Theoretical Study of Electronic Properties of Zintl Phase KSi

Abstract

We have studied the electronic properties of Zintl phase KSi, by the ab initio density functional pseudopotential method. Our interest in this Zintl compound is in its current use as a reagent in the synthesis of Si nanoclusters. The structure consists of isolated Si4 tetrahedra with K atoms situated above each face. The crystal system is cubic with the symmetry of the P4̄3m 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 between occupied and unoccupied energy levels in the Si4 cluster. The valence bandwidth lies between the valence bandwidth of crystalline Si with diamond structure and the width of the occupied energy levels of the Si4 cluster. 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 atoms is not the covalent bonding of sp3 hybridized orbitals, as found in diamond structure Si, but is rather a mutual overlap of s- and p-like mixed atomic states from each Si atom. These overlapped states form a bonding state located at the center of the tetrahedron. Furthermore, each K atom is ionized by the nearest Si4 tetrahedra, allowing the Si atom to fill its 3s and 3p shells.