Bonded Interactions in Silica Polymorphs, Silicates and Siloxane Molecules

Journal Article · Sat Aug 01 00:00:00 EDT 2009 · American Mineralogist

DOI:https://doi.org/10.2138/am.2009.3215· OSTI ID:965126

Gibbs, Gerald V. ^[1]; Wallace, Adam F. ^[1]; Cox, David F. ^[1]; Downs, R. T. ^[2]; Ross, Nancy L. ^[1]; Rosso, Kevin M. ^[3]

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1. Virginia Polytechnic Inst. and State Univ. (Virginia Tech), Blacksburg, VA (United States)
2. Univ. of Arizona, Tucson, AZ (United States)
3. Pacific Northwest National Lab. (PNNL), Richland, WA (United States)

Experimental model electron density distributions recorded for the silica polymorphs coesite and stishovite are comparable with electron density distributions calculated for a variety of silicates and siloxane molecules. The Si-O bond lengths and Si-O-Si angles calculated with first principles density functional theory methods as a function of pressure are also comparable with the bond lengths and angles observed for coesite and quartz within the experimental error. The similarity of the topological properties of the Si-O bonded interactions and the experimental and the geometry optimized structures for the silica polymorphs provides a basis for understanding the properties and crystal chemistry in terms of a molecular-based model. The agreement supports the argument that the bulk of the structural, physical and thermodynamic properties of the silica polymorphs are intrinsic properties of the molecular-like coordinated polyhedra such that the silica polymorphs can be pictured as ‘supermolecules’ of silica bound by the virtually same forces that bind the Si and O atoms in simple siloxane molecules. The topology of the electron density distribution is consistent with the assertion that the Si-O bonded interaction arises from the net electrostatic attraction exerted on the nuclei by the electron density accumulated between the Si and O atoms. The correlation between the Si-O bond length and Si-O-Si angle is ascribed to the progressive local concentration of the electron density in the nonbonded region of the O atom as the bond length increases and angle decreases rather then to bonded interactions involving the d-orbitals on Si. On the basis of the proximity of the bond critical point, rc, to the nodal surface of the Laplacian, (nabla)²ρ(r_c), and the values of ρ(r_c) and G(r_c)/ρ(r_c), the Si-O bond qualifies as an intermediate bonded interaction. For bonded interactions of intermediate character, (nabla)²ρ(r_c) increases linearly as ρ(r_c) increases, the greater the shared character, the larger the value of (nabla)²ρ(r_c). In addition, a mapping of (nabla)²ρ(r) serves to highlight those Lewis base domains that are susceptible to electrophilic attack by H like the O atom in coesite involved in bent Si-O-Si angles, the narrower the angle, the greater the affinity for H . On the basis of the net charges conferred on the Si and O atoms and the bonded radii of the two atoms, the Si-O bond of stishovite with six-coordinated Si and three-coordinated O is indicated to be more ionic in character than that in quartz with four-coordinated Si and two coordinated O. Unlike the conclusion reached for ionic and crystal radii (quantum mechanical unobservables), it is the bonded radius of the O atom that increases with the increasing coordination number of Si, not the radius of the Si atom. The modeling of the electron density distributions for quartz, coesite and beryl as a function of pressure indicates that the shared character of the bonded interactions in these minerals increases slightly with increasing pressure. The insight provided by the calculations and the modeling of the electron density distributions and the structures of the silica polymorphs bodes well for future Earth materials studies that are expected to improve and clarify our understanding of the connection between properties and structure within the framework of quantum mechanical observables, to find new and improved uses and to predict new properties for materials and to enhance our understanding of crystal chemistry and chemical reactions of materials in their natural environment at the atomic level

• Research Organization: Pacific Northwest National Lab. (PNNL), Richland, WA (United States). Environmental Molecular Sciences Lab. (EMSL)
• Sponsoring Organization: USDOE
• DOE Contract Number: AC05-76RL01830
• OSTI ID: 965126
• Report Number(s): PNNL-SA-66597; AMMIAY; 25629; KC0303020; TRN: US200920%%243
• Journal Information: American Mineralogist, Vol. 94, Issue 8-9; ISSN 0003-004X
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
SILICA
SILICATES
SILOXANES
ELECTRON DENSITY
BOND LENGTHS
DENSITY FUNCTIONAL METHOD
INTERACTIONS
Environmental Molecular Sciences Laboratory