Borate-Based Ultraviolet and Deep-Ultraviolet Nonlinear Optical Crystals

Borates have long been recognized as a very important family of nonlinear optical (NLO) crystals, and have been widely used in the laser frequency-converting technology in ultraviolet (UV) and deep-ultraviolet (DUV) regions. In this work, the borate-based UV and DUV NLO crystals discovered in the recent decade are reviewed, and the structure–property relationship in the representative borate-based UV and DUV NLO crystals is analyzed. It is concluded that the optical properties of these crystals can be well explained directly from the types and spatial arrangements of B-O groups. The deduced mechanism understanding has significant implications for the exploration and design of new borate-based crystals with excellent UV and DUV NLO performance.


Introduction
In 1961, Franken and his co-workers observed the phenomenon of second-harmonic generation (SHG) in quartz [1].Since then, nonlinear optics have attracted much attention and developed into one of the key branches of modern optics.A physicochemical phenomenon often leeches onto a material as the carrier, and in turn the materials usually find a practical application, benefiting from the attached property.Without exception, nonlinear optical (NLO) crystals, one of the most important optoelectronic functional material types, are now widely used in laser wavelength-converting technology [2][3][4][5][6][7].In particular, the NLO crystals are vital to the generation of coherent light in the ultraviolet (UV, λ < 400 nm) and deep-ultraviolet region (DUV, λ < 200 nm) regions by all-solid-state lasers, owing to its great application in many scientific and engineering fields such as semiconductor photolithography, micromachining and ultraprecise photoelectron spectrometry [8][9][10][11].Therefore, the exploration of NLO crystals in UV and DUV regions has been one of the research hotspots in the field of optoelectronic functional materials for decades.
From the viewpoint of practical applications, for a good UV (and DUV) NLO crystal, the following four conditions must be taken into consideration [12].Firstly, a short absorption cutoff (λ cutoff ) or wide energy bandgap (E g ) is necessary to guarantee the transmittance in the UV and DUV spectra.Moreover, the large bandgap can significantly decrease the occurrence possibility of two-photon absorption or multi-photon absorption by strong laser radiation, and thus great increase the laser-induced damage threshold in crystal.Secondly, the NLO response should be as large as possible for high NLO conversion efficiency.The SHG coefficient (d ij ) should be larger than 1 × KDP (KH 2 PO 4 , d 36 ~0. 39× pm/V) in the UV and DUV regions.Thirdly moderate birefringence (∆n ~0.06-0.10@ 400 nm) is important for the achievement of the phase-matching condition if quasi-phase matching is an infinite [Be 2 BO 3 F 2 ] ∞ layer.The layers are connected by electrostatic interaction between potassium and fluorine ions.Herein, the KBBF-type crystals are referred to the crystals in which the terminal oxygen atoms of (BO 3 ) 3− groups are exclusively connected with two (MO 3 X) (M = Be and Zn, X = F, Cl and Br) tetrahedra in the ratio 1:2 to form the [M 2 BO 3 X 2 ] ∞ layer, and the layers are linked by weak ionic bonds.
Crystals 2017, 7, 95 3 of 16 infinite [Be2BO3F2]∞ layer.The layers are connected by electrostatic interaction between potassium and fluorine ions.Herein, the KBBF-type crystals are referred to the crystals in which the terminal oxygen atoms of (BO3) 3− groups are exclusively connected with two (MO3X) (M = Be and Zn, X = F, Cl and Br) tetrahedra in the ratio 1:2 to form the [M2BO3X2]∞ layer, and the layers are linked by weak ionic bonds.BaBe2BO3F3 (BBBF): Much effort has been devoted to overcome the layering habit of KBBF family crystals.In 2016, Guo et al. proposed the strategy to improve the growth habit by enhancing electrostatic interaction, and successfully synthesized BaBe2BO3F3 (BBBF) [30].BBBF crystallizes in hexagonal space group of P63.In one unit cell, two [Be2BO3F2]∞ layers in KBBF are aligned vertical to c-axis in a nearly antiparallel manner, and the barium cations are located between the interstice between the layers (Figure 2).At the same time, isolated fluorine anions are also introduced to compensate the valence imbalance resulting from the substitution K + with Ba 2+ ions.Based on simple Coulomb model, the electrostatic interaction between the [Be2BO3F2]∞ layer and interstitial cations of BBBF is 1.5 times that of KBBF, indicating that the layered habit is largely overcome in BBBF.Benefiting from large birefringence originating from the parallel arrangement of (BO3) 3− plane, the phase-matching ability evaluation based on the first-principles refractive indices calculations revealed that the shortest SHG wavelength is 196 nm, implying that BBBF could be truly used in the SHG in the DUV region.However, suffering from the nearly antiparallel alignment of (BO3) 3− groups, the powder SHG response of BBBF is just 0.1 × KDP.  − and (BeO3F) 5− groups are represented by pink and green polyhedra respectively.(c) Phase-matching capabilities for BBBF.Adapted and reprinted with permission from ref [30], Copyright 2016 American Chemical Society.
AZn2BO3X2 (A = K, Rb, NH4; X = Cl, Br) Much effort has been devoted to overcome the layering habit of KBBF family crystals.In 2016, Guo et al. proposed the strategy to improve the growth habit by enhancing electrostatic interaction, and successfully synthesized BaBe 2 BO 3 F 3 (BBBF) [30].BBBF crystallizes in hexagonal space group of P6 3 .In one unit cell, two [Be 2 BO 3 F 2 ] ∞ layers in KBBF are aligned vertical to c-axis in a nearly antiparallel manner, and the barium cations are located between the interstice between the layers (Figure 2).At the same time, isolated fluorine anions are also introduced to compensate the valence imbalance resulting from the substitution K + with Ba 2+ ions.Based on simple Coulomb model, the electrostatic interaction between the [Be 2 BO 3 F 2 ] ∞ layer and interstitial cations of BBBF is 1.5 times that of KBBF, indicating that the layered habit is largely overcome in BBBF.Benefiting from large birefringence originating from the parallel arrangement of (BO 3 ) 3− plane, the phase-matching ability evaluation based on the first-principles refractive indices calculations revealed that the shortest SHG wavelength is 196 nm, implying that BBBF could be truly used in the SHG in the DUV region.However, suffering from the nearly antiparallel alignment of (BO 3 ) 3− groups, the powder SHG response of BBBF is just 0.1 × KDP.

