crystals Review
Borate-Based Ultraviolet and Deep-Ultraviolet Nonlinear Optical Crystals Yi Yang 1,2 , Xingxing Jiang 1, *, Zheshuai Lin 1,2, * and Yicheng Wu 1 1
2
*
Center for Crystal R&D, Key Lab of Functional Crystals and Laser Technology of Chinese Academy of Sciences, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China;
[email protected] (Y.Y.);
[email protected] (Y.W.) University of the Chinese Academy of Sciences, Beijing 100049, China Correspondence:
[email protected] (X.J.);
[email protected] (Z.L.); Tel.: +86-10-8254-3718 (Z.L.)
Academic Editors: Ning Ye and Rukang Li Received: 28 January 2017; Accepted: 21 March 2017; Published: 25 March 2017
Abstract: 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. Keywords: borates; nonlinear optical (NLO) crystals; structure-property relationship
ultraviolet and deep-ultraviolet;
1. 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 [2–7]. 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 [8–11]. 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 (λcutoff ) or wide energy bandgap (Eg ) 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 (dij ) should be larger than 1 × KDP (KH2 PO4 , d36 ~ 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 Crystals 2017, 7, 95; doi:10.3390/cryst7040095
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not available in the crystals [13]. Meanwhile, the refractive indices dispersion in the UV (and DUV) region should be low so as to match the fundamental wave with the SHG light [14]. Finally, good single crystal growth habit, chemical stability and mechanical properties are required. In borates, the large difference between the electronegativity of boron and oxygen atoms is very favorable for the transmittance of short-wavelength light. Moreover, boron atoms can be threeor four-coordinated with oxygen atoms, forming (BO3 )3− planar triangles and (BO4 )5− polyhedra. The presence of conjugated π-orbital and highly anisotropic electron distribution in the (BO3 )3− group is very beneficial to the generation of large microscopic second-order susceptibility and birefringence [15]. The elimination of dangling bonds on the oxygen anions in the B-O groups can effectively increase the energy bandgap and make the absorption edge blue-shifted in the borate crystals [14,15]. The spatial combinations of (BO3 )3 − and (BO4 )4 − groups contribute the diverse structural types in borate-based compounds. In the 1980s, two important NLO borates, β-BaB2 O4 (BBO) [16,17] and LiB3 O5 (LBO) [18], were discovered. These two crystals exhibit the excellent NLO performance in the visible and UV regions. Nevertheless, the relatively narrow energy bandgap (Eg ~ 6.5 eV or λcutoff ~190 nm) in BBO and the small birefringence value (∆n ~0.04 @ 400 nm) in LBO restrict their applications in the DUV region. In the 1990s, another important NLO borate KBe2 BO3 F2 (KBBF) [19–22] was discovered in Chen’s group. This crystal, and its analogs RbBe2 BO3 F2 (RBBF) [23] and CsBe2 BO3 F2 (CBBF) [24], possess a very large energy bandgap (Eg ~8.3 eV) and a relatively strong SHG effect (~1.2 × KDP). Moreover, the birefringence values for the three crystals are about 0.07 ~ 0.08 @ 400 nm, and the shortest SHG phase-matching wavelengths are 161 nm, 170 nm and 201 nm for KBBF, RBBF and CBBF, respectively [25]. This clearly indicates the excellence for the KBBF family crystals used as the DUV NLO crystals. However, due to the weak interaction between alkali metal and fluorine cations, the layering growth habit in the KBBF family crystals brings significant difficulty to single crystal growth and severely hiders their practical applications. Therefore, the explorations for new borate-based crystals with good UV (especially DUV) NLO performances are greatly desirable. Since the beginning of this century, quite a few new borate-based UV or DUV NLO compounds have been synthesized. They exhibit wide energy bandgaps and large NLO effects accompany with the diverse structural features. Considering that the number of UV and DUV NLO borates is still in a very small ratio compared with the whole number of borate compounds, it is important to know how microscopic structures determine the optical properties in these optoelectronic functional materials. In this review, we will focus on the developments of the borate-based UV and DUV NLO crystals in the recent decade. Some representative borate compounds are chosen according to the variability of microscopic B-O groups, and their structure–property relationship is analyzed. Based on the analysis and summarization, the prospects for UV and DUV NLO borates are discussed. 2. Results and Discussion The space group, UV edge, SHG effect and birefringence of all the mentioned representative borate-based UV and DUV NLO crystals are listed in Table 1. 2.1. Borate Crystals in Which the B-O Groups Are (BO3 )3− Groups Only According to their structural features, the types of borates discovered in the recent decade can mainly be catalogued into three classes: KBBF-type, Sr2 Be2 B2 O7 (SBBO)-type and three-dimensional network crystals. In most of these crystals, the NLO properties are mainly attributed to the (BO3 )3 − groups. 2.1.1. KBBF-Type Crystals KBe2 BO3 F2 (KBBF) is the unique NLO crystal that can generate coherent light at wavelengths below 200 nm by direct SHG method [22,26–29]. KBBF possesses a layer structure (Figure 1). Each terminal oxygen atom of the (BO3 )3− groups is shared with two (BeO3 F)5− tetrahedra, forming
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an infinite [Be2 BO3 F2 ]∞ layer. The layers are connected by electrostatic interaction between potassium and fluorine ions. the KBBF-type crystals are referred to the crystals in which the3 terminal infinite2017, [Be 2BO3Herein, Crystals 7, 95 F2]∞ layer. The layers are connected by electrostatic interaction between potassium of 16 3 − andatoms fluorine the KBBF-type crystals are referred to the in (M which theand terminal oxygen of ions. (BO3 Herein, ) groups are exclusively connected with twocrystals (MO3 X) = Be Zn, X = F, oxygen atoms 3)3− groups aretoexclusively two (MO 3X) (Mlayers = between Be and X = F,byClweak infinite [Be2BOof 3F2(BO ]in ∞ layer. The 1:2 layers are connected by3 X electrostatic interaction potassium Cl and Br) tetrahedra the ratio form the connected [M2 BO ]∞ layer, and the areZn, linked 2with Br) tetrahedra in the ratio 1:2 to formcrystals the [M2are BO3referred X2]∞ layer, the layers linked weak fluorine ions. Herein, the KBBF-type to and the crystals in are which the by terminal ionic and bonds. ionic bonds. 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.
3− Figure 1. (a)1.The structure of the KBeKBe BO3F32F]∞2 ]layer. Figure (a) crystal The crystal structure of the 2BO 2 (KBBF)crystal crystal and and (b) [Be [Be22BO (BO(BO 3)3− 3 ) ∞ layer. 2 BO 3 F32F(KBBF) 5− groups are represented by pink and green polyhedra respectively. 5 − and (BeO 3 F) and (BeO3 F) groups are represented by pink and green polyhedra respectively.
Figure 1. (a) The crystal structure of the KBe2BO3F2 (KBBF) crystal and (b) [Be2BO3F2]∞ layer. (BO3)3−
BaBeand 2BO(BeO 3F3 (BBBF): 3F)5− groups are represented by pink and green polyhedra respectively.
