SCF-Xα studies of the electronic structures of C,Si and Ge oxides |
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Authors: | J.A. Tossell |
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Affiliation: | Department of Chemistry, University of Maryland, College Park, MD 20742, U.S.A. |
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Abstract: | 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. |
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