Electron-density distribution in stishovite, SiO2: a new high-energy synchrotron-radiation study

• Published in: Acta crystallographica. Section A, Foundations of crystallography Vol. 57; no. 6; pp. 663 - 677
• Main Authors: Kirfel, A., Krane, H.-G., Blaha, P., Schwarz, K., Lippmann, T.
• Format: Journal Article
• Language: English
• Published: 5 Abbey Square, Chester, Cheshire CH1 2HU, England Munksgaard International Publishers 01-11-2001; Blackwell
• Subjects: Condensed matter: structure, mechanical and thermal properties; electron density; Exact sciences and technology; Inorganic compounds; Physics; silica; Single-crystal and powder diffraction; Structure of solids and liquids; crystallography; Structure of specific crystalline solids; synchrotron radiation; topological analysis; Water, oxides, hydroxides, peroxides; X-ray diffraction and scattering; Inorganic compounds; Theoretical study; Oxides; XRD; Topology; Experimental study; Charge distribution; Stishovite; High energy; Silicon oxides; Electron density; Comparative study; Synchrotron radiation; Crystal structure
• ISSN: 0108-7673; 1600-5724
• DOI: 10.1107/S0108767301010698

The electron‐density distribution of the high‐pressure polymorph of SiO2, stishovite [a = 4.177 (1), c = 2.6655 (5) Å, space group P42/mnm, Z = 2], has been redetermined by single‐crystal diffractometry using synchrotron radiation of 100.42 and 30.99 keV, respectively, in order to obtain essentially absorption‐ and extinction‐free data. Room‐temperature diffraction experiments on two samples of irregular shape were carried out on two different diffractometers installed at HASYLAB/DESY, Hamburg, Germany. The structure refinement on the high‐energy data converged at R(F) = 0.0047, wR(F) = 0.0038, GoF = 0.78, for a multipole model with neutral atoms and multipole expansions up to seventh order. For each atom, the radial expansion coefficients of the multipole orders (l > 0) were constrained to a common value. The absence of extinction was indicated by a refined correction parameter equalling zero within error limit. The excellent quality of the data is also illustrated by a high‐order (HO) refinement (s > 0.7 Å−1) yielding R(F) = 0.0060, wR(F) = 0.0048, GoF = 0.85. Both static deformation electron‐density distribution and structure amplitudes compare well with corresponding results obtained from band‐structure calculations using the linearized‐augmented‐plane‐wave (LAPW) method. Ensuing topological analysis of the total model electron density distribution revealed bond critical point properties for the two unique Si—O bonds, indicating a predominantly closed‐shell interaction mixed with a significant shared interaction contribution that decreases with increasing interatomic distance. Calculation of atomic basins yielded charges of +3.39 e and −1.69 e for Si and O, respectively, in good agreement with the theoretically calculated values of +3.30 e and −1.65 e. The volumina of the Si and O basins are 2.32 and 10.48 Å3, corresponding to spheres with radii of 0.82 and 1.36 Å, respectively. The results also conform well with correlations between bond length and bond critical point properties reported in the literature for geometry‐optimized hydroxyacid molecules. Estimates of the Si cation electronegativity indicate that the change of Si coordination by oxygen from 4 to 6 is accompanied by an increase of the ionicity of the Si—O bond of about 7%.