BaBe2 BO3 F3 (BBBF):
been devotedtotoovercome overcome the the layering KBBF family crystals. In 2016, Much effort hashas been devoted layeringhabit habitofof KBBF family crystals. In 2016, BaBeMuch 2BO 3F3effort (BBBF): Guo et al. proposed the strategy to improve the growth habit by enhancing electrostatic interaction, Guo et al. proposed the strategy to improve the growth habit by enhancing electrostatic interaction, and Much successfully synthesized BaBe2BO 3F3 (BBBF) the [30].layering BBBF crystallizes in hexagonal space group of effort has been devoted to overcome habit of KBBF family crystals. In 2016, and successfully synthesized BaBe 2 BO3 F3 (BBBF) [30]. BBBF crystallizes in hexagonal space group P63. et In al. one unit cell, [Be2BO F2]∞ layers KBBFhabit are aligned vertical to c-axis in a nearly Guo proposed thetwo strategy to 3improve thein growth by enhancing electrostatic interaction, of P63 . In one unit cell, two [Be2 BO3 F2 ]∞ layers in KBBF are aligned vertical to c-axis in a nearly antiparallel manner, and the BaBe barium are[30]. located between the interstice between layers and successfully synthesized 2BOcations 3F3 (BBBF) BBBF crystallizes in hexagonal spacethe group of antiparallel manner, and the barium cations are located between the interstice between the layers (Figure the cell, sametwo time,[Be isolated are are also aligned introduced to compensate theavalence P6 3. In 2). oneAtunit 2BO3F2]fluorine ∞ layers anions in KBBF vertical to c-axis in nearly (Figure 2). At the same time, isolated fluorine alsoBased introduced to compensate valence 2+ ions. imbalance resulting from the substitution K+ anions withlocated Baare oninterstice simple Coulomb the antiparallel manner, and the barium cations are between the betweenmodel, thethe layers + 2+ imbalance resulting substitution Baareand ions. Based on simple model, electrostatic between the [Be 2K BO3with Fanions 2]∞ layer cations of Coulomb BBBF is times the (Figure 2). Atinteraction thefrom same the time, isolated fluorine alsointerstitial introduced to compensate the1.5 valence electrostatic interaction between thelayered [Be2 BO layer and interstitial cations of BBBF is large 1.5 that of KBBF, indicating that habit overcome in simple BBBF. Benefiting from imbalance resulting from the the substitution K3+Fwith Ba2+ ions. Based on Coulomb model, thetimes ∞ largely 2 ]is birefringence originating from parallel arrangement of overcome (BO 3)3− plane, the phase-matching ability that of KBBF, indicating that thethe layered largely in BBBF. electrostatic interaction between the [Be2habit BO 3F2]is ∞ layer and interstitial cations ofBenefiting BBBF is 1.5from timeslarge evaluation based on the first-principles refractive revealed that the shortest SHG that of KBBF, indicating thatthe theparallel layered habit isindices largely overcome in BBBF. from large birefringence originating from arrangement ofcalculations (BO3 )3− plane, theBenefiting phase-matching ability 3− plane, wavelength is 196 nm, implying that BBBF could be truly used in the SHG in the DUV region. birefringence originating from the parallel arrangement of (BO 3 ) the phase-matching ability evaluation based on the first-principles refractive indices calculations revealed that the shortest SHG However, suffering from the nearly antiparallel alignment of (BOrevealed 3)3− groups, the SHG evaluation based the first-principles calculations that the powder shortest wavelength is 196 nm,on implying that BBBFrefractive could beindices truly used in the SHG in the DUV region. However, response of BBBF is just 0.1 × KDP. wavelength is 196 nm, implying that BBBF could be truly used in the SHG in the DUV region. suffering from the nearly antiparallel alignment of (BO3 )3− groups, the3−powder SHG response of BBBF However, suffering from the nearly antiparallel alignment of (BO3) groups, the powder SHG is just 0.1 × KDP. response of BBBF is just 0.1 × KDP.
Figure 2. (a) The structure of BaBe2BO3F3 (BBBF). (b) The adjacent [Be2BO3F2]∞ layers, the (BO3)3− and (BeO3F)5− groups are represented by pink and green polyhedra respectively. (c) Phase-matching capabilities BBBF . of Adapted and reprinted with permission ref [30], Copyright Figure (a) for The structure of BaBe 2BO3F adjacent [Befrom 2BO3F 2]∞ layers, the (BOthe 3)3−2016 and Figure 2. (a)2.The structure BaBe (BBBF).(b) (b)The The adjacent [Be (BO3 )3− 2 BO 3 F33 (BBBF). 2 BO 3 F2 ]∞ layers, 5− 5 − American Chemical Society. byby pink and green polyhedra respectively. (c) Phase-matching (BeO3F) and (BeO F) groups groupsare arerepresented represented pink and green polyhedra respectively. (c) Phase-matching 3
capabilities for BBBF . Adapted and reprinted with permission ref Copyright [30], Copyright 2016 capabilities for BBBF. Adapted and reprinted with permission from from ref [30], 2016 American AZn2American BO3X2 (AChemical = K, Rb, Society. NH4; X = Cl, Br) Chemical Society.
AZn2BO3X2 (A = K, Rb, NH4; X = Cl, Br)
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AZn2 BO3 X2 (A = K, Rb, NH4 ; X = Cl, Br) Crystals 2017, 7, 95
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AZn2 BO3 X2 (A = K, Rb, NH4 ; X = Cl, Br can be regarded as the production of the “transgenosis” Crystals 2017, 7, 95 4 of 16 AZn2BO3X 2 (A = K, 4; X = Cl, Br can be regarded as the production the “transgenosis” on KBBF crystals) [31,32]. InRb, theNH “transgenosis” process, potassium cations areofreplaced by A (K+ , Rb+ +, Rb+ 5In − the 5− tetrahedra on +KBBF crystals) “transgenosis” potassium are replaced by 3). A (K or NH ions, and groups replaced by (ZnO Due to the 2BO 3X2 (BeO (A[31,32]. = K, Rb, NH 4; X = are Cl, Br canprocess, be regarded as the cations production of (Figure the “transgenosis” 4 )AZn 3 F) 3 X) + 5− 5− or KBBF NH4 ) of ions, and (BeO F) the groups are by (ZnO 3X) tetrahedra (Figure 3). Due to the+ 5− 3In +, Rb introduction (ZnO the replaced shortest absorption edge of are AZn red-shifted on crystals) [31,32]. “transgenosis” process, potassium cations replaced A (K 3 X) 5−tetrahedra, 2 BO3 X2byare introduction of (ZnO 3X) tetrahedra, edge of AZn2BO 3X2 are red-shifted to +) ions, 5− groups the 5− tetrahedra or NH4nm. 3F) are shortest replacedabsorption byprocess (ZnO3X) (Figure 3). Due to the to 190–214 Forand the (BeO same reason, “transgenosis” doubles the SHG effect of AZn 2 BO3 X2 190–214 nm. For the same reason, “transgenosis” process doubles the SHG effect of AZn2BO3X2 5− 5 − introduction of (ZnO 3X) tetrahedra, the shortest absorption edge of AZn 2BO 3X2 are red-shifted to compared with KBBF, and the mechanism analysis revealed that (ZnO3 X) tetrahedra dominantly compared with KBBF, and the mechanism analysis process revealeddoubles that (ZnO 3X)5− tetrahedra dominantly 190–214 nm. For the same reason, “transgenosis” the SHG effect of AZn 2BO3X2 contributes to the SHG effect in the series of materials. This is the first case where [ZnO groups 3 Cl/Br] contributeswith to the SHGand effect the seriesanalysis of materials. Thisthat is the first case where [ZnO 3Cl/Br] compared KBBF, the in mechanism revealed (ZnO 3X)5− tetrahedra dominantly can begroups used as the NLO-active structural units in NLO materials. More importantly, in NH Zn 4 2 BO can to be the used as the NLO-active structural units in NLO materials. More importantly, in 3 X2 , contributes SHG effect in the series of materials. This is the first case where [ZnO3Cl/Br] the hydrogen bonds between ammonia and halogen ionsand largely reduce layered habit improve NH4Zn2BO 2, the hydrogen bonds between ammonia ionsthe largely reduce theand layered groups can3Xbe used as the NLO-active structural units inhalogen NLO materials. More importantly, in habit and improve the growth habit. the growth habit. NH4Zn2BO3X2, the hydrogen bonds between ammonia and halogen ions largely reduce the layered habit and improve the growth habit.