BaBe2BO3F3 (BBBF):
Much effort has been devoted to overcome the layering habit of KBBF family crystals.In 2016, Guo et al. proposed the strategy to improve the growth habit by enhancing electrostatic interaction, and successfully synthesized BaBe2BO3F3 (BBBF) [30].BBBF crystallizes in hexagonal space group of P63.In one unit cell, two [Be2BO3F2]∞ layers in KBBF are aligned vertical to c-axis in a nearly antiparallel manner, and the barium cations are located between the interstice between the layers (Figure 2).At the same time, isolated fluorine anions are also introduced to compensate the valence imbalance resulting from the substitution K + with Ba 2+ ions.Based on simple Coulomb model, the electrostatic interaction between the [Be2BO3F2]∞ layer and interstitial cations of BBBF is 1.5 times that of KBBF, indicating that the layered habit is largely overcome in BBBF.Benefiting from large birefringence originating from the parallel arrangement of (BO3) 3− plane, the phase-matching ability evaluation based on the first-principles refractive indices calculations revealed that the shortest SHG wavelength is 196 nm, implying that BBBF could be truly used in the SHG in the DUV region.However, suffering from the nearly antiparallel alignment of (BO3) 3− groups, the powder SHG response of BBBF is just 0.1 × KDP.AZn 2 BO 3 X 2 (A = K, Rb, NH 4 ; X = Cl, Br) AZn 2 BO 3 X 2 (A = K, Rb, NH 4 ; X = Cl, Br can be regarded as the production of the "transgenosis" on KBBF crystals) [31,32].In the "transgenosis" process, potassium cations are replaced by A (K + , Rb + or NH 4 + ) ions, and (BeO 3 F) 5− groups are replaced by (ZnO 3 X) 5− tetrahedra (Figure 3).Due to the introduction of (ZnO 3 X) 5− tetrahedra, the shortest absorption edge of AZn 2 BO 3 X 2 are red-shifted to 190-214 nm.For the same reason, "transgenosis" process doubles the SHG effect of AZn 2 BO 3 X 2 compared with KBBF, and the mechanism analysis revealed that (ZnO 3 X) 5− tetrahedra dominantly contributes to the SHG effect in the series of materials.This is the first case where [ZnO 3 Cl/Br] groups can be used as the NLO-active structural units in NLO materials.More importantly, in NH 4 Zn 2 BO 3 X 2 , the hydrogen bonds between ammonia and halogen ions largely reduce the layered habit and improve the growth habit.AZn2BO3X2 (A = K, Rb, NH4; X = Cl, Br can be regarded as the production of the "transgenosis" on KBBF crystals) [31,32].In the "transgenosis" process, potassium cations are replaced by A (K + , Rb + or NH4 + ) ions, and (BeO3F) 5− groups are replaced by (ZnO3X) 5− tetrahedra (Figure 3).Due to the introduction of (ZnO3X) 5− tetrahedra, the shortest absorption edge of AZn2BO3X2 are red-shifted to 190-214 nm.For the same reason, "transgenosis" process doubles the SHG effect of AZn2BO3X2 compared with KBBF, and the mechanism analysis revealed that (ZnO3X) 5− tetrahedra dominantly contributes to the SHG effect in the series of materials.This is the first case where [ZnO3Cl/Br] groups can be used as the NLO-active structural units in NLO materials.More importantly, in NH4Zn2BO3X2, the hydrogen bonds between ammonia and halogen ions largely reduce the layered habit and improve the growth habit.[32], Copyright 2016 American Chemical Society.

SBBO-Type Crystals
Sr2Be2B2O7 (SBBO) was designed by molecule engineering method in 1995 [33,34].In the crystal structure of SBBO (Figure 4), the terminal oxygen atoms of (BO3) 3− group are bonded with one beryllium atoms, forming a [BeBO3O]∞ single layer.Two [BeBO3O]∞ single layers connect each other via the bridge oxygen atoms, forming the [Be2B2O7]∞ double layers, and the strontium cations are intercalated in the interstices within and between the [Be2B2O7]∞ double layers.Herein the SBBO-type crystals are referred to the crystals with [M2B2O6X]∞-type double layers, and the A-site cations are embedded in the interspace inter-or intra-double layers.The measurement based on powder samples revealed that SBBO has the potential for DUV NLO crystals.However, the problem of structure instability has been always an issue for SBBO, which severely hinders its practical application [33,35,36].

NaCaBe2B2O6F
NaCaBe2B2O6F crystallizes in the monoclinic space group Cc, exhibiting an two-dimensional [Be2B2O6F]∞ double-layer structure, with bridge oxygen in the double layer of SBBO substituted by Adapted and reprinted with permission from ref [32], Copyright 2016 American Chemical Society.

