Model structures and properties of the electron density distribution for low quartz at pressure: a study of the SiO bond

The crystal structure, the electron density distribution and the topologica l properties of the distribution for low quartz were modeled at pressure, u sing first principles calculations. The geometry optimized nonequivalent Si O bond lengths and the SiOSi angles of the model structures match those obs erved at pressure to within a few percent. As the bond lengths and angles d ecrease with compression, the electron density distribution at the bond cri tical points, rho(r(c)), along the bonds increases slightly whereas the bon ded radii of both Si and O decrease with the radius of the oxide anion comp ressing about twice as fast as that of the Si cation. The magnitudes of the net charges on both Si and O obtained in a virial partitioning of the elec tron density distribution also decrease slightly. The significance of secon dary bond critical points and bond paths displayed between the oxide anions of adjacent silicate tetrahedra at pressures of 2.5 GPa and greater is dis cussed. The nature of these interactions is not clear, particularly since t hey are also exhibited by procrystal representations of the electron densit y distributions. As the rho(r(c)) values observed for the SiO bonds in sili cate minerals with four-coordinate Si are substantially greater than those reported for bonded interactions close to the closed shell ionic limit but less than those close to the shared covalent limit, the bond is indicated t o be more intermediate in character than either ionic or covalent, despite claims to the contrary. Negative values of the Laplacian, del(2)rho(r(c)), for MO bonds of first- and second-row M-atoms are not always typical of pre dominantly covalent bonds. (C) 1999 Elsevier Science B.V. All rights reserv ed.