Figure (a) Crystal structure KZn 2BO3Cl2, (b) the layers of [Zn2BO3Cl2]∞, (c) the electronic Figure 3. (a)3. Crystal structure of of KZn 2 BO3 Cl2 , (b) the layers of [Zn2 BO3 Cl2 ]∞ , (c) the electronic densities of NH4Zn2BO3Cl2 and RbZn2BO3Cl2. Adapted and reprinted with permission from ref [32], densities of NH Cl32Cl . Adapted reprinted with from ref [32], Figure 3. (a) Crystal structure of KZn 2, (b) the and layers of [Zn2BO 3Cl2permission ]∞, (c) the electronic 4 Zn 2 BO3 Cl 2 and RbZn 2 BO23BO Copyright 2016 American Chemical Society. densities of American NH4Zn2BO3Chemical Cl2 and RbZn 2BO3Cl2. Adapted and reprinted with permission from ref [32], Copyright 2016 Society. Copyright 2016 American Chemical Society.
2.1.2. SBBO-Type Crystals
2.1.2. SBBO-Type Crystals
2.1.2.Sr SBBO-Type Crystals 2Be2B2O7 (SBBO) was designed by molecule engineering method in 1995 [33,34]. In the crystal
Sr B2 O was designed by molecule method in 1995 [33,34]. with In theone crystal structure of7 (SBBO) SBBO (Figure 4), the terminal oxygenengineering atoms of (BO 3)3− group are bonded 2 Be Sr2 2Be 2B2O7 (SBBO) was designed by molecule engineering method in 1995 [33,34]. In the crystal 3− group beryllium atoms, forming a [BeBO 3O]∞ single layer. Two [BeBO 3(BO O] ∞3 single layers connect each other structure of SBBO (Figure 4), the terminal oxygen atoms of ) are bonded with structure of SBBO (Figure 4), the terminal oxygen atoms of (BO3)3− group are bonded with one one via the bridge oxygen forming the [Be 2B2O7Two ]∞ double layers, the strontium cations areother beryllium atoms, forming aatoms, [BeBO layer. [BeBO single layers connect ∞ ∞single ∞and 3 O] 3 O] beryllium atoms, forming a [BeBO 3O] single layer. Two [BeBO 3O] ∞ single layers connect eacheach other intercalated in the interstices within and between the [Be 2B2O7]∞ double layers. Herein the via the oxygen atoms, forming the doublelayers, layers, and strontium cations viabridge the bridge oxygen atoms, forming the[Be [Be 2B O77]]∞∞double and thethe strontium cations are are 2B 22O SBBO-type crystals are referred within to the crystals with [Mthe 2B2O6X]∞-type double layers, and the A-site intercalated in the interstices and between [Be 2 B 2 O 7 ] ∞ double layers. Herein the intercalated in the interstices within and between the [Be2 B2 O7 ]∞ double layers. Herein the SBBO-type cations are crystals embedded in the interspace inter- or intra-double layers. The measurement based on SBBO-type to with the crystals 2B2O6X] ∞-type double and A-site the A-site crystals are referred toare thereferred crystals [M2 B2with O6 X][M double layers,layers, and the cations ∞ -type powder samples revealed that SBBO has the potential for DUV NLO crystals. However, the cations are in embedded in the interspace inter- or intra-double layers. The measurement based on are embedded the interspace interor intra-double layers. The measurement based on powder problem of structure instability has been always an issue for SBBO, which severely hinders its powder samples revealed that SBBO has the potential for DUV NLO crystals. However, the samples revealed that SBBO has the potential for DUV NLO crystals. However, the problem of structure practical application [33,35,36]. problem of structure instability has been always an issue for SBBO, which severely hinders its instability has been always an issue for SBBO, which severely hinders its practical application [33,35,36]. practical application [33,35,36].
Figure 4. (a) The structure of Sr2Be2B2O7 (SBBO) crystal, (b) the [Be3B3O6]∞ layer. Figure (a) The structure of 2Sr 2BeB 2O7 (SBBO) the [Be[Be 3B3O Figure 4. (a)4.The structure of Sr Be crystal,(b) (b) the 2 2B 2O 7 (SBBO) crystal, 3 B6]3∞Olayer. 6 ]∞ layer. NaCaBe2B2O6F
NaCaBe 2B2O62F NaCaBe B2O6F crystallizes in the monoclinic space group Cc, exhibiting an two-dimensional [Be2BNaCaBe 2O6F]∞ double-layer structure, with bridge oxygen in the double layer of SBBO substituted by 2B2O6F 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
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NaCaBe2 B2 O6 F NaCaBe2 B2 O6 F crystallizes in the monoclinic space group Cc, exhibiting an two-dimensional [Be2 B2 O6 F]∞ double-layer structure, with bridge oxygen in the double layer of SBBO substituted by Crystals 2017, 7, 95 5 of 16 fluorine atoms [37]. The sodium ions are intercalated in the interstice within [Be2 B2 O6 F]∞ double layer, whilefluorine calciumatoms ions are embedded in the cavity between the double layers (Figure 5). Transmittance Crystals 2017, 7, 95 [37]. The sodium ions are intercalated in the interstice within [Be2B2O6F]∞ double 5 of 16 test revealed that the absorption of NaCaBe 185 nm, from the incomplete 2 O6 F is layer, while calcium ions are edge embedded in the2 Bcavity between theresulting double layers (Figure 5). 3− groups are aligned in nearly elimination dangling bonds at oxygen atoms. In the of lattice, Transmittance test revealed that theions absorption edge NaCaBe 2B(BO 2O6F nm,[Be resulting the 3 )is 185 fluorineofatoms [37]. The sodium are intercalated in thethe interstice within 2B2O6F]from ∞ double incomplete elimination ofthe dangling bondsoffset at atoms. In second-order the the lattice, thesusceptibility (BO 3)3− groups are antiparallel orientation, and geometrical of microscopic results layer, while calcium ions are embedded in oxygen the cavity between double layers (Figure 5). in aligned in nearly antiparallel orientation, and of the geometrical microscopic second-order Transmittance testeffect revealed that thethird absorption edge of NaCaBeoffset 2B2O6Fof is 185 nm, resulting from the a small powder SHG about one that KDP. susceptibility results in a of small powder SHG at effect aboutatoms. one third thatlattice, of KDP. incomplete elimination dangling bonds oxygen In the 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.
Figure 5. (a) The structure of NaCaBe2 B2B22O F, (b) double-layer structure viewed along the c-axis. Figure 5. (a) The structure of NaCaBe O66F, (b) double-layer structure viewed along the c-axis.
