SCF-Xα studies of the electronic structures of C,Si and Ge oxides

SCF-Xα scattered wave cluster MO calculations are presented for the oxyanions CO₃^?2, CO₄^?4, SiO₃^?2, SiO₄^?4, SiO₆^?8, GeO₄^?4 and GeO₆^?8. A comparison of the calculations with available X-ray spectral data shows good agreement. The MO diagrams for CO₃^?2 and CO₄^?4 are found to be quite similar, suggesting that the four coordinate oxyanion is not inherently unstable; therefore a quartz polymorph of CO₂ may become stable at high pressure. On the other hand, the valence region width of SiO₃^?2 is much smaller than that of SiO₄^?4; thus the three-coordinate Si oxyanion is expected to be highly unstable. For the Ge oxyanions the average valence region width is substantially larger in the six-coordinate than in the four-coordinate form, suggesting a preference for octahedral coordination. These observations indicate that although the valence region width is only one factor affecting the total energy of a system, it is predictive of favored coordination number according to the following principle: a change from a higher to a lower coordination number oxyanion of a given cation, Mⁿ⁺, will be favorable if the lower coordinate form has a wider valence region at its equilibrium internuclear distance.