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Because mid‐ and far‐infrared (IR) nonlinear optical (NLO) materials play an important role in military and civil fields, plenty of excellent IR NLO materials have been discovered in recent decades, such as chalcogenides with T2 supertetrahedra. Pnictides with T2 supertetrahedra instead of MQ4 (M = IIB, IIIA, IVA metals; Q = P, As) tetrahedra, may favor high second‐harmonic generation (SHG) efficiency due to their large hyperpolarizability, but they have been rarely reported. Therefore, in this study, two pnictides with T2 supertetrahedra, BaSi7P10 and SrSi7P10, are synthesized and studied. With three‐dimensional (3D) framework structures assembled from 1D zigzag chains of [Si7P16] dual‐T2 supertetrahedra, both BaSi7P10 and SrSi7P10 exhibit strong phase‐matching (PM) SHG responses (3.2‐ and 3.0‐times that of AgGaS2), large birefringences (0.077 and 0.112), and wide infrared transparency. In addition, theoretical calculation also reveals that the two‐coordinated P atom can enhance the SHG effects. This study provides a new strategy for exploring new IR NLO materials. [ABSTRACT FROM AUTHOR]

• A mid-infrared nonlinear optical crystal, HgSO 4 , was screened out and a single crystal (3 × 5 × 1 mm) was grown. • HgSO 4 exhibits a strong SHG response of 11 × KDP and a suitable birefringence for phase matching (0.093 @ 546.1 nm). • HgSO 4 exhibits a wide band gap of 4.13 and a large laser damage threshold (26.5 × AgGaS 2). • Theoretical calculations indicate that the strong SHG response of HgSO 4 originates from the introduction of Hg2+. [Display omitted] A mercury-based sulfate nonlinear optical (NLO) material, HgSO 4 , was successfully grown using a hydrothermal method. It crystallizes in the polar space group Pmn2 1 and forms a three-dimensional spatial network structure consisting of [HgO 8 ] polyhedras and [SO 4 ] polyhedras. The powder second harmonic generation (SHG) measurement showed that HgSO 4 is a phase-matching material and has a very strong SHG efficiency of 11 times that of KDP. Such a remarkable SHG response is due to the well-ordered arrangement of the distorted [HgO 8 ] and [SO 4 ] polyhedras. In parallel, large bandgap (4.13 eV) and laser damage threshold (26.5 × AgGaS 2) are exhibited. Further characterizations showed that this compound possesses a large birefringence and a wide transmission window from the near-UV to mid-IR, suggesting that HgSO 4 may be an excellent mid-IR NLO crystal. [ABSTRACT FROM AUTHOR]