Mid-infrared (IR) nonlinear optical (NLO) crystals are crucial for a wide range of applications in both military and civilian sectors, including laser guidance, electro-optical countermeasures, medical diagnostics, and environmental monitoring. With the rapid advancements in solid-state laser technology and the increasing demand for high-repetition-rate, high-power mid-infrared laser output, there is an urgent need to develop new long-wave infrared nonlinear optical crystals with superior overall performance, particularly those with high laser damage thresholds. Pnictides are considered one of the most promising material systems for IR NLO crystals. However, due to the limited understanding of the optical bandgap mechanism in pnictides and the absence of effective bandgap tuning strategies to address their narrow bandgaps, the exploration of high-performance pnictide-based NLO crystals remains a significant challenge. This work summarizes and analyzes the literature data, explaining the microscopic structural mechanism by which alkali and alkaline earth metals, with their large ionic radii, fail to effectively widen the bandgap of pnictides. It also outlines design strategies and future directions for the development of wide bandgap pnictide-based NLO crystals. This work explains the bandgap mechanism of pnictides and proposes an optical bandgap tuning strategy for pnictide-based IR NLO crystals through a comprehensive analysis and synthesis of existing literature.

Compared with those of sulfur and oxygen, the significantly weak electron affinity and electronegativity of pnictides are insufficient to stabilize non-bonding electron pairs formed by electron transfer from Alkali metals (IA)/ alkaline earth metals (IIA). As a result, these electron pairs become delocalized, leading to metal-metal interactions between the alkali/alkaline earth metals and adjacent P atoms, thus affecting the bandgap. These delocalized electrons occupy higher energy levels, with the conduction band maximum primarily determined by the non-bonding electron orbitals of P. Unlike oxides and chalcogenides, in pnictides-based compounds, the conduction band maximum is influenced not only by the P 3p orbitals but also by the (n-1)d empty orbitals of IA/IIA, which play a significant role in shaping the optical bandgap [Fig. 3(a)]. As the ionic radius of the IA/IIA decreases, its ionic polarization effect on adjacent P atoms intensifies (Table 3), and the interaction between them gradually transitions from a metal-metal interaction to a polar covalent interaction. The ionic nature of IA/IIA begins to positively influence the widening of the bandgap [Fig. 3(b)]. At this point, the valence band maximum and the conduction band minimum are primarily determined by the P 3p bonding orbitals in the covalent groups. Therefore, IA/IIA ions with small ionic radii and strong ionic polarization are effective in widening the bandgap of pnictides-based compounds. As the ionic radius increases, the ionic polarization ability of IA/IIA ions weakens, and the covalent interaction between IA/IIA and P disappears, leading to the formation of non-bonding P 3p electron pairs. Due to the weak electron affinity and electronegativity of P, these non-bonding electron pairs become delocalized, resulting in metal-metal interactions that reduce the bandgap (Fig. 5). Thus, regulating the bandgap in pnictides-based compounds should consider the delocalized distribution of valence electrons, due to the insufficient covalent coordination number of P atoms. This can be achieved through reasonable structural design and element coordination to control the ionic-covalent-metallic nature of the system. Based on this bandgap mechanism for pnictides, three approaches can be employed to design wide-bandgap pnictide-based NLO crystals: 1) Exploration of pnictides with P atoms having 3-coordinate (3CN) and 4-coordinate (4CN) structures; 2) Exploration of halopnictides containing halogens; 3) Exploration of pnictides with P—P homoatomic bonds.

as the ionic radius of IA/IIA ions increases, their ability to polarize the ions of adjacent P atoms weakens. The covalent interaction between IA/IIA and P disappears, and non-bonding electron pairs of the P atom form. However, the contractive electron affinity and electronegativity of P atoms are incapable of stabilizing multiple non-bonded electron pairs, resulting in their delocalized distribution. Consequently, the metallic interaction occurs between alkali/alkaline-earth metals and neighboring P atoms, reducing the bandgap. The design strategies and the exploration direction for wide bandgap pnictide-based nonlinear optical crystals are proposed: 1) Pnictides with P atoms having 3CN/4CN, such as conventional diamond-like pnictides; 2) Halopnictides; 3) Pnictides with P—P homoatomic bonds.