Effect of Rigid Units on the Symmetry of the Framework: Design andSynthesis of Centrosymmetric NaBa4(B5O9)2F2Cl andNoncentrosymmetric NaBa4(AlB4O9)2Br3Hongwei Yu,†,‡ Shilie Pan,*,† Hongping Wu,*,† Zhihua Yang,† Lingyun Dong,†,‡ Xin Su,†

Bingbing Zhang,†,‡ and Hongyi Li†,‡

†Key Laboratory of Functional Materials and Devices for Special Environments, CAS; Xinjiang Key Laboratory of ElectronicInformation Materials and Devices, Xinjiang Technical Institute of Physics & Chemistry, 40-1 South Beijing Road, Urumqi 830011,China‡University of Chinese Academy of Sciences, Beijing 100049, China

*S Supporting Information

ABSTRACT: Two similarly stoichiometric borate halides, Na-Ba4(AlB4O9)2Br3 and NaBa4(B5O9)2F2Cl, have been successfullydesigned and synthesized, and their structures were determined bysingle-crystal X-ray diffraction. Their crystal structures feature the[AlB4O9]∞ and [B5O9]∞ networks, respectively. NaBa4(AlB4O9)2Br3 isnoncentrosymmetric and crystallizes in polar space group P42nm, whileNaBa4(B5O9)2F2Cl is centrosymmetric and crystallizes in monoclinicspace group P21/n. Powder second-harmonic generation (SHG)measurements reveal that NaBa4(AlB4O9)2Br3 has an optical non-linearity comparable to that of KH2PO4 (KDP) and is type I phase-matchable. In addition, infrared and UV−Vis−NIR diffuse reflectancespectroscopy, as well as electronic band structure calculations, wereperformed on the reported materials.

■ INTRODUCTION

Nonlinear optical (NLO) crystals are of academic andcommercial interest attributable to their prominent applicationsin the laser science and optoelectronic devices.1−10 However,the basic requirement for NLO materials is that they mustcrystallize in the noncentrosymmetric (NCS) space group. Todesign and synthesize the NCS materials, various strategieshave been reported. In the strategies, the most considered oneis to adopt the NCS building units, which consist of distortedpolyhedra with a d0 cation center resulting from a second-orderJahn−Teller (SOJT) effect,11 polar displacement of a d10 cationcenter,12 or distortion from the stereochemically active lonepair (SCALP) effect of cation.13 However, the NCS buildingunits often exhibit the absorption in the UV region; therefore,they seem to be less suitable for the design of UV NLO crystals.In our group, by introducing the halogen atoms into borates,

some good NLO materials with wide transparence window andlarge second harmonic generation (SHG) response have beenobtained.5 Also, in the materials, K3B6O10Cl exhibits the largeSHG response and deep UV absorption edge, and is apromising SHG crystal.5c,14 Recently, we realized thatintroducing the relatively rigid units into the B−O frameworkcan lead to a large distortion of B−O groups, which tends togenerate large SHG responses for the materials. Based on theseunderstandings, we have successfully synthesized a new deep

UV NLO crystal Cs2B4SiO9, which exhibits the large SHGresponse and deep UV cutoff edge.5b

In this study, we intend to introduce a new rigid unit, AlO4tetrahedron, into B−O framework to design a [AlB4O9]∞framework similar to the [SiB4O10]∞ framework, which isresponsible for the large SHG response and short UV cutoffedge of Cs2B4SiO9. Also, to better understand the effect of therigid units on the symmetry of the framework, centrosymmetricNaBa4(B5O9)2F2Cl are also synthesized by using B atoms tosubstitute Al atoms.

