A modeling of the structure and favorable H-docking sites and defects for the high-pressure silica polymorph stishovite
- 66 Downloads
Employing first-principles methods, the docking sites for H were determined and H, Al, and vacancy defects were modeled with an infinite periodic array of super unit cells each consisting of 27 contiguous symmetry nonequivalent unit cells of the crystal structure of stishovite. A geometry optimization of the super-cell structure reproduces the observed bulk structure within the experimental error when P1 translational symmetry was assumed and an array of infinite extent was generated. A mapping of the valence electrons for the structure displays mushroom-shaped isosurfaces on the O atom, one on each side of the plane of the OSi3 triangle in the nonbonded region. An H atom, placed in a cell near the center of the super cell, was found to dock upon geometry optimization at a distance of 1.69 Å from the O atom with the OH vector oriented nearly perpendicular to the plane of the triangle such that the OH vector makes a angle of 91° with respect to . However, an optimization of a super cell with an Al atom replacing Si and an H atom placed nearby in a centrally located cell resulted in an OH distance of 1.02 Å with the OH vector oriented perpendicular to  as observed in infrared studies. The geometry-optimized position of the H atom was found to be in close agreement with that (0.44, 0.12, 0.0) determined in an earlier study of the theoretical electron density distribution. The docking of the H atom at this site was found to be ∼330 kJ mol−1 more stable than a docking of the atom just off the shared OO edge of the octahedra as determined for rutile. A geometry optimization of a super cell with a missing Si generated a vacant octahedra that is 20% larger than that of the SiO6 octahedra. The valence electron density distribution displayed by the two-coordinate O atoms that coordinate the vacant octahedral site is very similar to those displayed by the bent SiOSi angles in coesite. The internal distortions induced by the defect were found to diminish rather rapidly with distance, with the structure annealing to that observed in the bulk crystal to within about three coordination spheres.
KeywordsSpectral studies Diffraction methods Super cell structure
Unable to display preview. Download preview PDF.
The National Science Foundation (grants EAR–9627458, GVG and M. B. Boisen, Jr. and EAR–0229472, N. L. Ross, and GVG), The Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, Office of Science, US, Department of Energy (grant DE–FG02–97ER14751, DFC) and The US. Department of Energy (grant DE–FG 02–03ER15389, J. D. Rimstidt and GVG) are thanked for generously supporting this study. The study was also supported in part by the National Computational Science Alliance under a SURA Bloack Grant (project ndg) and utilizing the SGI Origin2000 at the National Center for Supercomputing Applications. G.V.G is pleased to thank George Rossman for generously sharing his extensive knowledge about H in rutile and stishovite. This paper was written in part while G.V.G was a Visiting Professor in the Chemistry Department at the University of New England in Armidale, Australia. The Faculty of the Sciences is thanked for awarding him a Visiting Distinguished Professor Scholarship. Professors Mark Spackman and Geoff Ritchie are also thanked for their kind hospitality and for making the visit a very stimulating and worthwhile experience. We are grateful to Monika Kock–Müller and Mauro Principe for their careful and valuable reviews of the manuscript; where their insightful comments and suggestions resulted in substantial improvements. Monika is also thanked for bringing to our attention a number of important references on the IR spectra of stishovite and rutile.