Electronic structure and bulk modulus of silicon dicarbide: a glitter phase

A hypothetical, metallic and hard three-dimensional extended structure is described with the chemical formula SiC2. This structure is achieved by replacing the tetrahedral carbon atoms in the previously described glitter lattice with tetrahedral silicon atoms. The new structure, like the parent carbon allotrope glitter, has polymeric substructures that can be carved from the three-dimensional extended structure. These substructures, which make use of the 1,4-disila-2,5-cyclohexadiene (1,4-disilaquinoid) moiety as a common theme, are of a polyquinoid, polyspiroquinoid and polyparaquinoid (polycyclophane) nature. Oligomers of the poly-1,4-disilaquinoid and poly-1,4disilaspiroquinoid substructures have been synthesized in the laboratory and some of their properties have been investigated. The present report describes theoretical calculations, at the semi-empirical (EHMO) and density functional (DFT) level of theory, of the electronic structure of the three-dimensional SiC2 lattice and of the synthetically realized poly-1,4-disilaspiroquinoid oligomers. It is seen that a relatively high-lying carbon-silicon sigma level causes the ordinarily insulated pi and pi* levels in SiC2 to be bridged, so that a conducting, metallic system is achieved. Finally, the bulk modulus of the three-dimensional SiC2 structure is evaluated at zero pressure using a semiempirical formula developed by Cohen. The zero-pressure bulk modulus Of SiC2 is one of the highest known in crystalline materials at 230 GPa. The zero-pressure bulk modulus of silicon dicarbide is comparable to that observed in the commonly used industrial abrasive called carborundum (SiC), which has an experimental zero-pressure bulk modulus of 211 GPa. In addition, it is estimated that the bulk modulus at pressure of the silicon dicarbide lattice can attain values in excess of 545 GPa by a mechanism of harmonic compression which has been described earlier for glitter.