■ EXPERIMENTAL SECTIONReagents. LiBr (Tianjin YaoHua Chemical Reagent Co., Ltd.,

99.0%), NaCl (Tianjin HongYan Chemical Reagent Co., Ltd., 99.0%),NaBr (Tianjin HongYan Chemical Reagent Co., Ltd., 99.0%), BaCO3(Tianjin Bodi Chemical Co., Ltd., 99.0%), BaF2 (Tianjin BodiChemical Co., Ltd., 99.0%), BaBr2 (Hebei Xingyinhe Chemical Co.,Ltd., 99.0%), and H3BO3 (Tianjin HongYan Chemical Co., Ltd.,99.5%) are used as received.

Crystal Growth. Single crystals of NaBa4(AlB4O9)2Br3 were grownfrom a high temperature solution by using Li2O−Na2O as the fluxsystem. The solution was prepared in a platinum crucible by melting amixture of Na2CO3, BaCO3, Al2O3, H3BO3, LiBr, and NaBr at a molar

Received: April 1, 2013Revised: June 26, 2013Published: July 1, 2013

Article

pubs.acs.org/crystal

© 2013 American Chemical Society 3514 dx.doi.org/10.1021/cg4004774 | Cryst. Growth Des. 2013, 13, 3514−3521

ratio of 0.5:4:1:8:2:1. The mixture (20 g) was heated in aprogrammable temperature electric furnace at 850 °C and held atthis temperature for 20 h until the solution became transparent andclear. The homogenized solution was then cooled rapidly. During theprocess of cooling, a platinum wire was intermittently dipped into thesolution to try the initial crystallization temperature. When thetemperature was cooled to 750 °C, there were some small crystalsgenerated on the surface of solution. It then was further cooled to 500°C at a rate of 2 °C/h. Finally, it was allowed to cool to roomtemperature by powering off the furnace.Similarly, single crystals of NaBa4(B5O9)2F2Cl were also grown by

spontaneous crystallization with NaF−BaF2−BaCl2 as the flux system.A mixture of NaF, BaF2, BaCl2, and H3BO3 with a molar ratio of4:1.5:1.2:4 melts in a platinum crucible, which was placed in the centerof a programmable temperature electric furnace. The furnace wasgradually heated to 800 °C, held at this temperature for 20 h, and thencooled rapidly to 700 °C; an initial crystallization temperature wastried in the same way with NaBa4(AlB4O9)2Br3. It then was furthercooled to 550 °C at a rate of 2 °C/h, and then the furnace waspowered off.X-ray Crystallographic Studies. A few millimeter-sized blocky

crystals of NaBa4(AlB4O9)2Br3 and NaBa4(B5O9)2F2Cl have beenobtained by the above spontaneous crystallization technique for thestructure determination (Figure S1 in the Supporting Information).The single crystals of NaBa4(AlB4O9)2Br3 and NaBa4(B5O9)2F2Cl withdimensions 0.198 mm × 0.128 mm × 0.059 mm and 0.120 mm ×0.086 mm × 0.079 mm were selected for the structure determination,respectively. Their crystal structures were determined by single-crystalX-ray diffraction on an APEX II CCD diffractometer usingmonochromatic Mo Kα radiation (λ = 0.71073 Å) at 296(2) K andintergrated with the SAINT program.15 Numerical absorptioncorrections were carried out using the SCALE program for areadetector.15 All calculations were performed with programs from theSHELXTL crystallographic software package.16 All atoms were refinedusing full matrix least-squares techniques; final least-squares refine-ment is on Fo

2 with data having Fo2 ≥ 2σ(Fo

2). The structures werechecked for missing symmetry elements by the program PLATON.17

Crystal data and structure refinement information are presented inTable 1. The final refined atomic positions and isotropic thermalparameters are listed in Table S1 in the Supporting Information.Selected bond distances (Å) and angles (deg) for NaBa4(AlB4O9)2Br3and NaBa4(B5O9)2F2Cl are given in Table S2 in the SupportingInformation.

