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We herein report two asymmetric germanate crystals, KNbGe3O9 and K3Nb3Ge2O13, with different structures and optical properties derived from divergent polymerized forms of GeO4 and NbO6 groups. Remarkably, K3Nb3Ge2O13 achieved a rare combination of the strongest second harmonic generation (SHG) response of 17.5×KDP @ 1064 nm and the largest birefringence of 0.196 @ 546 nm in germanates. It features unique [Nb3O12]∞ tubular chains constructed by circular Nb3O15 tripolymers. Theoretical calculations reveal that the d‐p interactions in the Nb3O15 group are responsible for outstanding optical properties. This work emphasizes the significance of the polymerizable functional units in obtaining high‐performance nonlinear optical (NLO) crystals. [ABSTRACT FROM AUTHOR]

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]

For the first time, the polar covalent tetrahedron [SO2(NH2)2] is revealed as a new deep‐ultraviolet (DUV) nonlinear optical (NLO)‐active unit according to theoretical calculations. Furthermore, sulfamide consisting of polar [SO2(NH2)2] units was confirmed as an excellent candidate as a DUV NLO crystal. Sulfamide provides the optimal balance between composition, structure, and properties, in addition to a very short absorption of 160 nm. It achieves multiple optical performance records for non‐π‐conjugated DUV NLO materials, including the strongest second harmonic generation (SHG) efficiency (about 4 times that of KDP), the largest birefringence (obv.: 0.07@589.3 nm) and the shortest SHG wavelength predicted as 188 nm. [ABSTRACT FROM AUTHOR]

Searching on the functional building units with ultrawide band gaps, large hyperpolarizabilities, and strong anisotropies is the key to finding new deep-ultraviolet (DUV) nonlinear-optical (NLO) materials. In this study, the polar covalent tetrahedral unit [SO 3 (OH)] - is studied by theoretical calculation and its excellent DUV optical properties are revealed. Moreover, we thoroughly investigate the known bisulfates, finding a promising DUV NLO crystal, NaHSO 4 ·H 2 O. It exhibits a large second-harmonic-generation (SHG) response of 1.5 times that of KH 2 PO 4 and a short cutoff edge (shorter than 190 nm). Meanwhile, it has the largest birefringence of 0.042 at 546 nm among the DUV NLO sulfates. Our study suggests that bisulfates should be fresh and ideal DUV NLO candidates.

Studies on new functional structural units with both large hyperpolarizability and high anisotropy are essentially important for finding high-performance nonlinear optical (NLO) materials and enriching the material systems. Under the guidance of the "structure-analogue" strategy, the work utilizes the molecular engineering approach to direct the construction of target units, BC 2 N 5 O 2 and B(C 2 N 5 ) 2 units. The BC 2 N 5 O 2 unit with a highly analogous structure to the B 3 O 7 group and its derivate B(C 2 N 5 ) 2 unit with a configuration of B 5 O 10 group are designed as NLO-active units. Furthermore, two compounds with these new NLO-active units, BC 2 N 5 H 6 (OH) 2 ·H 2 O ( I ) and B(C 2 N 5 H 6.5 ) 2 (NO 3 ) 2 ( II ), are synthesized, successfully. These compounds exhibit excellent properties with second-harmonic generation (SHG) responses ranging from 0.5 to 5.9 times that of KDP and large birefringence (Δ n I = 0.181 @ 546.1 nm and Δ n II = 0.148 @ 546.1 nm). Theoretical calculations prove that the BC 2 N 5 O 2 and B(C 2 N 5 ) 2 units make great contributions to the SHG effects and birefringence, which confirms that the BC 2 N 5 O 2 and B(C 2 N 5 ) 2 units are novel NLO bifunctional units and could be excellent fundamental building blocks to construct amounts of novel NLO and birefringence crystals. Our studies would enlighten the research studies on biguanide complexes of boron.