Tw om ixed-metal gallium iodate fluorides,n amely, a-and b-Ba 2 [GaF 4 (IO 3 ) 2 ](IO 3 )(1 and 2), have been designed by the aliovalent substitutions of a-a nd b-Ba 2 [VO 2 F 2 (IO 3 ) 2 ]-(IO 3 )( 3 and 4)i nvolving one cationic and two anionic sites. Both 1 and 2 display large second-harmonic generation responses ( % 6 KH 2 PO 4 (KDP)), large energy band gaps (4.61 and 4.35 eV), wide transmittance ranges ( % 0.27-12.5 mm), and high relevant laser-induced damage thresholds (29.7 and 28.3 AgGaS 2 ,r espectively), whichi ndicates that 1 and 2 are potential second-order nonlinear optical materials in the ultraviolet to mid-infrared. Our studies propose that three-site aliovalent substitution is af acile route for the discovery of good NLO materials.Noncentrosymmetric (NCS) oxide materials have extensively excited academic interest owing to their probable properties with applications in second-order nonlinear optics (NLO) as well as in pyroelectrics,f erroelectrics,a nd piezoelectrics. [1][2][3][4][5][6] To the best of our knowledge,a ne xcellent second-order NLO material should feature al arge second harmonic generation (SHG) coefficient, ahigh relevant laserinduced damage threshold (LDT), appropriate optical transmittance range,m oderate birefringence for achieving phase matching,and good thermal stability.Inaddition, it should be free of moisture and accessible by af acile synthetic method. [7][8][9] Metal iodates currently attract extensive research interest due to their potential outstanding SHG response. [10] Up to now,several effective synthetic routes have been reported for exploring the NCS metal iodates featuring novel crystal structures and remarkable SHG performance,s uch as condensing IO 3 À and IO 4 3À units to form polyiodates, [11][12][13] combining six-or five-coordinate d 0 transition metal (TM) cations (e.g.T i 4+ ,V 5+ ,M o 6+ ,a nd Nb 5+ ) [14][15][16][17][18][19][20][21][22] or metal cations (e.g. Bi 3+ and Pb 2+ ) [23][24][25][26] having as tereochemically active lone-pair (SCALP) with iodates,a nd introducing fluoride anions into iodates. [27][28][29][30][31][32] Anumber of excellent SHG materials have been discovered, for instance,NaI 3 O 8 , [11] Cs 2 I 4 O 11 , [13] Li 2 M(IO 3 ) 6 (M = Ti 4+ ,Sn 4+ ,Ge 4+ ), [33][34][35] A(VO) 2 O 2 (IO 3 ) 3 (A = K, Rb,C s, NH 4 ), [15,20] AMoO 3 IO 3 (A = Rb,C s), [14] BaNbO-(IO 3 ) 5 , [17] BiOIO 3 , [23] PbPt(IO 3 ) 6 , [25] Bi(IO 3 )F 2 , [28] and KBi 2 -(IO 3 ) 2 F 5 , [30] which can produce very strong SHG effects (> 10 KH 2 PO 4 (KDP)).Recently,anumber of new NCS crystals were obtained by the aliovalent substitution of known parent compounds to produce materials with strong SHG response. [36][37][38][39][40][41] For example,S e IV O 3 2À has typically been used as as urrogate for I V O 3 À ,w ith substitution of the oxo anion by ah alide anion (usually fluoride anion) to maintain charge balance.T he first bismuth selenite fluoride,B iFSeO 3 ,w ith the strongest SHG performance of all metal selenites,was designed by aliovalent substitution of laye...

Metrics & MoreArticle RecommendationsCONSPECTUS: Second-harmonic-generation (SHG) causes the frequency doubling of light, which is very useful for generating high-energy lasers with specific wavelengths. Noncentrosymmetry (NCS) is the first requirement for an SHG process because the SHG coefficient is zero (χ 2 = 0) in all centrosymmetric structures. At this stage, developing novel NCS crystals is a crucial scientific topic. Assembling polar units in an addictive fashion can facilely form NCS crystals with outstanding SHG performance. In this way, our group has obtained many different NCS crystals with extremely large SHG intensities (>5 × KDP or 1 × KTP). In this Account, we first provide a brief review of the development of SHG materials and concisely highlight the features of the excellent SHG materials. Then, we present four facile and rational molecular design strategies: (1) Traditional BO 3 3− -based crystals feature short absorption edges but usually suffer from relatively weak SHG performance (<5 × KDP). The combination of two types of pure π-conjugated anions (BO 3 3− and NO 3 − ) in a parallel fashion in the same compound has afforded a metal borate nitrate with a strong SHG effect. (2) To overcome the problems of the weak SHG effect and small birefringence in the less anisotropic QO 4 -based compounds, highly polarizable cations such as Hg 2+ and Bi 3+ are introduced into these systems, which greatly enhances both SHG effects and birefringence. (3) Iodate anions can be condensed into polynuclear iodate anions with a higher density of I 5+ per unit cell, hence polyiodate anions can serve as excellent SHG-active groups. We developed a novel synthesis method for hydrothermal reactions under a phosphoric acid medium and obtained a series of metal polyiodates with strong SHG effects. In addition, as the number of iodate groups increases, the structural configuration of the polyiodate anion changes from linear to bent. (4) We introduce the concept of aliovalent substitution which features site-to-site atomic displacement at the structural level. Such aliovalent substitution led to new materials that have the same chemical stoichiometries or structural features as their parent compounds. Thus, aliovalent substitution can provide more experimental opportunities and afford new high-performance SHG materials. The introduction of a fluoride anion and the replacement of metal cations in the MO 6 octahedron can result in new metal iodates with balanced properties including a large SHG effect, a wide band gap, and a high laser-induced damage threshold (LIDT) value. Finally, we briefly discuss several problems associated with the studies of SHG materials and give some prospects for SHG materials in the future.