Compound Synthesis. The polycrystalline powders of Na-Ba4(AlB4O9)2Br3 and NaBa4(B5O9)2F2Cl were synthesized by tradi-tional solid-state reaction techniques. The stoichiometric mixtures ofNaBr, BaBr2, BaCO3, Al2O3, and H3BO3 at a molar ratio 1:1:3:1:8 forNaBa4(AlB4O9)2Br3 and NaCl, BaF2, BaCO3, and H3BO3 at a molarratio 1:1:3:10 for NaBa4(B5O9)2F2Cl were ground well and packedinto Pt crucibles, respectively. The raw materials were heated to 400°C, and held for 10 h to decompose the carbonate and eliminate thewater. They were then heated to 800 °C (650 °C forNaBa4(B5O9)2F2Cl) and held for 48 h. The two mixtures wereground between all heatings. The powders of NaBa4(AlB4O9)2Br3 andNaBa4(B5O9)2F2Cl were obtained. The powder X-ray diffraction datawere carried out with a Bruker D2 PHASER diffractometer equippedwith a diffracted beam monochromator set for Cu Kα radiation (λ =1.5418 Å). The diffraction patterns were taken from 10° to 70° (2θ)with a scan step width of 0.02° and a fixed counting time of 1 s/step.The diffraction patterns are well agreeable with the calculated ones(Figure 1).

Infrared Spectroscopy. IR spectra of NaBa4(AlB4O9)2Br3 andNaBa4(B5O9)2F2Cl were recorded on a Shimadzu IR Affinity-1 Fouriertransform infrared spectrometer in the 400−4000 cm−1 range. Thesamples were mixed thoroughly with dried KBr, and the characteristic

Table 1. Crystal Data and Structure Refinement for NaBa4(AlB4O9)2Br3 and NaBa4(B5O9)2F2Cl

empirical formula NaBa4(AlB4O9)2Br3 NaBa4(B5O9)2F2Clformula weight 1240.52 1041.90crystal system tetragonal monoclinicspace group, Z P42nm, 2 P21/n, 2unit cell dimensions a = 12.144(5) Å a = 6.7397(17) Å

b = 11.422(3) Åc = 6.824(6) Å c = 11.448(3) Å

β = 91.906(3)°volume 1006.4(10) Å3 880.8(4) Å3

density (calcd) 4.094 Mg/m3 3.929 Mg/m3

θ range for data collection 2.37−27.45° 2.52−27.70°limiting indices −15 ≤ h ≤ 15, −8 ≤ k ≤ 15, −8 ≤ l ≤ 8 −8 ≤ h ≤ 8, −14 ≤ k ≤ 14, −11 ≤ l ≤ 14reflns collected/unique 5826/1212 [R(int) = 0.0325] 5317/2052 [R(int) = 0.0324]completeness to θ 99.8% 99.5%GOF on F2 1.137 1.027final R indices [Fo

2 > 2σ(Fo2)]a R1 = 0.0316, wR2 = 0.0986 R1 = 0.0258, wR2 = 0.0547

R indices (all data)a R1 = 0.0328, wR2 = 0.0998 R1 = 0.0318, wR2 = 0.0574absolute structure parameter 0.04(4)extinction coefficient 0.0094(7) 0.0072(3)largest diff. peak and hole 1.388 and −2.829 e Å−3 1.196 and −0.875 e Å−3

aR1 = ∑∥Fo| − |Fc∥/∑|Fo| and wR2 = [∑w(Fo2 − Fc

2)2/∑wFo4]1/2 for Fo

2 > 2σ(Fo2).

Figure 1. Experimental and calculated X-ray diffraction patterns ofNaBa4(AlB4O9)2Br3 and NaBa4(B5O9)2F2Cl. The black curves areexperimental patterns; the red ones are calculated ones.