K3 BaK 3Ba 3Li42B Al64O B620 O5.F20(a) F The structure of NaCaBe2B2O6F, (b) double-layer structure viewed along the c-axis. 3 Li 2 Al Figure 3Ba3Al B620 OF 20F crystallizes in in hexagonal hexagonal space P-62c. Its Its exhibits the the mostmost complicated K3 BaK3 Li B64O crystallizes spacegroup group P-62c. exhibits complicated 2 Li42Al K32Ba 3Li2Al4B6O20F [Li Al 4B6O20F]∞ double-layer structure (Figure 6) in the SBBO-type crystals[38]. [Li2Al4B6O20F]∞ [Li2 Al4 B6 O20 F]∞ double-layer structure (Figure 6) in the SBBO-type crystals[38]. [Li2 Al4 B6 O20 F]∞ double layers substituting one space of (BeOP-62c. 4)6− with (LiO3F)6−the and two 6Its − with 6− complicated K3Ba 3Li2Al 4B6generated O20F crystallizes in hexagonal exhibits most double layers are are generated bybysubstituting onethird thirdgroup of (BeO (LiOtetrahedra tetrahedra and 4) 3 F) 6− with (AlO4)5− tetrahedra. Regardless of the complicated crystal structure, it keeps thirds of (BeO 4 ) [Li2Al4of B6O(BeO 20F]∞ double-layer structure (Figure 6) Regardless in the SBBO-type crystals[38]. [Li 2Al4B6O20F]∞ 6 − with (AlO 5 − tetrahedra. two thirds of the complicated crystal structure, 4) 4) the similar spatial arrangement of (BO3)3− groups in SBBO, and a strong SHG effect (1.5 × double layers are generated by substituting one third of (BeO 4)6−exhibits with (LiO 3F)6− tetrahedra and two 3− groups it keeps theDue similar spatial arrangement of (BO in SBBO, and exhibits a 4B strong SHG 3 ) electronegativity 6− 5− KDP). to the introduction of lithium with low and valence in [Li 2 Al 6 O 20 F] ∞ thirds of (BeO4) with (AlO4) tetrahedra. Regardless of the complicated crystal structure, it keeps effectlayers, (1.5 ×theKDP). Due toorbitals the introduction of atoms lithium low electronegativity and valence nonbonding ofofsome arewith almost reserved, theSHG absorption edge× in the similar spatial arrangement (BO3oxygen )3− groups in SBBO, and exhibits a and strong effect (1.5 [Li2 Alof B O F] layers, the nonbonding orbitals of some oxygen atoms are almost reserved, and ∞ 4 6 20 K3BaDue 3Li2Al 6O20introduction F is located at nm. with At the same time, the element the KDP). to4Bthe of 190 lithium low electronegativity and substitution valence in [Lipreserve 2Al4B6O20 F]∞ the absorption edge of K Ba Li Al B O F is located at 190 nm. At the same time, the element substitution strong 4B 6O20F] ∞ double layers, K3Ba 3Li2Al4B6and O20Fthe shows no layered 3 between 3 2orbitals 4[Li62Al layers, interaction the nonbonding of20 some oxygen atoms are and almost reserved, absorption edge habit in 3the crystal process. More importantly, should beelement emphasized thatKthe insertion preserve strong interaction between [Li O20itF]time, layers, and Bof6 O20 F of K3the Ba Li 2Al 4 B6 O 20growth F is located at 190 nm. At the the substitution preserve ∞ double 2 Al 4 B6same 3 Ba 3 Li2 Al4 the lithium largelyhabit improves of∞ process. [Li2Al4Blayers, 6O 20F]∞ and double itbeaccommodate strong interaction between [Liflexibility 2Al4growth B6O20F] double K3Balayers, 3Li2Al Bshould 6O20F shows no layered that shows no layered in thethe crystal More importantly, it4making emphasized large ions. Therefore, theprocess. problem ofthe structural instability SBBO is completely eliminated in habit K in+ the crystal growth More importantly, itofshould that the insertion of it the insertion of lithium largely improves flexibility [Li2of Albe layers, making 4 Bemphasized 6 O20 F]∞ double K 3Ba3Li2Al 4B6O20F, as demonstrated by theofphonon spectrum (Figurelayers, 6b). making it accommodate + lithium largely improves the flexibility [Li 2Al4B 6O20F]∞ double accommodate large K ions. Therefore, the problem of structural instability of SBBO is completely large K+ ions. Therefore, the problem of structural instability of SBBO is completely eliminated in eliminated in K3 Ba3 Li2 Al4 B6 O20 F, as demonstrated by the phonon spectrum (Figure 6b).
K3Ba3Li2Al4B6O20F, as demonstrated by the phonon spectrum (Figure 6b).
Figure 6. (a) Structure of K3Ba3Li2Al4B6O20F, (b) phonon spectrum of K3Ba3Li2Al4B6O20F. Adapted and reprinted with permission from ref [38], Copyright 2016 American Chemical Society. (a) Structure K3Ba 3LiAl B66O spectrum of Kof 3Ba 2Al4Li 20F.BAdapted and FigureFigure 6. (a)6.Structure of Kof3 Ba O2020F,F,(b) (b)phonon phonon spectrum K3Li Al 3 Li 2 2Al44B 3 Ba 3 B62O 4 6 O20 F. Adapted Rb3Alreprinted 3B3O10F with permission refref [38], Copyright 20162016 American Chemical Society.Society. and reprinted with permissionfrom from [38], Copyright American Chemical Rb3Al3B3O10F crystallizes in the trigonal space group of P31c, and possess a three-dimensional Rb3Al3B3O(Figure 10F structure 7，ref [39]). Each (BO3)3− is linked with one (AlO3F)4− and two (AlO4)5− tetrahedra, forming the alveolate [Al3(BO3)in 3OF] layer in the (a,b) plane. O/F atoms of (AlO4/AlO3F) Rb3Al3B3O10F crystallizes the∞ trigonal space group of The P31c,apical and possess a three-dimensional 3− 4− tetrahedra pointing upward and download are shared by the adjacent layer to serve as interlayer structure (Figure 7，ref [39]). Each (BO3) is linked with one (AlO3F) and two (AlO4the )5− tetrahedra, bridge, constructing the three-dimensional framework. Rubidium cations reside in the interstice forming the alveolate [Al3(BO3)3OF]∞ layer in the (a,b) plane. The apical O/F atoms of (AlO4/AlO3of F)
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
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Rb3 Al3 B3 O10 F Rb3 Al3 B3 O10 F crystallizes in the trigonal space group of P31 c, and possess a three-dimensional structure (Figure 7, ref [39]). Each (BO3 )3− is linked with one (AlO3 F)4− and two (AlO4 )5− tetrahedra, forming the alveolate [Al3 (BO3 )3 OF]∞ layer in the (a,b) plane. The apical O/F atoms of (AlO4 /AlO3 F) tetrahedra pointing upward and download are shared by the adjacent layer to serve as the interlayer bridge, the three-dimensional framework. Rubidium cations reside in the interstice Crystals 2017,constructing 7, 95 6 of 16 of the framework to counterbalance the valence states. UV−vis near-infrared diffuse spectrum the framework the3 Al valence UV−vis spectrum revealed revealed that to thecounterbalance absorption of Rb F is below 200 near-infrared nm, consistentdiffuse with the coordination with 3 B3 O10states. − group. that the absorption of Rb3oxygen Al3B3O10atoms F is in below 2003 )3nm, consistent thedirect coordination aluminum of the dangling the (BO Inducedwith by the connectionwith of the aluminum the∞dangling atoms in thecompensates (BO3)3− group. by theofdirect connection of [Al3 (BO3 )of layer, theoxygen high spatial density theInduced deterioration crossing arrangement 3 OF] 3 − the 3)groups 3OF]∞ layer, the high spatial density compensates the deterioration of crossing of [Al (BO3(BO ) to the SHG response, and the powder SHG effect is about 1.2 × KDP. Moreover, 3 arrangement (BO3)3−adhesion groups to theof SHG response, and the powder is about 1.2 × KDP. owing to theofstrong force Al-O/F bond between [Al3 (BO3SHG )3 OF]effect no layered habit is ∞ layer, Moreover, to the strong adhesion observed owing in the growth of Rb3 Al 3 B3 O10 F. force of Al-O/F bond between [Al3(BO3)3OF]∞ layer, no layered habit is observed in the growth of Rb3Al3B3O10F.