SBBO-Type Crystals
Sr 2 Be 2 B 2 O 7 (SBBO) was designed by molecule engineering method in 1995 [33,34].In the crystal structure of SBBO (Figure 4), the terminal oxygen atoms of (BO 3 ) 3− group are bonded with one beryllium atoms, forming a [BeBO 3 O] ∞ single layer.Two [BeBO 3 O] ∞ single layers connect each other via the bridge oxygen atoms, forming the [Be 2 B 2 O 7 ] ∞ double layers, and the strontium cations are intercalated in the interstices within and between the [Be 2 B 2 O 7 ] ∞ double layers.Herein the SBBO-type crystals are referred to the crystals with [M 2 B 2 O 6 X] ∞ -type double layers, and the A-site cations are embedded in the interspace inter-or intra-double layers.The measurement based on powder samples revealed that SBBO has the potential for DUV NLO crystals.However, the problem of structure instability has been always an issue for SBBO, which severely hinders its practical application [33,35,36].AZn2BO3X2 (A = K, Rb, NH4; X = Cl, Br can be regarded as the production of the "transgenosis" on KBBF crystals) [31,32].In the "transgenosis" process, potassium cations are replaced by A (K + , Rb + or NH4 + ) ions, and (BeO3F) 5− groups are replaced by (ZnO3X) 5− tetrahedra (Figure 3).Due to the introduction of (ZnO3X) 5− tetrahedra, the shortest absorption edge of AZn2BO3X2 are red-shifted to 190-214 nm.For the same reason, "transgenosis" process doubles the SHG effect of AZn2BO3X2 compared with KBBF, and the mechanism analysis revealed that (ZnO3X) 5− tetrahedra dominantly contributes to the SHG effect in the series of materials.This is the first case where [ZnO3Cl/Br] groups can be used as the NLO-active structural units in NLO materials.More importantly, in NH4Zn2BO3X2, the hydrogen bonds between ammonia and halogen ions largely reduce the layered habit and improve the growth habit.[32], Copyright 2016 American Chemical Society.

SBBO-Type Crystals
Sr2Be2B2O7 (SBBO) was designed by molecule engineering method in 1995 [33,34].In the crystal structure of SBBO (Figure 4), the terminal oxygen atoms of (BO3) 3− group are bonded with one beryllium atoms, forming a [BeBO3O]∞ single layer.Two [BeBO3O]∞ single layers connect each other via the bridge oxygen atoms, forming the [Be2B2O7]∞ double layers, and the strontium cations are intercalated in the interstices within and between the [Be2B2O7]∞ double layers.Herein the SBBO-type crystals are referred to the crystals with [M2B2O6X]∞-type double layers, and the A-site cations are embedded in the interspace inter-or intra-double layers.The measurement based on powder samples revealed that SBBO has the potential for DUV NLO crystals.However, the problem of structure instability has been always an issue for SBBO, which severely hinders its practical application [33,35,36].

NaCaBe2B2O6F
NaCaBe2B2O6F crystallizes in the monoclinic space group Cc, exhibiting an two-dimensional [Be2B2O6F]∞ double-layer structure, with bridge oxygen in the double layer of SBBO substituted by  F] ∞ double-layer structure, with bridge oxygen in the double layer of SBBO substituted by fluorine atoms [37].The sodium ions are intercalated in the interstice within [Be 2 B 2 O 6 F] ∞ double layer, while calcium ions are embedded in the cavity between the double layers (Figure 5).Transmittance test revealed that the absorption edge of NaCaBe 2 B 2 O 6 F is 185 nm, resulting from the incomplete elimination of dangling bonds at oxygen atoms.In the lattice, the (BO 3 ) 3− groups are aligned in nearly antiparallel orientation, and the geometrical offset of microscopic second-order susceptibility results in a small powder SHG effect about one third that of KDP.
Crystals 2017, 7, 95 5 of 16 fluorine atoms [37].The sodium ions are intercalated in the interstice within [Be2B2O6F]∞ double layer, while calcium ions are embedded in the cavity between the double layers (Figure 5).Transmittance test revealed that the absorption edge of NaCaBe2B2O6F is 185 nm, resulting from the incomplete elimination of dangling bonds at oxygen atoms.In the lattice, the (BO3) 3− groups are aligned in nearly antiparallel orientation, and the geometrical offset of microscopic second-order susceptibility results in a small powder SHG effect about one third that of KDP.

K3Ba3Li2Al4B6O20F
K3Ba3Li2Al4B6O20F crystallizes in hexagonal space group P-62c.Its exhibits the most complicated [Li2Al4B6O20F]∞ double-layer structure (Figure 6) in the SBBO-type crystals [38].[Li2Al4B6O20F]∞ double layers are generated by substituting one third of (BeO4) 6− with (LiO3F) 6− tetrahedra and two thirds of (BeO4) 6− with (AlO4) 5− tetrahedra.Regardless of the complicated crystal structure, it keeps the similar spatial arrangement of (BO3) 3− groups in SBBO, and exhibits a strong SHG effect (1.5 × KDP).Due to the introduction of lithium with low electronegativity and valence in [Li2Al4B6O20F]∞ layers, the nonbonding orbitals of some oxygen atoms are almost reserved, and the absorption edge of K3Ba3Li2Al4B6O20F is located at 190 nm.At the same time, the element substitution preserve the strong interaction between [Li2Al4B6O20F]∞ double layers, and K3Ba3Li2Al4B6O20F shows no layered habit in the crystal growth process.More importantly, it should be emphasized that the insertion of lithium largely improves the flexibility of [Li2Al4B6O20F]∞ double layers, making it accommodate large K + ions.Therefore, the problem of structural instability of SBBO is completely eliminated in K3Ba3Li2Al4B6O20F, as demonstrated by the phonon spectrum (Figure 6b).

Rb3Al3B3O10F
Rb3Al3B3O10F crystallizes in the trigonal space group of P31c, and possess a three-dimensional structure (Figure 7，ref [39]).Each (BO3) 3− is linked with one (AlO3F) 4− and two (AlO4) 5− tetrahedra, forming the alveolate [Al3(BO3)3OF]∞ layer in the (a,b) plane.The apical O/F atoms of (AlO4/AlO3F) tetrahedra pointing upward and download are shared by the adjacent layer to serve as the interlayer bridge, constructing the three-dimensional framework.Rubidium cations reside in the interstice of   fluorine atoms [37].The sodium ions are intercalated in the interstice within [Be2B2O6F]∞ double layer, while calcium ions are embedded in the cavity between the double layers (Figure 5).Transmittance test revealed that the absorption edge of NaCaBe2B2O6F is 185 nm, resulting from the incomplete elimination of dangling bonds at oxygen atoms.In the lattice, the (BO3) 3− groups are aligned in nearly antiparallel orientation, and the geometrical offset of microscopic second-order susceptibility results in a small powder SHG effect about one third that of KDP.