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absorption peaks were shown in Figure S2 in the SupportingInformation.UV−Vis−NIR Diffuse Reflectance Spectrum. Optical diffuse

reflectance spectra of NaBa4(AlB4O9)2Br3 and NaBa4(B5O9)2F2Clwere measured at room temperature with a Shimadzu SolidSpec-3700DUV spectrophotometer. Data were collected in the wavelengthrange 190−2600 nm. Also, reflectance spectra were converted toabsorbance with the Kubelka−Munk function.18,19

Second-Order NLO Measurements. The SHG test wasperformed on the powder sample of NaBa4(AlB4O9)2Br3 by theKurtz−Perry method.20 The polycrystalline NaBa4(AlB4O9)2Br3 wasground and sieved into distinct particle size ranges, <20, 20−38, 38−55, 55−88, 88−105, 105−150, and 150−200 μm, and the micro-crystalline KDP was also served as a reference. The sample was thenplaced in a 0.2 mm thick quartz cell and irradiated by a Q-switchedNd:YAG solid-state laser (1064 nm, 10 kHz, 10 ns). The intensity ofthe frequency-doubled output emitted from the sample using aphotomultiplier tube was measured.Numerical Calculation Details. To investigate a deep structure−

property relationship, the electronic structures were obtained usingdensity functional theory (DFT)-based ab initio calculationsimplemented in the CASTEP package.21 The exchange-correlationpotential was calculated using Perdew−Burke−Ernzerhof (PBE)function within the Generalized Gradient Approximation (GGA)scheme.22 For the purpose of achieving energy convergence, a plane-wave basis set energy cutoff was 570.0 eV within ultrasoftpseudopotential. Also, the Monkhorst−Pack scheme was set at 3 ×3 × 5 in the Brillouin zone (BZ) of the primitive cell for the totalenergy calculations. The following orbital electrons were treated asvalence electrons: Na, 2s22p63s1; Ba, 5s25p66s2; Al, 3s23p1; B, 2s22p1;O, 2s22p4; F, 2s22p5; Cl, 3s23p5; Br, 4s24p5. Other parameters used inthe calculations were set by the default values of the CASTEP code.

■ RESULTS AND DISCUSSION

Crystal Structure. NaBa4(AlB4O9)2Br3 crystallizes in spacegroup P42nm of the tetragonal system. In the asymmetric unit,there is one unique Na atom, one unique Ba atom, one uniqueAl atom, three unique B atoms, six unique O atoms, and two Bratoms (Table S1a in the Supporting Information). The basicbuilding units of NaBa4(AlB4O9)2Br3 are the isolated AlO4tetrahedra and B4O9 groups, which are composed of two BO3triangles and two BO4 tetrahedra by sharing O atoms. EachB4O9 group is linked to four different AlO4 tetrahedra throughits terminal O atoms, and likewise each AlO4 tetrahedron sharesits four vertices with four neighboring B4O9 groups to form 3D

[AlB4O9] backbone. The Na, Ba, and Br atoms are filled in thecavities of [AlB4O9] backbone (Figure 2). The Na atoms arecoordinated by four O atoms and one Br atom with Na−Obond lengths and Na−Br bond lengths 2.617(6) and 2.953(16)Å, respectively. The Ba atoms are coordinated by seven Oatoms and three Br atoms with Ba−O bond lengths and Ba−Brbond lengths ranging from 2.713(7) to 3.315(7) Å and3.226(3) to 3.352(4) Å, respectively. The Al atoms arecoordinated by four O atoms with the Al−O bond lengthsranging from 1.733(7) to 1.763(9) Å. The B atoms have twocoordinated environments, BO3 and BO4. For the BO3

triangles, the B−O bond lengths range from 1.349(12) to1.381(12) Å, and for BO4 tetrahedra, the B−O bond lengthsrange from 1.408(15) to 1.543(17) Å. All of the bond lengthsare consistent with those observed in other compounds.23−25