− groups 3−)3groups Figure Crystalstructure structureofofRb Rb3Al (b)the the[Al [Al3(BO )3 OF] the(BO (BO Figure 7. 7. (a)(a)Crystal 3B 3O 10F, (b) 3)3 3OF] ∞ layer, the 3)3 areare ∞ layer, 3 Al 3B 3O 10 F, 3 (BO represented pink triangle. represented byby pink triangle.
2.1.3. Three-Dimensional (3D) Network Crystals 2.1.3. Three-Dimensional (3D) Network Crystals − groups InIn this type 3)3−)3groups areare connected with other groups to to form the this typeofofborate boratecrystals, crystals,the the(BO (BO connected with other groups form the 3 3D3D networks. networks.
LiSr(BO LiSr(BO 3)23 )2 LiSr(BO crystallizes in monoclinic group Cc, which shows shows a three-dimensional framework LiSr(BO 3)23 )2 crystallizes monoclinicspace space group Cc, which a three-dimensional 14 − structure. Strontium is eight-coordinated with oxygen atoms, formingatoms, (SrO8 )forming polyhedral [40]. framework structure. atom Strontium atom is eight-coordinated with oxygen (SrO8)14− 3 − 14 − 3− triangles The (BO3[40]. ) triangles (SrO8 ) and polyhera are alternatively connected in the inratio of 1:1, polyhedral The (BO3)and (SrO8)14− polyhera are alternatively connected the ratio the infinite [SrBO[SrBO Meanwhile, the strontium atomsatoms are bonded with the oxygen ofconstructing 1:1, constructing the infinite 3]∞ layer. Meanwhile, the strontium are bonded with the 3 ]∞ layer. 3 − 3− atomsatoms of the of (BO groups outside of [SrBO and and connect the layer together, giving rise to oxygen the 3) groups outside of [SrBO 3]∞ layer connect the layer together, giving 3 ) (BO 3 ]∞ layer thetothree-dimensional [SrB2 O[SrB network (Figure 8). The ions are inserted in the interstice rise the three-dimensional 6]∞ network (Figure 8).lithium The lithium ions are inserted in the 6 ]∞2O within the [SrB2the O6 ][SrB to keeptothe electron neutrality. One dangling oxygen oxygen atom is atom left at is the interstice within 2O6]∞ network keep the electron neutrality. One dangling ∞ network (BO )3− (BO groups outsideoutside of [SrBO the so absorption edge ofedge LiSr(BO 186 at nm. left at 3the 3)3− groups of3 ][SrBO 3]∞ so layer, the absorption of LiSr(BO 2 justat ends ∞ layer, 3 )2 just3)ends 3− groups The (BO within the [SrBO are are aligned in ainnearly parallel manner, while the 186 nm. The 3)3− groups within the [SrBO 3]∞ layer aligned a nearly parallel manner, while 3 ) (BO 3 ]∞ layer outside (SrBO areare almost arranged in an in antiparallel configuration. Therefore, the “effective” the outside (SrBO ∞ layers almost arranged an antiparallel configuration. Therefore, the 3 )∞3)layers 3 − groups (BO3 )3 − groups those inthose the [SrBO ] layer, and high spatial density of those (BO ) “effective” (BO3)3− are groups are in the [SrBO 3 ] ∞ layer, and high spatial density of those (BO 3)3− ∞ 3 3 results in the large SHG response (2 × KDP) of LiSr(BO ) . Meanwhile, the ordered arrangement groups results in the large SHG response (2 × KDP)3 of 2 LiSr(BO3)2. Meanwhile, the orderedof 3 − 3− (BO3 ) groups within the [SrBO also give rise to thegive largerise birefringence (0.056, calculated arrangement of (BO 3) groups within the [SrBO 3]∞ layer also to the large birefringence 3 ]∞ layer value), making LiSr(BO )2 a potential NLO crystal for the output of the coherent laser. (0.056, calculated value), 3making LiSr(BO 3) 2 a potential NLO crystal for266-nm the output of the 266-nm coherent laser.
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 Crystals 2017, 7, 95 (0.056, calculated value), making LiSr(BO3)2 a potential NLO crystal for the output of the 266-nm coherent laser.
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3 − groups 8. (a)structure The structure of the LiSr(BO)3)2 crystal crystal and 3)3− groups are represented by pink by pink Figure Figure 8. (a) The of the LiSr(BO andthe the(BO (BO are represented 3 2 3) 3− 14− 3) groups coordinated to a (SrO8) polyhedron, (c) the [SrBO3]∞ layer. triangle, (b) (BO 3 − 14 − triangle, (b) (BO3 ) groups coordinated to a (SrO8 ) polyhedron, (c) the [SrBO3 ]∞ layer.
NaSr3 BeCrystals 3 B3 O2017, 9 F4 7, 95
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NaSr3 Be3 B3 O9 F4 crystallizes in trigonal space group R3m. By sharing the fluorine ions, three NaSr 3Be3B3O9F4 5− tetrahedra constituted a (Be O F)19− cluster (Figure 9, ref [41]). The oxygen atoms of (BeO3 F)Crystals 3 12 2017, 7, 95 7 of 16 NaSr3Be3B3O9F4 crystallizes in trigonal space3 −group R3m. By sharing the fluorine ions, three 10− (Be3 O12 F)19 − cluster are connected by three (BO ) groups, forming (Be B O F) building block. 3 3 3 12 (BeO33Be F)5−3Btetrahedra constituted a (Be3O12F)19− cluster 9, ref [41]). The oxygen atoms of 104 − building 3 − are(Figure NaSr 3O9F In the (Be B O F) block, the (BO ) almost parallel with each other, except the 3 F)12 19− cluster are connected by three (BO33)3− groups, forming (Be3B3O12F)10− building block. In (Be33O12 10 − building slight deviation from (a,b) plane. The (Be B O F) blocks are interconnected in almost NaSr 3Be 3B310− O 9F4 crystallizes in trigonal space group R3m. By sharing the fluorine ions, three 3 3)33− are 12 almost parallel with each other, except the slight the (Be3B3O12F) building block, the (BO + and Sr 2+ (BeO 3F)5− tetrahedra constituted a 3B (Be 1210− F)19− cluster (Figure refare [41]). Theinoxygen atoms of the same orientation, generating the(Be 3D The Na located in the cavity of the 3D deviation from (a,b) plane. The 3network. O312OF) building blocks are9,interconnected almost the same (Be 3O12F)19− cluster are connected by three (BO3+)3− groups, forming (Be 3B3O12F)10− building block. In 2+ orientation, generating the 3D network. The Na and Sr are located in the cavity of the 3D network. network. The absorption edge of NaSr Be B O F is 170nm, originating from single-coordination of 3 3 3 9 4 the (Be 3B3O12F)10− 3 building block, the (BO3)3− are almost parallel with each other, except the slight − 3 − The absorption edge of NaSr 3Be3B3O9F4 is 170nm, originating from single-coordination of oxygen oxygen atoms of (BO3 ) groups with beryllium atoms. The subparallel geometry of (BO3 ) groups deviation from (a,b) plane. The (Be3B3O12F)10− building blocks are interconnected in almost the same atoms of (BO 3)3− groups with beryllium atoms. The subparallel geometry of (BO3)3− groups in the in the lattice leads to a large SHG response (4~5 × KDP) and birefringence (0.061@400 nm), showing + 2+ are orientation, 3D network. and Srand located in the(0.061@400 cavity of the 3D network. lattice leadsgenerating to a largethe SHG response The (4~5Na × KDP) birefringence nm), showing potential for UV NLO crystals for the generation of coherent laser at 266 nm as with LiSr(BO The absorption NaSr3Be 3O9F4 is 170nm, originating from single-coordination of oxygen 3 )2 . potential for UVedge NLOof crystals for3Bthe generation of coherent laser at 266 nm as with LiSr(BO3)2. 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.