K3Ba3Li2Al4B6O20F
K3Ba3Li2Al4B6O20F crystallizes in hexagonal space group P-62c.Its exhibits the most complicated [Li2Al4B6O20F]∞ double-layer structure (Figure 6) in the SBBO-type crystals [38].[Li2Al4B6O20F]∞ double layers are generated by substituting one third of (BeO4) 6− with (LiO3F) 6− tetrahedra and two thirds of (BeO4) 6− with (AlO4) 5− tetrahedra.Regardless of the complicated crystal structure, it keeps the similar spatial arrangement of (BO3) 3− groups in SBBO, and exhibits a strong SHG effect (1.5 × KDP).Due to the introduction of lithium with low electronegativity and valence in [Li2Al4B6O20F]∞ layers, the nonbonding orbitals of some oxygen atoms are almost reserved, and the absorption edge of K3Ba3Li2Al4B6O20F is located at 190 nm.At the same time, the element substitution preserve the strong interaction between [Li2Al4B6O20F]∞ double layers, and K3Ba3Li2Al4B6O20F shows no layered habit in the crystal growth process.More importantly, it should be emphasized that the insertion of lithium largely improves the flexibility of [Li2Al4B6O20F]∞ double layers, making it accommodate large K + ions.Therefore, the problem of structural instability of SBBO is completely eliminated in K3Ba3Li2Al4B6O20F, as demonstrated by the phonon spectrum (Figure 6b).

Rb3Al3B3O10F
Rb3Al3B3O10F crystallizes in the trigonal space group of P31c, and possess a three-dimensional structure (Figure 7，ref [39]).Each (BO3) 3− is linked with one (AlO3F) 4− and two (AlO4) 5   In this type of borate crystals, the (BO3) 3− groups are connected with other groups to form the 3D networks.

NaBeB3O6
NaBeB3O6 crystallizes in orthorhombic space group Pna21 [42].By sharing the corner oxygen atoms, three (BO3) 3− triangles and two (BeO4) 6− tetrahedra form nearly coplanar double six-membered ring (Be2B3O11) 9− anions, similar to naphthalene.The (Be2B3O11) 9− building blocks are connected with each other in a crossed manner by sharing the (BeO4) 6− , forming the endless cruciate chains along the c-axis.The adjacent cruciate chains are connected to each other via sharing the O atoms of (BO3) 3− triangles generating the open framework with Na + ions residing in the tunnels (Figure 10).UV−Vis−NIR diffuse reflectance spectrum revealed that NaBeB3O6 exhibits good transmittance below 200 nm.The (Be2B3O11) 9− triangles result the very condensed spatial arrangement of (BO3) 3− groups, leading to the large SHG response (1.6 × KDP).More importantly, it should be noted that the collinear alignment of NaBeB3O6 also gives rise to the large birefringence (0.08@400 nm, calculated values [14]), which is the first case in borate-based NLO crystals.NaSr3Be3B3O9F4 crystallizes in trigonal space group R3m.By sharing the fluorine ions, three (BeO3F) 5− tetrahedra constituted a (Be3O12F) 19− cluster (Figure 9, ref [41]).The oxygen atoms of (Be3O12F) 19− cluster are connected by three (BO3) 3− groups, forming (Be3B3O12F) 10− building block.In the (Be3B3O12F) 10− building block, the (BO3) 3− are almost parallel with each other, except the slight deviation from (a,b) plane.The (Be3B3O12F) 10− building blocks are interconnected in almost the same orientation, generating the 3D network.The Na + and Sr 2+ are located in the cavity of the 3D network.The absorption edge of NaSr3Be3B3O9F4 is 170nm, originating from single-coordination of oxygen atoms of (BO3) 3− groups with beryllium atoms.The subparallel geometry of (BO3) 3− groups in the lattice leads to a large SHG response (4~5 × KDP) and birefringence (0.061@400 nm), showing potential for UV NLO crystals for the generation of coherent laser at 266 nm as with LiSr(BO3)2.

CsZn2B3O7
CsZn2B3O7 crystallizes in orthorhombic space group Cmc21 [45,46].Its structure can be regarded as the variant of γ-KBe2B3O7 [42] with beryllium and potassium substituted by zinc and cesium atoms (Figure 12).Due the introduction of zinc atoms with d-orbital, the absorption edge of CsZn2B3O7 is red-shifted to 218 nm.Moreover, it also should be noted that (ZnO4) 6− also has a considerable contribution to the SHG effect and enhances the SHG effect to 1.5 × KDP (that of γ-KBe2B3O7 is 0.68 KDP).

Borate Crystals in Which the B-O Groups Are (BO4) 5− Groups Only
The most famous NLO borates in which the B-O groups are (BO4) 5− groups are BPO4 [47][48][49][50] and SrB4O7 [51-54].Both of them have very large energy bandgap (Eg ~ 9.5 eV) or extremely short absorption edge (λcutoff ~ 130 nm).However, due to very small optical anisotropy in (BO4) 5− groups, this type of UV NLO borates has too small a birefringence (typically ∆n < 0.01) to achieve the phase-matching condition, and has been less focused on in the recent decade.To our best knowledge, the only known compound in this type of borates is LaBeB3O7, which was synthesized in  [45,46].Its structure can be regarded as the variant of γ-KBe 2 B 3 O 7 [42] with beryllium and potassium substituted by zinc and cesium atoms (Figure 12).Due the introduction of zinc atoms with d-orbital, the absorption edge of CsZn 2 B 3 O 7 is red-shifted to 218 nm.Moreover, it also should be noted that (ZnO 4 ) 6− also has a considerable contribution to the SHG effect and enhances the SHG effect to 1.5 × KDP (that of γ-KBe 2 B 3 O 7 is 0.68 KDP).

