Superalkali nature of the SiM (M = Li, Na, and K) Zintl clusters: a theoretical study on electronic structure and dynamic nonlinear optical properties.

Zintl clusters have attracted widespread attention because of their intriguing bonding and unusual physical properties. We explore the Si and SiM (where M = Li, Na, and K) Zintl clusters using the density functional theory combined with other methods. The exothermic nature of the SiM cluster formation is disclosed, and the interactions of alkali metals with pristine Si are shown to be noncovalent. The reduced density gradient analysis is performed, in which increased van der Waals interactions are observed with the enlargement of the size of alkali metals. The influence of the implicit solvent model is considered, where the hyperpolarizability () in the solvent is found to be about 83 times larger than that in the gas phase for SiK. The frequency-dependent nonlinear optical (NLO) response for the dc-Kerr effect is observed up to 1.3 × 10 au, indicating an excellent change in refractive index by an externally applied electric field. In addition, natural bonding orbitals obtained from the second-order perturbation analysis show the charge transfer with the donor-acceptor orbitals. Electron localization function and localized orbital locator analyses are also performed to better understand the bonding electrons in designed clusters. The studied Zintl clusters demonstrate the superalkali character in addition to their remarkable optical and nonlinear optical properties.