Figure 9. (a) The of NaSr (b)the the F)10− cluster. 3 Be 3 B3B 3 3O 3B Figure 9. (a)structure The structure of NaSr 3Be O99FF44, ,(b) (Be(Be 3B3O 123 F)O10−12cluster.
NaBeB3NaBeB O6 3O6
Figure 9. (a) The structure of NaSr3Be3B3O9F4, (b) the (Be3B3O12F)10− cluster.
3O6 crystallizes in orthorhombic space group Pna21 [42]. By sharing the corner oxygen NaBeBNaBeB in orthorhombic space group Pna21 [42]. By sharing the corner oxygen 3 O6 crystallizes atoms, three 3)3− triangles and two (BeO4)6− tetrahedra form nearly coplanar double 3 − (BO NaBeB 3O6 atoms, three (BO ) triangles and two (BeO4 )6− tetrahedra form nearly coplanar double six-membered 3 six-membered ring (Be2B3O11)9− anions, similar to naphthalene. The (Be2B3O11)9− building blocks are 9 − 9By − sharing ring (Beconnected anions, similar to naphthalene. The building are connected NaBeB 3O 6 crystallizes inaorthorhombic space group(Be Pna2 [42]. theblocks corner cruciate oxygen 2 B3 O 11 ) with 2 B(BeO 31 O 114))6− each other in crossed manner by sharing the , forming the endless 3− 6− 6 − atoms, three 3) triangles and two (BeO 4) (BeO tetrahedra form nearly coplanar double with each other in the a (BO crossed manner bycruciate sharing the ) , forming the endless cruciate chains along c-axis. The adjacent chains are connected to each other via sharing the O chains 4 six-membered ring (Be 2B3O11)9− anions, similar to naphthalene. The (Be 2B3O11)9− building blocks are 3− triangles + ions atoms of (BO 3 ) generating the open framework with Na residing in the tunnels along the c-axis. The adjacent cruciate chains are connected to each other via sharing the O atoms of connected with each other indiffuse a crossed manner by sharing the 4)6−, forming the endless cruciate + (BeO 10). UV−Vis−NIR spectrum revealed that NaBeB O6 exhibits (BO3 )3 −(Figure triangles generating the open reflectance framework with Na ions residing in 3the tunnelsgood (Figure 10). chains along the c-axis. The adjacent cruciate chains are connected eachvery othercondensed via sharingspatial the O 9− transmittance below 200 nm. The (Be2B3O11) triangles resulttothe UV−Visatoms −NIRofdiffuse reflectance spectrum revealed that NaBeB O exhibits good transmittance 6 + ions residing in the tunnels below (BOof 3)3− triangles generating the open framework with3 Na arrangement (BO 3)3− groups, leading to the large SHG response (1.6 × KDP). More importantly, it 9 − 200 nm.(Figure The (Be B3UV−Vis−NIR O11that ) the triangles result the very condensed spatial arrangement of (BO3good )3− groups, 10). diffuse reflectance revealed that NaBeB3O6 exhibits 2noted should be collinear alignment of spectrum NaBeB 3O6 also gives rise to the large birefringence 9− below 200values nm.(1.6 The (Be2B3is OMore 11) triangles very leading transmittance to the large response × which KDP). importantly, it the should becondensed noted thatspatial the collinear (0.08@400 nm,SHG calculated [14]), the first case in result borate-based NLO crystals. arrangement (BOalso 3)3− groups, leading to the large SHG response (1.6 × KDP). More importantly, it alignment of NaBeBof3 O gives rise to the large birefringence (0.08@400 nm, calculated values [14]), 6 should be noted that the collinear alignment of NaBeB3O6 also gives rise to the large birefringence which is the first case in borate-based NLO crystals.
(0.08@400 nm, calculated values [14]), which is the first case in borate-based NLO crystals.
Figure 10. (a) The structure of NaBeB3O6, (b) double six-membered ring (Be2B3O11)9− anions.
Na2Be4B4O11 and LiNa5Be12B12O33 Figure 10. (a) The structure of NaBeB3O6, (b) double six-membered ring (Be2B3O11)9− anions.
9− anions. Figure (a) Thetostructure of the NaBeB double six-membered ring (Be2of B3 the O11 )B-O One10.strategy overcome layering habit of KBBF is the introduction groups 3 O6 , (b)
interconnecting the 2D [Be2BO3X2] layers. Accordingly, several borate NLO compounds have been Na 2Be4B4O11 and LiNa5Be12B12O33 discovered, including Na2Be4B4O11 [43], LiNa5Be12B12O33 [43], γ-KBe2B3O7 [42], β-KBe2B3O7 [42], strategy to overcome the layering habit of KBBF is the introduction of the B-O groups RbBeOne 2B3O7 [42] and Na2CsBe6B5O15 [44]. Herein, Na2Be4B4O11 and LiNa5Be12B12O33 are chosen as the interconnecting the 2D [Be2BO3X2] layers. Accordingly, several borate NLO compounds have been representative examples. discovered, including Na2Be 4B4O11 [43], LiNa5Be12B12O33 [43], γ-KBe2B3O7 [42], β-KBe2B3O7 [42], Na2Be4B4O11 and LiNa 5Be12B12O33 crystallize in triclinic space groups P1 and Pc, respectively. RbBe 2B3O7 [42] and Na2CsBe6B5O15 [44]. Herein, Na2Be4B4O11 and LiNa5Be12B12O33 are chosen as the Regardless of the different macroscopic symmetry, Na2Be4B4O11 and LiNa5Be12B12O33 share the
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Na2 Be4 B4 O11 and LiNa5 Be12 B12 O33 One strategy to overcome the layering habit of KBBF is the introduction of the B-O groups interconnecting the 2D [Be2 BO3 X2 ] layers. Accordingly, several borate NLO compounds have been discovered, including Na2 Be4 B4 O11 [43], LiNa5 Be12 B12 O33 [43], γ-KBe2 B3 O7 [42], β-KBe2 B3 O7 [42], RbBe2 B3 O7 [42] and Na2 CsBe6 B5 O15 [44]. Herein, Na2 Be4 B4 O11 and LiNa5 Be12 B12 O33 are chosen as the representative examples. Na2 Be4 B4 O11 and LiNa5 Be12 B12 O33 crystallize in triclinic space groups P1 and Pc, respectively. Crystals 2017, 7,of 95 the different macroscopic symmetry, Na Be B O and LiNa Be B O share 8 ofthe 16 Regardless 2 4 4 11 5 12 12 33 6− similar 3D 2017, framework structure. The (BO3 )3− group shares each oxygen atom with two 8(BeO Crystals 7, 95 of 16 4 ) tetrahedra, forming the infinite two-dimensional [Be2BO5]∞ layers. The adjacent [Be2BO5]∞ layers are tetrahedra, forming the infinite two-dimensional [Be2 BO5 ]∞ layers. The adjacent [Be2 BO5 ]∞ layers further bridgedforming together distorted (B2O[Be 5)4− groups by sharing O atoms to layers build the 3D tetrahedra, thethrough infinite two-dimensional ∞ layers.by Thesharing adjacent 2BO5]∞ are further bridged together through distorted (B2 O52BO )4−5]groups O[Be atoms to buildare the 3D framework, with the sodium or lithium cations residing in the tunnel of the framework (Figure 11). 4− further bridged together through distorted (B 2O5) 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 the dangling bonds ofthe the oxygen atoms within the [Be 2BO5]∞ framework, with the sodium or of lithium cations residing of the framework Despite the absolute elimination of the dangling bondsin of thetunnel oxygen atoms within(Figure the [Be11). 