CsZn2B3O7
CsZn2B3O7 crystallizes in orthorhombic space group Cmc21 [45,46].Its structure can be regarded as the variant of γ-KBe2B3O7 [42] with beryllium and potassium substituted by zinc and cesium atoms (Figure 12).Due the introduction of zinc atoms with d-orbital, the absorption edge of CsZn2B3O7 is red-shifted to 218 nm.Moreover, it also should be noted that (ZnO4) 6− also has a considerable contribution to the SHG effect and enhances the SHG effect to 1.5 × KDP (that of γ-KBe2B3O7 is 0.68 KDP).The most famous NLO borates in which the B-O groups are (BO 4 ) 5− groups are BPO 4 [47][48][49][50] and SrB 4 O 7 [51][52][53][54].Both of them have very large energy bandgap (E g ~9.5 eV) or extremely short absorption edge (λ cutoff ~130 nm).However, due to very small optical anisotropy in (BO 4 ) 5− groups, this type of UV NLO borates has too small a birefringence (typically ∆n < 0.01) to achieve the phase-matching condition, and has been less focused on in the recent decade.To our best knowledge, the only known compound in this type of borates is LaBeB 3 O 7 , which was synthesized in 2013.
LaBeB 3 O 7 LaBeB 3 O 7 crystallizes in the orthorhombic space group Pnm2 1 [55].Two symmetrically independent B sites exist in the lattice.The B(1) sites are fully occupied by boron atoms, while B(2) sites are disordered occupied by boron and beryllium atoms in the molar ratio of 1:1.Via sharing the corner oxygen atoms, two (B(1)O 4 ) 5− tetrahedra form an (B 2 O 7 ) 9− dimmer.(XO 4 ) (X = B and Be) tetrahedra are connected by the (B 2 O 7 ) 9− dimmer, and give rise to the three-dimensional network with lanthanum cations located at the channel of the framework (Figure 13).The anionic tetrahedra are aligned almost in the same direction.Regardless of the small second-order susceptibility of (BO 4 ) 5− groups, the favorable superposition also results in the relatively large SHG response (1 ~2 × KDP).Moreover, the powder SHG test revealed that LaBeB 3 O 7 is phase-matching for the light at 1064 nm.It is speculated that the Be/B disorder occupation brings about a relatively large birefringence (0.03@1634 nm) and improves the phase-matching ability.
Crystals 2017, 7, 95 9 of 16 LaBeB3O7 crystallizes in the orthorhombic space group Pnm21 [55].Two symmetrically independent B sites exist in the lattice.The B(1) sites are fully occupied by boron atoms, while B(2) sites are disordered occupied by boron and beryllium atoms in the molar ratio of 1:1.Via sharing the corner oxygen atoms, two (B(1)O4) 5− tetrahedra form an (B2O7) 9− dimmer.(XO4) (X = B and Be) tetrahedra are connected by the (B2O7) 9− dimmer, and give rise to the three-dimensional network with lanthanum cations located at the channel of the framework (Figure 13).The anionic tetrahedra are aligned almost in the same direction.Regardless of the small second-order susceptibility of (BO4) 5− groups, the favorable superposition also results in the relatively large SHG response (1 ~ 2 × KDP).Moreover, the powder SHG test revealed that LaBeB3O7 is phase-matching for the light at 1064 nm.It is speculated that the Be/B disorder occupation brings about a relatively large birefringence (0.03@1634 nm) and improves the phase-matching ability.

Borate Crystals Containing the B-O Combinational Groups
In some UV and DUV NLO borates synthesized in the last decade, the B-O groups are not merely (BO3) 3− groups or (BO4) 5− groups, but the combination of these two types of basic B-O groups.In this section, several representative NLO compounds in this type of borates are selected.The combinational B-O groups can also result in the good NLO properties of borate crystals in the UV and even DUV regions.
K3B6O10Br and K3B6O10Cl with (B6O13) 8− K3B6O10Br and K3B6O10Cl crystallize in space groups of trigonal R3m, and the crystals with different halogen anions are isostructural with each other [56,57].By sharing the common (BO4) 5− , their (B3O8) 7− groups are interconnected with each other, forming the (B6O13) 8− building block.(B6O13) 8− groups are further connected, generating the 3D framework, with potassium and halogen ions located in the interstice of the framework (Figure 14).Except some slant off the (a,b) plane, almost all the (BO3) 3− groups are aligned in the same orientation.The favorable superposition of second-order susceptibility and optical anisotropic bring about the large SHG response and birefringence [58][59][60].Moreover, the partial elimination of dangling bonds at oxygen atoms result in the good transmittance in DUV region with the absorption 180 nm [60] and 182 nm [58] for K3B6O10Cl and K3B6O10Br respectively.All these properties indicate that K3B6O10X could be a good candidate for second-and third-harmonic generation for laser at 1064 nm [61,62].

Borate Crystals Containing the B-O Combinational Groups
In some UV and DUV NLO borates synthesized in the last decade, the B-O groups are not merely (BO 3 ) 3− groups or (BO 4 ) 5− groups, but the combination of these two types of basic B-O groups.In this section, several representative NLO compounds in this type of borates are selected.The combinational B-O groups can also result in the good NLO properties of borate crystals in the UV and even DUV regions.[56,57].By sharing the common (BO 4 ) 5− , their (B 3 O 8 ) 7− groups are interconnected with each other, forming the (B 6 O 13 ) 8− building block.(B 6 O 13 ) 8− groups are further connected, generating the 3D framework, with potassium and halogen ions located in the interstice of the framework (Figure 14).Except some slant off the (a,b) plane, almost all the (BO 3 ) 3− groups are aligned in the same orientation.The favorable superposition of second-order susceptibility and optical anisotropic bring about the large SHG response and birefringence [58][59][60].Moreover, the partial elimination of dangling bonds at oxygen atoms result in the good transmittance in DUV region with the absorption 180 nm [60] and 182 nm [58] for K 3 B 6 O 10 Cl and K 3 B 6 O 10 Br respectively.All these properties indicate that K 3 B 6 O 10 X could be a good candidate for second-and third-harmonic generation for laser at 1064 nm [61,62].1.5 × KDP [46] (3.3 × KDP) [45] 0.056(cal) [46] Borate crystals in which the B-O groups are (BO 4 ) 5− groups only LaBeB 3 O 7 [55] Pnm2 sites are disordered occupied by boron and beryllium atoms in the molar ratio of 1:1.Via sharing the corner oxygen atoms, two (B(1)O4) 5− tetrahedra form an (B2O7) 9− dimmer.(XO4) (X = B and Be) tetrahedra are connected by the (B2O7) 9− dimmer, and give rise to the three-dimensional network with lanthanum cations located at the channel of the framework (Figure 13).The anionic tetrahedra are aligned almost in the same direction.Regardless of the small second-order susceptibility of (BO4) 5− groups, the favorable superposition also results in the relatively large SHG response (1 ~ 2 × KDP).Moreover, the powder SHG test revealed that LaBeB3O7 is phase-matching for the light at 1064 nm.It is speculated that the Be/B disorder occupation brings about a relatively large birefringence (0.03@1634 nm) and improves the phase-matching ability.

