Recent Achievements in Lone-Pair Cation-Based Infrared Second-Order Nonlinear Optical Materials

Abstract

In the past 20 years, fundamental research on novel nonlinear optical (NLO) compounds has been extensively performed, which has built the important background for the application of NLO materials. In particular, for NLO materials in the middle-infrared (MIR) and deep-ultraviolet regions, fundamental exploration is crucial in view of the insufficiency of available materials. To address the urgent issues encountered during the application of MIR NLO materials, hundreds of potential materials have been studied, and many design strategies have been introduced to obtain these materials. These contents have been well summarized in reviews by our group and others. However, there has not been a specific review to summarize the MIR NLO materials containing lone-pair cations. Although the contribution of lone-pair cations on the NLO effects has been discussed separately in several publications, a systematic survey of them has not been published. Therefore, a complete survey on this field is needed. In this review, MIR NLO materials containing lone-pair cations are summarized, and the materials are organized based on their chemical compositions. They are introduced based on the different lone-pair cations, including As 3+ , Sb 3+ , Bi 3+ , Pb 2+ , and Sn 2+ cations. Though iodates are also important IR NLO material candidates and most of them contain low-valent iodide ions with lone pairs, the iodates without non-iodine lone-pair cations are not included in this work, as they have been systematically summarized in a specific review of metal-iodate-based NLO materials previously. Apart from the structural chemistry and NLO properties of the materials included in this work, the contribution of lone-pair cations to the NLO properties is also discussed, and the future directions of this field are also proposed.

Unprecedented mid-infrared nonlinear optical materials achieved by crystal structure engineering, a case study of (KX)P2S6 (X = Sb, Bi, Ba)

“…The trivalent cations containing stereochemically active lone pair electrons (SCALP) such as Sb and Bi were also incorporated into the system. Many previous works have demonstrated that SCALP play an important role in enhancing NLO properties [33][34][35][36][37][38] . Herein, in this study, we verified that a new strategy of applying crystal structure engineering via mixed cation plays a dual-role in three emerging MIR NLO materials, KSbP 2 S 6 , KBiP 2 S 6 , and K 2 BaP 2 S 6 .…”

“…Many previous studies have demonstrated that SCALP play an important role in enhancing NLO properties. [33][34][35][36][37][38] Herein, in this study, we veried that a new strategy of applying crystal structure engineering via mixed cations plays a dual-role in three emerging MIR NLO materials, KSbP 2 S 6 , KBiP 2 S 6 , and K 2 BaP 2 S 6 . The mixed cations, K/Sb, K/Bi and K/Ba, align the [P 2 S 6 ] structure motifs, contributing to both a high SHG coefficient and enlarging the bandgap.…”

Sn7Br10S2: The First Ternary Halogen‐Rich Chalcohalide Exhibiting a Chiral Structure and Pronounced Nonlinear Optical Properties

“…Generally, the outmost s 2 electrons for metal lone-pair cations cannot be shared with the adjacent anions. [19] What's more, these lone-pair electrons have the potential to hinder the formation of chemical bonds between metal cations and ligands on the same side, resulting in the pyramidal coordination geometries (often [MX 3 ] [20] or [MX 5 ] [14,21] motifs). The structures with pyramidal units can usually exhibit strong NLO responses under an external optoelectronic field.…”

Sn7Br10S2: The First Ternary Halogen‐Rich Chalcohalide Exhibiting a Chiral Structure and Pronounced Nonlinear Optical Properties

“…On the other hand, NCS d 10 or lone‐pair cations are competitive chemical components for NLO materials. Generally, the outmost s 2 electrons for metal lone‐pair cations cannot be shared with the adjacent anions [19] . What's more, these lone‐pair electrons have the potential to hinder the formation of chemical bonds between metal cations and ligands on the same side, resulting in the pyramidal coordination geometries (often [MX 3 ] [20] or [MX 5 ] [14, 21] motifs).…”