2 BO5 ]∞ the absolute elimination of (B the dangling bondsbonded of the oxygen atoms within theatoms, [Be2BOleaving 5] ∞ layer,Despite the terminal oxygen atoms of 2O 5− )4− are only with one beryllium 4 layer, the terminal oxygen atoms of (B2 O5 ) are only bonded with one beryllium atoms, leaving some the terminal oxygen atoms of Na (B22O 5)4− beryllium leavingedge somelayer, nonbonding orbitals. Therefore, Be 4Bare 4O11only and bonded LiNa5Bewith 12B12one O33 exhibit theatoms, absorption nonbonding orbitals. Therefore, Na2 Be4 BNa and LiNa5 Be12 B12 O33 exhibit thethe absorption edge at 171 4 O211 some nonbonding orbitals. Therefore, Be 4B4O11 and LiNa5Be12B12O33 4− exhibit absorption 3− at 171 and 169 nm respectively. 3The (BO3) groups within (B2O5) groups are almostedge aligned − groups 4 − groups and 169 nm respectively. The (BO ) within (B O ) are almost aligned head-to-head 3− 4− 3 2 within 5 at 171 and nm respectively. 3) groups (B2O5)nothing groupstoarethe almost head-to-head in 169 a crossing manner,The and(BO they contribute almost SHGaligned effect and in a crossing manner, they contribute almost to the SHGnothing effect and head-to-head in aand crossing manner, and theynothing contribute almost to birefringence. the SHG effect However, and birefringence. However, on the other hand, the connection of [Be2BO 5]∞ layers via (B2O5)4 groups 4 on the other hand, the connection of [Be layers viaof(B[Be O2BO ) 5]groups birefringence. However, on the other hand, ∞ layers largely via (B2Opreserve 5)4 groups high 2 BOthe 5 ]∞connection 2 5 3− group for SHG effect and birefringence. largely preserve highhigh density of ofthe effective (BO 3)3− 3− group largely preserve the (BOand 3) group for SHG Therefore, effect and both birefringence. density of the effective (BOdensity foreffective SHG effect birefringence. Na2 Be4 B4 O11 3) Therefore, bothboth Na2Be 4B4O11 and LiNa5Be12B12O33 show high SHG efficiency (1.3 and 1.4 times that of Therefore, 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 and and LiNa5 Be12 B12 O33 show high SHG efficiency (1.3 and 1.4 times that of KDP, respectively) KDP,KDP, respectively) andand phase-matching ability. respectively) phase-matching ability. phase-matching ability.
Figure 11.the (a) structures the structures of Na 2Be4B4O11, (b) the 2D infinite planar [Be2BO5]∞ layer. Figure 11. (a) of Na 2 Be4 B4 O11 , (b) the 2D infinite planar [Be2 BO5 ]∞ layer. Figure 11. (a) the structures of Na2Be4B4O11, (b) the 2D infinite planar [Be2BO5]∞ layer. CsZn2B3O7
CsZn2 B3 O7 CsZn2B3OCsZn 7 2B3O7 crystallizes in orthorhombic space group Cmc21 [45,46]. Its structure can be regarded CsZn2 B3 O7 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 CsZn 2B3O7 crystallizes in orthorhombic space group Cmc21 [45,46]. Its structure can be regarded as the variant of γ-KBe [42]introduction with beryllium and atoms potassium byabsorption zinc and cesium 3 O7 the atoms (Figure 12).2 B Due of zinc with substituted d-orbital, the edge ofatoms as the variant of the γ-KBe 2B3O7 [42] with beryllium and potassium substituted by zinc and cesium (Figure 12). Due introduction of zinc atoms with d-orbital, the absorption edge of 6− CsZn2B3O7 is red-shifted to 218 nm. Moreover, it also should be noted that (ZnO4) alsoCsZn has 2aB3 O7 atoms (Figure 12). Due the introduction of zinc atoms with d-orbital, the absorption edge of 6 − is red-shifted to 218 nm. Moreover, it also should be notedthe that (ZnO a considerable considerable contribution to the SHG effect and enhances SHG effect 1.5 ×has KDP (that of 4 ) toalso 6− CsZn 2B3O7 is red-shifted to 218 nm. Moreover, it also should be noted that (ZnO4) alsoB has a γ-KBe 2 B3 O 0.68 KDP). contribution to7 isthe SHG effect and enhances the SHG effect to 1.5 × KDP (that of γ-KBe 2 3 O7 is considerable contribution to the SHG effect and enhances the SHG effect to 1.5 × KDP (that of 0.68 KDP). γ-KBe2B3O7 is 0.68 KDP).
Figure 12. The structure of CsZn2B3O7.
2.2. Borate Crystals in Which the B-O Groups Are (BO4)5− Groups Only Figure of O77.4). 5− groups are BPO4 [47–50] and Figure 12. The the structure of CsZn CsZn 33O The most famous NLO borates in12. which B-O groups are2B(BO SrB4O7 [51–54]. Both of them have very large energy bandgap (Eg ~ 9.5 eV) or extremely short
2.2. Borate Crystals in(λ Which Aredue (BOto4)5− Groups absorption edge cutoff ~ the 130 B-O nm).Groups However, very smallOnly optical anisotropy in (BO4)5− groups, this type of UV NLO borates has too small a birefringence (typically ∆n < 0.01) to achieve the
The most famous NLO borates in which the B-O groups are (BO4)5− groups are BPO4 [47–50] and phase-matching condition, and has been less focused on in the recent decade. To our best SrB4O7 [51–54]. Both of them have very large energy bandgap (Eg ~ 9.5 eV) or extremely short knowledge, the only known compound in this type of borates is LaBeB3O7, which was synthesized in absorption edge (λcutoff ~ 130 nm). However, due to very small optical anisotropy in (BO4)5− groups,
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2.2. 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 [47–50] and SrB4 O7 [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 LaBeB3 O7 , which was synthesized in 2013. LaBeB O7 7, 95 Crystals 32017,
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LaBeB3 O7 crystallizes in the orthorhombic space group Pnm21 [55]. Two symmetrically 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 independent B sites exist in the lattice. The B(1) sites are fully occupied by boron atoms, while B(2) are disordered occupied by boron and beryllium atoms in the molar ratio of 1:1. Via sharing the corner sites are disordered occupied by boron and beryllium atoms in the molar ratio of 1:1. Via sharing the oxygen atoms, two (B(1)O4 )5− tetrahedra form an (B2 O7 )9− dimmer. (XO4 ) (X = B and Be) tetrahedra corner oxygen atoms, two9 −(B(1)O4)5− tetrahedra form an (B2O7)9− dimmer. (XO4) (X = B and Be) are connected by the (B2 O7 ) dimmer, and give rise to the three-dimensional network with lanthanum tetrahedra are connected by the (B2O7)9− dimmer, and give rise to the three-dimensional network cations located at the channel of the framework (Figure 13). The anionic tetrahedra are aligned almost with lanthanum cations located at the channel of the framework (Figure 13). The anionic tetrahedra in the same direction. Regardless of the small second-order susceptibility of (BO4 )5− groups, the are aligned almost in the same direction. Regardless of the small second-order susceptibility of favorable superposition also results in the relatively large SHG response (1 ~ 2 × KDP). Moreover, the (BO4)5− groups, the favorable superposition also results in the relatively large SHG response (1 ~ 2 × powder SHG test revealed that LaBeB3 O7 is phase-matching for the light at 1064 nm. It is speculated KDP). Moreover, the powder SHG test revealed that LaBeB3O7 is phase-matching for the light at 1064 that the Be/B disorder occupation brings about a relatively large birefringence (0.03@1634 nm) and nm. It is speculated that the Be/B disorder occupation brings about a relatively large birefringence improves the phase-matching ability. (0.03@1634 nm) and improves the phase-matching ability.