The results of bond valence calculations (Na, 1.138; Ba, 2.238;Al, 3.104; B, 3.029−3.048; Br, 0.970−1.299) indicate that theNa, Ba, Al, B, and Br atoms are in oxidation states of +1, +2, +3,+3, and −1, respectively.26,27NaBa4(B5O9)2F2Cl crystallizes in space group P21/n of the

monoclinic system. In the asymmetric unit, there is one uniqueNa atom, two unique Ba atoms, three unique B atoms, nineunique O atoms, one F atom, and one Cl atom (Table S1b inthe Supporting Information). Its crystal structure is also the 3D[B5O9]∞ framework, which is composed of the basic buildingunit B5O12. The Na atoms, Ba atoms, F atoms, and Cl atomsare located in the space of the [B5O9]∞ network (Figure 3).The Na atoms are coordinated by four O atoms and two Fatoms with the Na−O bond lengths ranging from 2.415(3) to2.566(4) Å. The Ba atoms have two coordinated environments:Ba(1) are coordinated by seven O atoms, one F atom, and oneCl atom; Ba(2) are coordinated by six O atoms, two F atoms,and one Cl atom. The Ba−O bond lengths range from 2.663(3)to 3.089(3) Å, the Ba−F bond lengths range from 2.663(3) to3.089(3) Å, and the Ba−Cl bond lengths range from 3.224(6)to 3.493 (9) Å. For the BO3 triangles, the B−O bond lengthsrange from 1.338(6) to 1.375(6) Å, and for BO4 tetrahedra, theB−O bond lengths range from 1.426(6) to 1.507(6) Å. Thebond lengths are also consistent with those observed in othercompounds.23−25 The results of bond valence calculations (Na,1.044; Ba, 2.013−2.230; B, 3.025−3.226; F, 0.700; Cl, 0.993)

Figure 2. The structure of NaBa4(AlB4O9)2Br3.

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indicate that the Li, Ba, B, F, and Cl atoms are in oxidationstates of +1, +2, +3, −1, and −1, respectively.26,27Effect of Rigid AlO4 Tetrahedra on the Symmetry of

the Framework. NaBa4(AlB4O9)2Br3 and NaBa4(B5O9)2F2Clhave similar stoichiometry, and from the formula, they seem tobe the simple substitution in same group elements (B→Al; Fand Cl→Br). However, they exhibit distinctly different crystalstructures. NaBa4(AlB4O9)2Br3 crystallizes in polar space groupP42nm, while NaBa4(B5O9)2F2Cl crystallizes in centrosymmet-ric space group P21/n. As described above, NaBa4(AlB4O9)2Br3contains two kinds of basic building units, B4O9 and AlO4tetrahedra. The B/Al molar ratio in the structure is 4. That is,the amount of B4O9 groups and AlO4 tetrahedra is equal, and itis worth noting that both B4O9 groups and AlO4 tetrahedrahave the four unsaturated terminal O atoms. Thus, when theB4O9 and AlO4 tetrahedra connect with each other to form theframework, the connection between B4O9 and AlO4 units mustbe that each B4O9 group is linked to four different AlO4tetrahedra through its terminal O atoms, and likewise eachAlO4 tetrahedron shares its four vertices with four neighboringB4O9 groups, because the connection can make B4O9 groupsand AlO4 tetrahedra spread out as much as possible. The singleconnection reduces the possibility that the material crystallizesin the centrosymmetric space group. However, in Na-Ba4(B5O9)2F2Cl, the basic building units are the B5O12 groups,which have more terminal O atoms. More importantly, there

are no other heteroatom groups in the [B5O9]∞ framework ofNaBa4(B5O9)2F2Cl. Therefore, when B5O12 groups connectwith each other to form the [B5O9]∞ framework, they canchoose more flexible connections, which favor the centrosym-metric structure of NaBa4(B5O9)2F2Cl.To better understand the effect of AlO4 tetrahedra on the

