Theoretical study of electronic properties of Zintl phase KSi

We have studied the electronic properties of Zintl phase KSi, by the ab ini tio density functional pseudopotential method. Our interest in this Zintl c ompound is in its current use as a reagent in the synthesis of Si nanoclust ers. The structure consists of isolated Sir tetrahedra with K atoms situate d above each face. The crystal system is cubic with the symmetry of the P ( 4) over bar 3m space group. Band structure calculations show a band gap of 1.3 eV. The presence of K atoms has widened the band gap over that found be tween occupied and unoccupied energy levels in the Si-4 cluster. The valenc e bandwidth lies between the valence bandwidth of crystalline Si with diamo nd structure and the width of the occupied energy levels of the Si-4 cluste r. The density of states shows four major structures for the occupied bands . The lowest energy band of conduction states is also given in the density of states plot. The nature of the bonding in the crystal is revealed by an examination of the charge density associated with each of the structures in the density of states. It is found that the dominant bonding between Si at oms is not the covalent bonding of sp(3) hybridized orbitals, as found in d iamond structure Si-4 but is rather a mutual overlap of s- and p-like mixed atomic states from each Si atom. These overlapped states form a bonding st ate located at the center of the tetrahedron. Furthermore, each K atom is i onized by the nearest Si-4 tetrahedra, allowing the Si atom to fill its 3s and 3p shells.