MINERALS AND MOLECULES AND THEIR BONDED INTERACTIONS AND NEW SILICA PHASES

The radii of bonded atoms have been widely used by geochemists, crystal chemists, mineralogists, material scientists, chemists, physicists and others in the prediction of the coordination numbers, isomorphic substitutions, cation conductivity and diffusion, chemical zoning, size discrimination, the leaching of ions, among other things in crystals, together with the successful reproduction of precise bond lengths. Since Bragg (1920) derived his famous set of atomic radii, assuming that the bonded atoms are additive impenetrable spheres in cubic and hexagonal materials, a relatively large number of sets of radii have been derived for oxides, sulfides, nitrides and other materials. Nonetheless, despite the success and widespread use of crystal radii, soft anions like the oxide anion in a crystal may not be impenetrable spheres of electron density as conventionally assumed, but may be polarized and distorted from spherical symmetry by the electric fields of the bonded metal atoms. If true, then the derivation of a set of radii based on the bond lengths alone may result in a fallacious set of radii. The purpose of this presentation is to evaluate the experimental and theoretical electron density distributions of the bonded oxygen atoms for a variety of oxide crystal structures. The oxygen atoms comprising oxides display a relatively wide range of bonded radii, revealing that the electron density distributions of the individual atoms are not in general spherical and characterized by a well-defined set of crystal radii. As such, the assumption that accurate sets of crystal cation radii for oxides can be derived by assuming a given additive radius for the oxygen atom is clearly open to question.