symmetry of the [AlB4O9] and [B5O9] net, we further calculatethe dipole moments of all of the B−O groups in the unit cellswith a simple bond-valence approach,28 and the results areshown in Table 2. It is clear that the [AlB4O9] frameworkgenerates a net dipole moment 0.92 D along the c axis in theunit cell, while in NaBa4(B5O9)2F2Cl, the dipole momentsalong all directions are canceled. The results are consistent withthe polarization observed from their crystal structures (Figure4). Furthermore, the effect of the rigid units on the distortionsof the B−O groups in framework can be made clear bycomparing the dipole moments of B−O groups inNaBa4(AlB4O9)2Br3, NaBa4(B5O9)2F2Cl, and Cs2SiB4O10. InCs2SiB4O10, all of the Si−O bonds have the same bond lengths,which indicate that SiO4 tetrahedra are absolutely rigid. Theabsolutely rigid SiO4 tetrahedra lead to the considerably largeB−O group distortions (7.44 D for the BO4 tetrahedra).

5b InNaBa4(AlB4O9)2Br3, the AlO4 tetrahedra are also rigid becausethe local dipole moment of the AlO4 tetrahedra is obviouslysmaller than all of the B−O groups (Table 2). It is also foundthat the local dipole moments of the B−O groups inNaBa4(AlB4O9)2Br3 are a little larger than those inNaBa4(B5O9)2F2Cl (Table 2). Those indicate that the rigidunits do have the effects on the distortions of B−O groups andthe more rigid units will lead to larger distortions of B−Ogroups.

Infrared Spectroscopy. The IR spectra of Na-Ba4(AlB4O9)2Br3 and NaBa4(B5O9)2F2Cl are shown in FigureS2 in the Supporting Information, and the assignments for theabsorption peaks are listed in Table 3.29−31 It is clear that thetwo spectra both exhibit similar B−O vibrations. However, inthe IR spectrum of NaBa4(AlB4O9)2Br3, two other strong peaksat 480 and 443 cm−1 are observed. They can be assigned to thestretching-bending vibrations of AlO4 units. The results areconsistent with their crystal structures.

UV−Vis−NIR Diffuse Reflectance Spectrum. The opticaldiffuse reflectance spectra of NaBa4(AlB4O9)2Br3 and Na-Ba4(B5O9)2F2Cl in the region 190−2600 nm are shown inFigure 5. Absorption (K/S) data were calculated from theKubelka−Munk function: F(R) = (1 − R)2/2R = K/S. It is clearthat there is no absorption from 200 to 2600 nm for

Figure 3. The structure of NaBa4(B5O9)2F2Cl.

Table 2. Dipole Moment Calculations of NaBa4(AlB4O9)2Br3 and NaBa4(B5O9)2F2Cl

magnitude

compounds species x(a) y(b) z(c) debye 10−4 esu·cm/A3

NaBa4(AlB4O9)2Br3 B(1)O3 −0.40 −0.65 0.61 0.98 63. 99B(2)O4 1.55 −1.55 1.22 2.51 80.79B(3)O4 0.02 −0.02 −2.71 2.71 87.34AlO4 −0.14 0.14 0.50 0.54 17.42unit cell 0 0 0.92 7.39

NaBa4(B5O9)2F2Cl B(1)O4 1.62 −1.07 0.75 2.06 79.18B(2)O3 −1.23 1.08 0.66 1.79 68.53B(3)O4 0.16 1.55 −0.67 1.69 65.02B(4)O3 1.52 0.14 0.37 1.56 59.98B(5)O4 0.02 −0.61 0.69 0.92 35.35unit cell 0 0 0

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NaBa4(B5O9)2F2Cl and from 248 to 2600 nm for Na-Ba4(AlB4O9)2Br3. In their (K/S)-versus-E plots, extrapolatingthe linear part of the rising curve to zero provides their onsetsof absorption (4.13 eV for NaBa4(AlB4O9)2Br3 and 5.12 eV forNaBa4(B5O9)2F2Cl).SHG Measurements. NaBa4(AlB4O9)2Br3 possesses the

