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 topological properties of the distribution for low quartz were modeled at pressure, using first principles calculations. The geometry optimized nonequivalent SiO bond lengths and the SiOSi angles of the model structures match those observed at pressure to within a few percent. As the bond lengths and angles decrease with compression, the electron density distribution at the bond critical points, rho(r(c)), along the bonds increases slightly whereas the bonded radii of both Si and O decrease with the radius of the oxide anion compressing 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 electron density distribution also decrease slightly. The significance of secondary bond critical points and bond paths displayed between the oxide anions of adjacent silicate tetrahedra at pressures of 2.5 GPa and greater is discussed. The nature of these interactions is not clear, particularly since they are also exhibited by procrystal representations of the electron density distributions. As the rho(r(c)) values observed for the SiO bonds in silicate 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 to 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 predominantly covalent bonds. (C) 1999 Elsevier Science B.V. All rights reserved.