Hg-based chalcogenides: An intriguing class of infrared nonlinear optical materials

• The first comprehensive review about Hg-based IR NLO chalcogenides. • Hg-based chalcogenides possess large NLO response, wide IR transparent window , and suitable birefringence. • The structures and optical performances of Hg-based IR NLO chalcogenides were summarized, compared and discussed. • The directions for the design and exploration of Hg-based IR NLO chalcogenides in the future were discussed in detail. Infrared (IR) nonlinear optical (NLO) crystals with excellent overall performances are critical for medical devices, deep space detection, photoelectric countermeasure. Due to the unique bonding characteristics of Hg element, Hg-based chalcogenides emerge as an appealing class of IR NLO crystals with rich structures, large NLO responses, wide IR transparent ranges, and suitable birefringence, which make them intrinsically satisfy numerous crucial criteria for practical application. The investigation of Hg-based IR NLO chalcogenides began from the last century and many new members have been constantly discovered. But the detailed and systematic review of this seductive system remains vacant. Therefore, we decided to review this meaningful work by dividing the existing Hg-based IR NLO chalcogenides into two groups on the basis of their chemical components to introduce their crystal structures and performances and structure-property relationships deeply. Besides, some summaries are given as follows: (i) The number of Hg-based IR NLO chalcogenides embodying HgQ 4 polyhedra is much larger than that containing HgQ 3 or HgQ 2 (Q = chalcogen atom). (ii) There are more quaternary Hg-based IR NLO chalcogenides than the ternary. (iii) The cations that make up the Hg-based IR NLO chalcogenides are mainly concentrated in Li, Na, K, Sr, Ba, Si, Ge, Sn, P, Ag, Cu, while the anions are only S and Se. (iv) The synergistic effect of Hg with tetra-coordinated cations, such as Ag, Cu, Si, Ge, and P, tends to produce diamond-like (normal + defect) structures, and further accelerate the generation of large NLO response. Finally, some directions for further investigation have been put forward including how to select further exploration systems and to optimize the NLO properties of Hg based chalcogenides, how to realize the practical application of these new materials, and how to deepen the understanding of the composition-structure-property relationship among Hg-based IR NLO chalcogenide materials.