Borate Crystals Containing the B-O Combinational Groups
In some UV and DUV NLO borates synthesized in the last decade, the B-O groups are not merely (BO3) 3− groups or (BO4) 5− groups, but the combination of these two types of basic B-O groups.In this section, several representative NLO compounds in this type of borates are selected.The combinational B-O groups can also result in the good NLO properties of borate crystals in the UV and even DUV regions.
K3B6O10Br and K3B6O10Cl with (B6O13) 8− K3B6O10Br and K3B6O10Cl crystallize in space groups of trigonal R3m, and the crystals with different halogen anions are isostructural with each other [56,57].By sharing the common (BO4) 5− , their (B3O8) 7− groups are interconnected with each other, forming the (B6O13) 8− building block.(B6O13) 8− groups are further connected, generating the 3D framework, with potassium and halogen ions located in the interstice of the framework (Figure 14).Except some slant off the (a,b) plane, almost all the (BO3) 3− groups are aligned in the same orientation.The favorable superposition of second-order susceptibility and optical anisotropic bring about the large SHG response and birefringence [58][59][60].Moreover, the partial elimination of dangling bonds at oxygen atoms result in the good transmittance in DUV region with the absorption 180 nm [60] and 182 nm [58] for K3B6O10Cl and K3B6O10Br respectively.All these properties indicate that K3B6O10X could be a good candidate for second-and third-harmonic generation for laser at 1064 nm [61,62].[63,64].Two (B 3 O 8 ) 3− are linked by sharing the oxygen of (BO 4 ) 5− to form the (B 6 O 14 ) 10− building block.The (B 6 O 14 ) 10− groups are further interconnected to generate the 3D framework with Ba 2+ (or Sr 2+ ) and F + residing in the nine-number tunnels in the framework (Figure 15).Even if some rotation occurs between the (B 6 O 14 ) 10− groups, (BO 3 ) 3− , the basic unit mostly contributing to SHG effect, is almost in the same spatial orientation.Therefore, both crystals exhibit large SHG response (3 and 2.5 times that of KDP for Ba Ba3B6O11F2 and Sr3B6O11F2 with (B6O14) 10− Ba3B6O11F2 and Sr3B6O11F2 are isomorphism and crystallize in monoclinic space group P21 [63,64].Two (B3O8) 3− are linked by sharing the oxygen of (BO4) 5− to form the (B6O14) 10− building block.The (B6O14) 10− groups are further interconnected to generate the 3D framework with Ba 2+ (or Sr 2+ ) and F + residing in the nine-number tunnels in the framework (Figure 15).Even if some rotation occurs between the (B6O14) 10− groups, (BO3) 3− , the basic unit mostly contributing to SHG effect, is almost in the same spatial orientation.Therefore, both crystals exhibit large SHG response (3 and 2.5 times that of KDP for Ba3B6O11F2 and Sr3B6O11F2, respectively).Besides, the formation of B-O bond between the oxygen atoms at (BO3) 3− group and four-coordinated boron atoms result in the absorption edge below 190 nm.16).The rare earth cations (Y 3+ or Gd 3+ ) reside in the interspace between the layers (Figure 16).Due to the absence of d-d or f-f electronic transition in Y 3+ and Gd 3+ cations, the absorption edges of ReBe2B5O11 are all below 200 nm.Powder SHG measurement showed that their SHG efficiencies are comparable with KDP.The theoretical analysis-based first-principles calculation elucidated that the large SHG response of ReBe2B5O11 mainly originated from the relatively ordered spatial arrangement of (BO3) 3− group.

The Borates Containing B-O Groups and Other NLO Active Groups
Commonly, for the UV and DUV borate-based crystals the NLO properties mainly originate from the contribution of B-O groups.However, there exist some borates in which other types of microscopic groups also make considerable contributions to the NLO effect apart from the B-O groups.In this section, we select Ba3(ZnB5O10)PO4 and Cs2B4SiO9 as the representative examples.16).The rare earth cations (Y 3+ or Gd 3+ ) reside in the interspace between the layers (Figure 16).Due to the absence of d-d or f -f electronic transition in Y 3+ and Gd 3+ cations, the absorption edges of ReBe 2 B 5 O 11 are all below 200 nm.Powder SHG measurement showed that their SHG efficiencies are comparable with KDP.The theoretical analysis-based first-principles calculation elucidated that the large SHG response of ReBe 2 B 5 O 11 mainly originated from the relatively ordered spatial arrangement of (BO 3 ) 3− group.Ba3B6O11F2 and Sr3B6O11F2 with (B6O14) 10−   Ba3B6O11F2 and Sr3B6O11F2 are isomorphism and crystallize in monoclinic space group P21 [63,64].Two (B3O8) 3− are linked by sharing the oxygen of (BO4) 5− to form the (B6O14) 10− building block.The (B6O14) 10− groups are further interconnected to generate the 3D framework with Ba 2+ (or Sr 2+ ) and F + residing in the nine-number tunnels in the framework (Figure 15).Even if some rotation occurs between the (B6O14) 10− groups, (BO3) 3− , the basic unit mostly contributing to SHG effect, is almost in the same spatial orientation.Therefore, both crystals exhibit large SHG response (3 and 2.5 times that of KDP for Ba3B6O11F2 and Sr3B6O11F2, respectively).Besides, the formation of B-O bond between the oxygen atoms at (BO3) 3− group and four-coordinated boron atoms result in the absorption edge below 190 nm.16).The rare earth cations (Y 3+ or Gd 3+ ) reside in the interspace between the layers (Figure 16).Due to the absence of d-d or f-f electronic transition in Y 3+ and Gd 3+ cations, the absorption edges of ReBe2B5O11 are all below 200 nm.Powder SHG measurement showed that their SHG efficiencies are comparable with KDP.The theoretical analysis-based first-principles calculation elucidated that the large SHG response of ReBe2B5O11 mainly originated from the relatively ordered spatial arrangement of (BO3) 3− group.

