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 Si 4 tetrahedra with K atoms situated above each face. The crystal system is cubic with the symmetry of the P4 h3m 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 Si 4 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 Si 4 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 sp 3 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 Si 4 tetrahedra, allowing the Si atom to fill its 3s and 3p shells.

NMR Study of the Synthesis of Alkyl-Terminated Silicon Nanoparticles from the Reaction of SiCl4with the Zintl Salt, NaSi

“…The full width at half-maximum (fwhm) of these resonances are 233 and 145 Hz, respectively. The intensity of the spinning sidebands suggests large chemical shift anisotropies for both 29 Si sites (Δδ a = 214 (3) ppm, η a = 0.28 (6); Δδ b = 236 (3) ppm, η b = 0.29 (6)) , and the far upfield shift results from the donation of electrons from the electropositive Na to the electronegative Si, , as is common in Zintl salts. This is different from the clathrates, which exhibit Knight shifts. , The spectrum in Figure b is the first reported 29 Si NMR data for the Zintl salt NaSi and one would expect that similar phases will exhibit similarly extreme diamagnetic chemical shifts…”

“…The quadrupolar coupling parameters and chemical shifts were obtained from simulation of the MAS central transition line shape (Figure 1a), calculated using the method of Massiot et al 24 Note that the site yielding the resonance at δ i ) 49.5 ppm is most likely in a more electronically strained environment evidenced by the larger quadrupolar coupling constant C q , but the smaller asymmetry parameter η suggests more axial symmetry. Other than the spinning sidebands, no other resonances are observed in the 23 6)) 25,26 and the far upfield shift results from the donation of electrons from the electropositive Na to the electronegative Si, 27,28 as is common in Zintl salts. This is different from the clathrates, which exhibit Knight shifts.…”

Structural diversity and electronic properties in potassium silicides

“…The optimized lattice parameter (12.733 Å) agrees well with the experimental data (12.620 Å). 32,33 Two nonequivalent Si atoms locate at the 24i (0.562, 0.073, 0.183) and 8e (0.568, 0.568, 0.568) sites, respectively. Si1 atoms form slightly distorted tetrahedrons with two different Si-Si bond lengths (2.428 Å and 2.447 Å) at ambient pressure, while Si2 atoms form regular tetrahedrons with a bond length of 2.435 Å at ambient pressure [see Fig.…”

“…36 KSi crystallizes in a cubic KGetype structure (space group P-43n, 32 molecules per unit a) ylli@jsnu.edu.cn cell, i.e., Z = 32), in which both K and Si atoms form tetrahedrons. 32,33 KSi can be written as K 4 Si 4 because of the existence of cluster Si 4 4− . KSi can absorb hydrogen to form the potassium silanide, KSiH 3 , a reversible hydrogen storage material.…”