| The ab initio local-orbital molecular dynamics method is used to analyze theoretically complex system in materials science. Predominantly tetrahedral materials ranging from the random semiconducting alloys to silica polymorphs, and zeolites are investigated using the unified approach of electronic structure theory.;Next, the unusual clathrate phases of Si have been studied. Si clathrates are found to be energetically competitive with the ground state diamond phase of Si. The band gaps of these materials are 0.7 eV wider than that of bulk Si. The effects of alkali-metal doping in clathrate Si solids were studied. At low concentrations, the resulting compounds are metallic in a one electron picture. However, for Na doping a flat and Jahn-Teller distorted band is found, which may produce an insulating structure when many electron effects are included. Formation of the insulating state is probably due to a combination of the Jahn-Teller and Mott transitions. The phonon frequencies at the r point have been calculated.;A new method suitable for the problems with a large transfer of charge between the atoms of the system has been developed. This new method is applied to study silica. In particular, the structural transition in high cristobalite is investigated. The results of the first-principles calculations are used to derive a simple model energy for this transition. The model enables us to obtain an analytical expression for the Si-O bond length as a function of the bond angle.;Finally, a class of complex materials known as zeolites is considered. Zeolites belong to the group of porous tectosilicates--materials based on 4-connected tetrahedral frameworks. Traditional applications of zeolites lie in the areas of oil refining and selective catalysis. Zeolites have very open low density structures build of cages connected with each other by channels. The electronic properties of the two zeolites--melanophlogite and silica-sodalite are studied. Both structures found to be only 10 kJ/mol above ;First the random Si-C-Ge alloys have been investigated. Contrary to early theoretical work the band gap of these materials is found to be smaller than that of bulk Si. The effects of geometrical relaxation around carbon impurities, and also the effects of ordering of carbon atoms have been studied. Our conclusion is that the closing of the band gap upon the adding of carbon is a purely chemical phenomenon, caused by the great difference in electronegativity between Si and C. For the atomic structure, it is found that there are two different types of Si-C bonds. |