The Borates Containing B-O Groups and Other NLO Active Groups
Commonly, for the UV and DUV borate-based crystals the NLO properties mainly originate from the contribution of B-O groups.However, there exist some borates in which other types of microscopic groups also make considerable contributions to the NLO effect apart from the B-O groups.In this section, we select Ba3(ZnB5O10)PO4 and Cs2B4SiO9 as the representative examples.

The Borates Containing B-O Groups and Other NLO Active Groups
Commonly, for the UV and DUV borate-based crystals the NLO properties mainly originate from the contribution of B-O groups.However, there exist some borates in which other types of microscopic new borate-based crystals with excellent UV NLO performance.Last, but not least, it should be emphasized that the NLO properties presented in this review are mainly based on powder samples, and re-determination by large-sized crystals is necessary in order to more accurately measure their optical performances, including birefringence values and SHG coefficients.Therefore, the crystal growth for large size crystals with high optical quality is vital for eventually evaluating the practical application prospect of NLO materials.This should also be an important research task for the practical applications of UV and DUV NLO borate in the future.
]∞ layer.The layers are connected by electrostatic interaction between potassium and fluorine ions.Herein, the KBBF-type crystals are referred to the crystals in which the terminal oxygen atoms of (BO3) 3− groups are exclusively connected with two (MO3X) (M = Be and Zn, X = F, Cl and Br) tetrahedra in the ratio 1:2 to form the [M2BO3X2]∞ layer, and the layers are linked by weak ionic bonds.

Figure 5 .
Figure 5. (a) The structure of NaCaBe2B2O6F, (b) double-layer structure viewed along the c-axis.

Figure 5 .
Figure 5. (a) The structure of NaCaBe 2 B 2 O 6 F, (b) double-layer structure viewed along the c-axis.

K 3
Ba 3 Li 2 Al 4 B 6 O 20 F K 3 Ba 3 Li 2 Al 4 B 6 O 20 F crystallizes in hexagonal space group P-62c.Its exhibits the most complicated [Li 2 Al 4 B 6 O 20 F] ∞ double-layer structure (Figure 6) in the SBBO-type crystals[38].[Li 2 Al 4 B 6 O 20 F] ∞ double layers are generated by substituting one third of (BeO 4 ) 6− with (LiO 3 F) 6− tetrahedra and two thirds of (BeO 4 ) 6− with (AlO 4 ) 5− tetrahedra.Regardless of the complicated crystal structure, it keeps the similar spatial arrangement of (BO 3 ) 3− groups in SBBO, and exhibits a strong SHG effect (1.5 × KDP).Due to the introduction of lithium with low electronegativity and valence in [Li 2 Al 4 B 6 O 20 F] ∞ layers, the nonbonding orbitals of some oxygen atoms are almost reserved, and the absorption edge of K 3 Ba 3 Li 2 Al 4 B 6 O 20 F is located at 190 nm.At the same time, the element substitution preserve the strong interaction between [Li 2 Al 4 B 6 O 20 F] ∞ double layers, and K 3 Ba 3 Li 2 Al 4 B 6 O 20 F shows no layered habit in the crystal growth process.More importantly, it should be emphasized that the insertion of lithium largely improves the flexibility of [Li 2 Al 4 B 6 O 20 F] ∞ double layers, making it accommodate large K + ions.Therefore, the problem of structural instability of SBBO is completely eliminated in K 3 Ba 3 Li 2 Al 4 B 6 O 20 F, as demonstrated by the phonon spectrum (Figure 6b).

Figure 5 .
Figure 5. (a) The structure of NaCaBe2B2O6F, (b) double-layer structure viewed along the c-axis.
− tetrahedra, forming the alveolate [Al3(BO3)3OF]∞ layer in the (a,b) plane.The apical O/F atoms of (AlO4/AlO3F) tetrahedra pointing upward and download are shared by the adjacent layer to serve as the interlayer bridge, constructing the three-dimensional framework.Rubidium cations reside in the interstice of
186 nm.The (BO3) 3− groups within the [SrBO3]∞ layer are aligned in a nearly parallel manner, while the outside (SrBO3)∞ layers are almost arranged in an antiparallel configuration.Therefore, the "effective" (BO3) 3− groups are those in the [SrBO3]∞ layer, and high spatial density of those (BO3)3−  groups results in the large SHG response (2 × KDP) of LiSr(BO3)2.Meanwhile, the ordered arrangement of (BO3) 3− groups within the [SrBO3]∞ layer also give rise to the large birefringence (0.056, calculated value), making LiSr(BO3)2 a potential NLO crystal for the output of the 266-nm coherent laser.

