Review of KBe2BO3F2-like second-order nonlinear optical materials and their structure-performance relationships

Nonlinear optical (NLO) materials have attracted widespread attention since they can greatly expand the spectral range of the laser output through second harmonic generation (SHG). Recently, employing advantageous structural templates to design and fabricate NLO materials has promoted their rapid development, compared with the traditional "trial-and-error" methodology. KBe2BO3F2, an outstanding usually can be as a good structure prototype for oriented exploring NLO materials. In the crystal structure of KBe2BO3F2, the it-conjugated planar triangle anionic group [BO3]3- and tetrahedra [BeO3F]5- can be replaced by other it-conjugated planar groups and tetrahedra/polyhedra to form KBe2BO3F2-like structures, respectively. After the literature survey, the it-conjugated planar groups are [BO3]3- itself or [BO2]-, [B2O5]4- , [B3O6]3- , [CO3]2- , [BeO3]4- , [BeN3]7-, [BS3]3- , [C(NH2)3]+, while the tetrahedra include [BeO3F]5- itself or [MO4]n- (M = B, P, S, Si, Li, Be, Al, Ga, Zn, Cl; n = 1, 2, 3, 4, 5, 6, 7), [SiN4]8- , [BO3T]4- (T = F, (OH)), [MO2F2]n- (M = P, Si; n = 1, 2), [MO3F]n- (M = Li, S, Y; n = 1, 4, 6), [BeO3(OH)]5-, [AlO3X]4- (X = F, Cl), [ZnO2(OH)2]4-, [ZnO3X]5- (X = (OH), F, Cl, Br), and [ZnS3Br]5-, as well as polyhedra [C3O4H2]2-, [C3H6NO2]-, [AlO3F2]5- , [MgO4F2]8- , [AIIO6]10- (AII = Mg, Ca), [LaO8]13- , [LaO9]15- , [PbO3X3]7- (X = Cl, Br, I), and [IO3]-. Accordingly, about 107 related compounds were collected: 37 borate-halides, 17 fluorooxoborates, 16 borates, 13 gamma-Be2BO3F analogues, seven hydroxyborates, four borosulfates, three borophosphates, two guanidiniums, two borosilicates, and one malonate, hydroxycarbonate, carbonate-halide, borate-iodate, sulfate, silicate. According to their relationships between crystal structures and NLO properties: (i) KBe2BO3F2-like materials that can achieve deep-ultraviolet NLO behavior all contain it-conjugated planar groups ([BO3]3-, [CO3]2-, [B3O6]3-) and fluorooxo-heteroleptic tetrahedra ([BO3F]4-, [LiO3F]6-, [SiO2F2]2-, [PO2F2]-). (ii) The combination of the it-conjugated planar groups with large polarization anisotropy and large first-order hyperpolarizability and great HOMO-LUMO energy gaps tetrahedra to design excellent NLO materials is the flexible and benign strategies. (iii) The emergence of the KBe2BO3F2-like infrared NLO structural template has expanded the study sights of infrared NLO materials. The intention is to help insights into the effective designing of novel NLO materials using KBe2BO3F2 as a structural template.