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

The electron-density distribution of the high-pressure polymorph of SiO2, stishovite [a = 4.177 (1), c = 2.6655 (5) A, space group P4(2)/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 A(-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 A3, corresponding to spheres with radii of 0.82 and 1.36 A, 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%.