Figure 13. 13.The The structure of LaBeB 3O7 along (a)direction [010] direction anddirection. (b) [001] The structure of LaBeB (a) [010] and (b) [001] Thedirection. ((Be/B(2))O 3 O7 along 4) 5 − 5− ((Be/B(2))O 4) group is by green the (B(1)O 4) group represented by group is represented byrepresented green tetrahedron andtetrahedron the (B(1)O4 ) and group is represented byis pink tetrahedron. pink tetrahedron.
2.3. Borate Crystals Containing the B-O Combinational Groups 2.3. 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 3UV − groups 5 −borates In (BO some and DUV NLO synthesized in the last of decade, groups are B-O not merely or (BO groups, but the combination these the twoB-O types of basic 3) 4) 3− groups or (BO4)5− groups, but the combination of these two types of basic B-O groups. merely (BO 3 ) groups. In this section, several representative NLO compounds in this type of borates are selected. In this section, several representative NLO in compounds in this type ofofborates are selected. The combinational B-O groups can also result the good NLO properties borate crystals in the The UV combinational B-O groups can also result in the good NLO properties of borate crystals in the UV and even DUV regions. and even DUV regions. K3 B6 O10 Br and K3 B6 O10 Cl with (B6 O13 )8− K3B6O10Br and K3B6O10Cl with (B6O13)8− K3 B6 O10 Br and K3 B6 O10 Cl crystallize in space groups of trigonal R3m, and the crystals with K3B6halogen O10Br and K3B6O 10Cl crystallize with in space of trigonal R3m, and the crystals different anions are isostructural eachgroups other [56,57]. By sharing the common (BO4with )5 − , 7− groups different anions isostructural with Bythe sharing the 4)5−, their (B3 Ohalogen areare interconnected with each each other other,[56,57]. forming (B6 O13 )8−common building(BO block. 8) 7− 8− 8 − their 8) groups are interconnected with each forming the 6O13) building block. (B ) 3Ogroups are further connected, generating theother, 3D framework, with(Bpotassium and halogen 6 O13(B (B6Olocated 13)8− groups further of connected, generating the 3DExcept framework, withoff potassium and halogen ions in theare interstice the framework (Figure 14). some slant the (a,b) plane, almost ions the interstice of the framework (Figure The 14). favorable Except some slant off the (a,b) plane, all thelocated (BO3 )3 −ingroups are aligned in the same orientation. superposition of second-order 3− almost all theand (BOoptical 3) groups are aligned the same orientation. The favorable superposition of susceptibility anisotropic bringin about the large SHG response and birefringence [58–60]. second-order susceptibility and optical anisotropic bring about the large SHG response and Moreover, the partial elimination of dangling bonds at oxygen atoms result in the good transmittance birefringence [58–60]. Moreover, the partial elimination of dangling atoms result in in DUV region with the absorption 180 nm [60] and 182 nm [58] bonds for K3at B6oxygen O10 Cl and K3 B 6 O10 Br 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].
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 Crystals aligned in the same direction. Regardless of the small second-order susceptibility of 2017,almost 7, 95 10 of 16 (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 respectively. All these properties indicate that K3 B6 O10 X could be a good candidate for second- and nm. It is speculated that the Be/B disorder occupation brings about a relatively large birefringence third-harmonic generation for laser at 1064 nm [61,62]. (0.03@1634 nm) and improves the phase-matching ability. Table 1. The ultraviolet (UV) edge, second-harmonic generation (SHG) coefficients and birefringence value of borate-based nonlinear optical (NLO) compounds. Chemical Formula (Abbreviation)
Space Group
UV Edge (nm)
SHG Coefficient (pm/V) or Powder SHG Efficiency )3−
∆n(@1064 nn)
Borate crystal in which B-O groups are (BO3 only R32 147 [26] d11 = 0.49 [68] 0.077 [36] R32 151 d11 = 0.50 0.058 R32 152 d11 = 0.45 ± 0.01 0.073(@694.3 nm) P63 147(cal) 0.32 × KDP 0.081(cal@200 nm) R32 190 2.82 × KDP 194 [32] 3.01 × KDP [32] KZn2 BO3 Cl2 [32] R32 (~200) [31] 1.3 × KDP [31] 2.85 × KDP [32] Figure 13. The structure of LaBeB 3O7 along (a) [010] direction and (b) [001] direction. The RbZn2 BO3 Cl2 [31,32] R32 198 [32] 1.17 × KDP [31] 5− ((Be/B(2))O 4) group is represented by green209 tetrahedron and2.68the (B(1)O by KZn2 BO3 Br R32 [32] × KDP [32] 4) group is represented 2 [31,32] RbZn BO Br [31,32] R32 214 [32] 2.53 × KDP [32] 2 3 2 pink tetrahedron. Sr2 Be2 B2 O7 (SBBO) [33] P-6c2 155 d22 = 2.0–2.48 0.062(@589 nm) NaCaBe2 B2 O6 F [37] Cc 190 1/3 × KDP K3 Ba3 Li2 Al4 B6 O20 F [38] P-62c 190 1.5 × KDP 0.052(cal@532 nm) 2.3. Borate Crystals Containing the B-O Combinational Groups 1.2 × KDP Rb3 Al3 B3 O10 F [39] P31 c < 200 deff = 0.76 LiSr (BO ) [40] Cc 186 0.056(cal) 3 2 In some UV and DUV NLO borates synthesized in the2 ×last KDP decade, the B-O groups are not NaSr3 Be3− R3m 170 5 × KDP 0.06 3 B3 O9 F4 [41] 5− merely (BO3) groups or (BO4) groups, but the combination dof these two types of basic B-O groups. eff = 0.62 NaBeB O [42] Pna21 170(cal) [14] × KDP In this section,3 6several representative NLO compounds in 1.6 this type of borates are selected. The Na2 Be4 B4 O11 [43] P1 171 1.3 × KDP combinational groups can Pc also result in the properties of borate crystals in the UV LiNa5 Be12 BB-O 169 good NLO 1.4 × KDP 12 O33 [43] deff = 0.27 O7 [42] P21 186(cal) [14] and evenγ-KBe DUV 2 B3regions. 0.68 × KDP deff = 0.29 β-KBe2 B3 O7 [42] Pmn21 187(cal) [14] 0.75 × KDP 8− K3B6O10Br and K3B6O10Cl with (B6O13) deff = 0.31 RbBe2 B3 O7 [42] Pmn21 179(cal) [14] 0.79 × KDP O15 [44] C2 192(cal) [14] groups of 1.17trigonal × KDP 2 CsBe 6 B5and K3BNa 6O 10Br K3B6O10Cl crystallize in space R3m, and the crystals with 218 [46] 1.5 × KDP [46] CsZn2 B3 O7 [45,46] Cmc21 0.056(cal) [46] different halogen anions are isostructural with each other(3.3 [56,57]. By sharing the common (BO4)5−, (