2
Be 4 B 4 O 11 and LiNa 5 Be 12 B 12 O 33 are chosen as the representative examples.Na 2 Be 4 B 4 O 11 and LiNa 5 Be 12 B 12 O 33 crystallize in triclinic space groups P1 and Pc, respectively.Regardless of the different macroscopic symmetry, Na 2 Be 4 B 4 O 11 and LiNa 5 Be 12 B 12 O 33 share the similar 3D framework structure.The (BO 3 ) 3− group shares each oxygen atom with two (BeO 4 ) 6− tetrahedra, forming the infinite two-dimensional [Be 2 BO 5 ] ∞ layers.The adjacent [Be 2 BO 5 ] ∞ layers are further bridged together through distorted (B 2 O 5 ) 4− groups by sharing O atoms to build the 3D framework, with the sodium or lithium cations residing in the tunnel of the framework (Figure 11).Despite the absolute elimination of the dangling bonds of the oxygen atoms within the [Be 2 BO 5 ] ∞ layer, the terminal oxygen atoms of (B 2 O 5 ) 4− are only bonded with one beryllium atoms, leaving some nonbonding orbitals.Therefore, Na 2 Be 4 B 4 O 11 and LiNa 5 Be 12 B 12 O 33 exhibit the absorption edge at 171 and 169 nm respectively.The (BO 3 ) 3− groups within (B 2 O 5 ) 4− groups are almost aligned head-to-head in a crossing manner, and they contribute almost nothing to the SHG effect and birefringence.However, on the other hand, the connection of [Be 2 BO 5 ] ∞ layers via (B 2 O 5 ) 4 groups largely preserve high density of the effective (BO 3 ) 3− group for SHG effect and birefringence.Therefore, both Na 2 Be 4 B 4 O 11 and LiNa 5 Be 12 B 12 O 33 show high SHG efficiency (1.3 and 1.4 times that of KDP, respectively) and phase-matching ability.Crystals 2017, 7, 95 8 of 16 tetrahedra, forming the infinite two-dimensional [Be2BO5]∞ layers.The adjacent [Be2BO5]∞ layers are further bridged together through distorted (B2O5) 4− groups by sharing O atoms to build the 3D framework, with the sodium or lithium cations residing in the tunnel of the framework (Figure 11).Despite the absolute elimination of the dangling bonds of the oxygen atoms within the [Be2BO5]∞ layer, the terminal oxygen atoms of (B2O5) 4− are only bonded with one beryllium atoms, leaving some nonbonding orbitals.Therefore, Na2Be4B4O11 and LiNa5Be12B12O33 exhibit the absorption edge at 171 and 169 nm respectively.The (BO3) 3− groups within (B2O5) 4− groups are almost aligned head-to-head in a crossing manner, and they contribute almost nothing to the SHG effect and birefringence.However, on the other hand, the connection of [Be2BO5]∞ layers via (B2O5) 4 groups largely preserve high density of the effective (BO3) 3− group for SHG effect and birefringence.Therefore, both Na2Be4B4O11 and LiNa5Be12B12O33 show high SHG efficiency (1.3 and 1.4 times that of KDP, respectively) and phase-matching ability.

Figure 11 .
Figure 11.(a) the structures of Na 2 Be 4 B 4 O 11 , (b) the 2D infinite planar [Be 2 BO 5 ] ∞ layer.CsZn 2 B 3 O 7 CsZn 2 B 3 O 7 crystallizes in orthorhombic space group Cmc2 1[45,46].Its structure can be regarded as the variant of γ-KBe 2 B 3 O 7[42] with beryllium and potassium substituted by zinc and cesium atoms (Figure12).Due the introduction of zinc atoms with d-orbital, the absorption edge of CsZn 2 B 3 O 7 is red-shifted to 218 nm.Moreover, it also should be noted that (ZnO 4 ) 6− also has a considerable contribution to the SHG effect and enhances the SHG effect to 1.5 × KDP (that of γ-KBe 2 B 3 O 7 is 0.68 KDP).
the infinite two-dimensional [Be2BO5]∞ layers.The adjacent [Be2BO5]∞ layers are further bridged together through distorted (B2O5) 4− groups by sharing O atoms to build the 3D framework, with the sodium or lithium cations residing in the tunnel of the framework (Figure11).Despite the absolute elimination of the dangling bonds of the oxygen atoms within the [Be2BO5]∞ layer, the terminal oxygen atoms of (B2O5) 4− are only bonded with one beryllium atoms, leaving some nonbonding orbitals.Therefore, Na2Be4B4O11 and LiNa5Be12B12O33 exhibit the absorption edge at 171 and 169 nm respectively.The (BO3) 3− groups within (B2O5) 4− groups are almost aligned head-to-head in a crossing manner, and they contribute almost nothing to the SHG effect and birefringence.However, on the other hand, the connection of [Be2BO5]∞ layers via (B2O5)4 groups largely preserve high density of the effective (BO3) 3− group for SHG effect and birefringence.Therefore, both Na2Be4B4O11 and LiNa5Be12B12O33 show high SHG efficiency (1.3 and 1.4 times that of KDP, respectively) and phase-matching ability.

Figure 14 .
Figure 14.The structure of the K 3 B 6 O 10 Br and the (B 6 O 13 ) 8− group.
3 B 6 O 11 F 2 and Sr 3 B 6 O 11 F 2 , respectively).Besides, the formation of B-O bond between the oxygen atoms at (BO 3 ) 3− group and four-coordinated boron atoms result in the absorption edge below 190 nm.Crystals 2017, 7, 95 10 of 16

Figure 15 .
Figure 15.The structure of Ba3B6O11F2 and the (B6O14) 10− group.The (BO4) 5− group is represented by green tetrahedra and the (BO3) 3− group is represented by the pink triangle.

Figure 15 .
Figure 15.The structure of Ba 3 B 6 O 11 F 2 and the (B 6 O 14 ) 10− group.The (BO 4 ) 5− group is represented by green tetrahedra and the (BO 3 ) 3− group is represented by the pink triangle.

Figure 15 .
Figure 15.The structure of Ba3B6O11F2 and the (B6O14) 10− group.The (BO4) 5− group is represented by green tetrahedra and the (BO3) 3− group is represented by the pink triangle.
NaCaBe 2 B 2 O 6 F crystallizes in the monoclinic space group Cc, exhibiting an two-dimensional [Be 2 B 2 O 6 Be 4 B 4 O 11 and LiNa 5 Be 12 B 12 O 33