Chalcogenides and pnictides are both excellent Infrared (IR) nonlinear optical (NLO) material candidates. For decades, NLO chalcogenide crystals have been extensively investigated. Numerous state-of-the-art IR NLO materials and material design methods have been developed. 

Recently, the exploration on pnictide NLO crystals has emerged after years of silence due to their remarkable NLO performance and much higher thermal conductivity than those of chalcogenide. One of main challenges during the exploration of NLO pnictides is lack of effective and rational material design strategies. 

In a study published in Advanced Science, Dr. LUO Min's group from Fujian Institute of Research on the Structure of Matter of the Chinese Academy of Sciences uncovered a vital band gap mechanism of pnictides by combining first-principles calculation with the classical Zintl-Klemm concept. 

The researchers investigated the AII-Si-Pn₂ family and its structural variation of AI₂-Si-Pn₂ family (AI = Li, Na, K, Rb and Cs; AII = Cd, Mg, Ca, Sr; Pn= P and As). An abnormal ‘‘band gap reduction’’ was observed among them. MgSiP₂, Li₂SiP₂ and CdSiP₂ exhibited wider calculated band gap of 2.41, 2.32 and 2.13 eV, and the band gap decreases from 2.41 eV of MgSiP₂ to 1.81 eV of Cs₂SiP₂with the increase of electropositivity and atomic radius of A site. 

They then analyzed their crystal structures. MgSiP₂ and CdSiP₂ have classical chalcopyrite structure (I-42d) with three-dimensional (3D) covalent framework consisting of [MgP₄]/[CdP₄] and [SiP₄] tetrahedra while the residual pnictides have lower-dimensional structure. 

The researchers also calculated the electronic structure of pnictides based on HSE06 exchange correlation function. Counterintuitively, the introduction of Ca, Sr, Na, K, Rb and Cs with stronger ionicity did not broaden but decrease the band gap. Intriguingly, this is contrasting with alkali/alkalineearth metal NLO chalcogenides and oxides, where the orbitals of strongly ionic A site atom generally occupy higher energy level of conduction band, and the band gap mainly depends on anionic framework, and the wavefunction overlap of A cation and neighboring anion is negligible.  

According to Zintl-Klemm concept, the electropositive cation donates its valence electrons to a negatively charged polyanion network for which the octet closed shell is fulfilled. However, P anions which require larger charge transfer cannot stabilize that large charge without suitable cation environment, and on loosing close cation contacts they become highly reductive. 

Besides, the researchers analyzed the electron localization function, electron density difference, partial density of state and electronic structure. The electron pairs between IA/IIA and P are often defined as nearly free electrons (NFE) while that between IIIA/IVA-P are tight-binding electrons (TBE). As the size and mass of A site cation improve, the NFE and repulsive effect enlarge, may be responsible for the degradation of band gap. Based on this discovery, they proposed the ionicity–covalency–metallicity regulation for designing wide-band gap NLO pnictides. 

This study provides an essential guidance for the future design and synthesis of NLO pnictides.