An organic-inorganic hybrid polyiodate, namely, [o-C 5 H 4 NHOH] 2 [I 7 O 18 (OH)]•3 H 2 O (I), featuring a novel branched polyiodate chain has been obtained by evaporation method. [o-C 5 H 4 NHOH] 2 [I 7 O 18 (OH)]•3 H 2 O (I) crystalizes in the polar space group Ia and features an [I 7 O 18 (OH)] 2À 1 branched polyiodate chain in which [I 3 O 9 ] 3À trimers are grafted on the same side of the one-dimensional (1D) chain based on [I 4 O 11 (OH)] 3À tetramers. The asymmetric organic amine groups are beneficial to the polymerization of iodate groups and inducing the formation of the non-centrosymmetric (NCS) structure. Compound I exhibits a rather large Second-Harmonic-Generation (SHG) signal of 8.5 KH 2 PO 4 (KDP) upon 1064 nm laser radiation, a moderate band gap of 3.90 eV and a high laser-induced-damage-threshold (LIDT) of 182.34 MW cm À2 , hence it is a promising new SHG material.The relationship between the structures of the organic amine groups and the overall structures has been also analyzed.Second-harmonic-generation (SHG) materials harboring the capacity of generating coherent light are widely used in solid-state lasers in recently years. [1][2][3] The basic requirements for brilliant SHG materials include the NCS structures, strong SHG effects, high LIDT values, wide band gaps, phasematching capabilities, facile synthetic methods, etc. Metal iodates are a class of excellent SHG crystals due to the existence of the stereochemically active-lone-pair (SCALP) electrons of I 5+ cation which can undergo second-order Jahn-Teller (SOJT) distortion. [4,5] At present, several common strategies have been utilized to design new SHG iodates: (a) Introduction of SOJT distorted d 0 transitional metal (TM) cations (Mo 6+ , V 5+ , W 6+ , Nb 5+ , Ti 4+ , etc.). [6][7][8][9][10][11][12] The additive polarizations from both models of SOJT cations can result in strong SHG effects as exampled by A(VO) 2 O 2 (IO 3 ) 3 (A = K, Rb, Cs, NH 4 ) (500 SiO 2 and 3.6 KTiOPO 4 (KTP)); [7,8] (b) Introduction of cations with also SCALP electrons (Bi 3+ , Pb 2+ , etc.) as in BiO(IO 3 ) (12.5 KDP) and PbPt(IO 3 ) 6 (H 2 O) (8 KDP); [13,14] (c) Introduction of the strong electronegative F À anion which can widen the band gaps of iodates and