Relation of Infrared Spectra to Coordination in Quartz and Two High-Pressure Polymorphs of SiO2.

Infrared spectra of the four-coordinated quartz and coesite polymorphs of SiO(2), the rutile six-coordinated (stishovite) polymorph of SiO(2), and the quartz and rutile polymorphs of GeO(2) show that a change from tetrahedral to octahedral cation coordination results in (i) a 23-per-cent increase in the wavelength of the main absorption band for both the SiO(2) and GeO(2) polymorphs and (ii) a significant increase in the force constant of the same magnitude for the SiO(2) and GeO(2) polymorphs. The quartz and the rutile isostructural pairs for SiO(2) and GeO(2) show that the effect of increasing mass is to increase proportionally the wavelength of the respective main absorption bands. The infrared data for the rutile form of SiO(2) fit the empirical equation of Dachille and Roy relating cation coordination, mass, atomic number, valence, and main absorption wavelength.