Vibronic Interactions in Silicon Polyhedra of the Si₄₆ Clathrate Compound

Abstract

The vibronic (vibrational−electronic) interactions in the Si46 clathrate compound are discussed using the highly symmetric (SiH)20 and (SiH)24 cluster models to look at a possible correlation between the Jahn−Teller effect and the superconductivity. The vibronic coupling constants for six Hg modes in the mono-, tri-, and pentaanions of (SiH)20 and for twelve E4 modes in the tri- and pentaanions of (SiH)24 are calculated using the MNDO-PM3 method, one of the most reliable semiempirical molecular orbital methods, to know which mode of vibration plays a governing role in the Jahn−Teller distortions of these interesting hypothetical multianions. The lowest mode of about 100 cm-1 appears to significantly couple with their degenerate electronic states to exhibit a large coupling constant in both silicon clusters. This type of low-frequency mode is characteristic of nanosized molecular systems and would have relevance to acoustic phonon modes in the solid. If the Jahn−Teller distortions play a role in the appearance of the molecular superconductivity in the silicon clathrate compound as suggested in the fullerene A3C60 complexes, the lowest mode of vibration in the Si20 moiety should have a significant effect on the interesting solid-state properties.