Exploring new horizons in mid-to-far infrared nonlinear optical crystals: the significant potential of trigonal pyramidal [TeS₃]²⁻ functional units

Traditional tetrahedral-based mid-to-far infrared (MFIR) nonlinear optical (NLO) crystals often face limitations due to the optical anisotropy constraints imposed by their highly symmetric structures. In contrast, the relatively rare trigonal pyramidal [TeS₃]^2- functional unit characterized by its asymmetric structure and stereochemically active lone pair (SCALP), offers improved optical anisotropy, hyperpolarizability and a broader IR transparency range. Despite its potential, synthetic challenges have hindered the development of MFIR NLO crystals that incorporate this unit, with only one example reported to date. Herein, an innovative MFIR NLO crystal, Cu₁₀Te₄S₁₃ has been successfully constructed using the trigonal pyramidal [TeS₃]^2- units, <i>via</i> a simple high-temperature solid-state method. The novel three-dimensional structure of Cu₁₀Te₄S₁₃ is interconnected by butterfly-orchid-like [Cu₆Te₄S₁₃] anionic clusters and [CuS₄] groups, where the former are composed of trigonal pyramidal [TeS₃]^2- groups and [Cu₆S₁₃] hexamers. Cu₁₀Te₄S₁₃ exhibits a remarkable second harmonic generation effect, approximately 3.75 times that of AgGaS₂ at 2900 nm in the particle size range of 30-45 μm. Additionally, it demonstrates favorable crystal growth habits, producing single crystals with maximum dimensions of about 7 × 3 × 2 mm³. This polished single crystal appears to exhibit complete transparency within the MFIR spectral window ranging from 2.5 to 25 μm, representing the widest IR transmission in all reported NLO chalcogenides. Furthermore, the structure-property relationship is also elucidated through first-principles analysis. This work confirms the potential of the unique trigonal pyramidal [TeS₃]^2- as a MFIR NLO functional unit, paving the way for the development of unconventional MFIR NLO materials.