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

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 As3+, Sb3+, Bi3+, Pb2+, and Sn2+ cations. Though iodates are also important MIR 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.