. The behavior of bcp properties of the points is similar t that exhibited by the primary critical points between Si and O. In contrast, the value of the electron density at the points is about one order of magnitude less. In several cases, the points disappear with increasing pressure. Despite their closer contacts, no critical points were found between the anions that comprise the individual silicate tetrahedra nor are they likely to be found. The presence of the secondary points in procrystal representations of the electron density distributions suggests that the formation of the points is more an artifact of the spherical electron density distribution of the core electrons than one of the valence electrons.