NCS crystal structure and may have the potential application asUV NLO materials. Powder SHG measurements using 1064nm radiation reveal that NaBa4(AlB4O9)2Br3 possesses a SHGefficiency comparable to that of KDP (Figure 6a), and is type Iphase-matchable (Figure 6b).20 According to the anionic grouptheory of NLO activity in borates,2,5,6 the nonlinearity of aborate crystal originates from the B−O groups. Therefore, asdescribed in the crystal structure, the B−O groups inNaBa4(AlB4O9)2Br3 are pulled by the rigid AlO4 tetrahedra togenerate a large distortion, and the distortion is enhanced alongthe c axis (Table 2). Therefore, we can draw the conclusion thatthe SHG efficiency of NaBa4(AlB4O9)2Br3 mainly comes fromthe distorted B−O groups.Comparing the SHG response of NaBa4(AlB4O9)2Br3 with

that of Cs2B4SiO9, which also contains the rigid units in theframework of structure and is obtained in our group, it is foundthat the SHG response of Cs2B4SiO9 (∼4.6 × KDP) isobviously larger than that of NaBa4(AlB4O9)2Br3 (∼1 × KDP).

The difference can be ascribed to the different rigid degree ofthe rigid units in their frameworks on SHG responses. Asdescribed above, in Cs2B4SiO9, the SiO4 tetrahedra areabsolutely rigid, which lead to a considerably large distortionof the B−O groups. While in NaBa4(AlB4O9)2Br3, although theAlO4 tetrahedra are also rigid, a little variation of Al−O bondlength can still be observed. Therefore, the relatively strongerrigid degree of SiO4 tetrahedra in Cs2B4SiO9 makes it generatemore distortions of the B−O groups than that ofNaBa4(AlB4O9)2Br3. According to the anionic group theory,the larger distortion of B−O groups will favor generating alarger SHG response.

Band Structures and Density of States. The bandstructures of NaBa4(AlB4O9)2Br3 and NaBa4(B5O9)2F2Cl alongthe high symmetry lines in the unit cell are shown in Figure S3in the Supporting Information. It can be seen thatNaBa4(AlB4O9)2Br3 and NaBa4(B5O9)2F2Cl are both the directgap crystals with band gap values of 3.386 and 5.072 eV,respectively. The value for NaBa4(AlB4O9)2Br3 is smaller thanthe experimental optical band gap 4.13 eV due to thediscontinuity in the derivative of exchange-correlation energywithin density-functional theory.32 Therefore, during calculat-ing optical responses of NaBa4(AlB4O9)2Br3, an energy shift inthe conduction bands or a so-called scissors operator is

Figure 4. (a) The [AlB4O9] framework with the polarization along c axis and (b) the centrosymmetric [B5O9] framework.

Table 3. Assignments of the Infrared Absorption Peaks for NaBa4(AlB4O9)2Br3 and NaBa4(B5O9)2F2Cl

mode description NaBa4(AlB4O9)2Br3 (cm−1) NaBa4(B5O9)2F2Cl (cm

−1)

asymmetric stretching of B3−O 1381, 1313 1415, 1290asymmetric stretching of B4−O 1163, 1041 1095, 1010symmetric stretching of B3−O 983 958symmetric stretching of B4−O 858 837out-of-plane bending of B3−O 707, 669, 628 731, 700bending of B4−O and B3−O 582, 528 594, 557stretching-bending vibrations of AlO4 units 480, 443

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introduced to overcome such a difference.33 To understand thecomposition and origin of the calculated bands, their total andpartial densities of states (DOS and PDOS) are analyzed(Figure 7), and the core states are removed because of theirlittle interest for the optical properties in the visible and UVspectrum.34 For NaBa4(AlB4O9)2Br3, the top of the valenceband is mostly from contributions of O 2p states with a small

amount of Br 3p orbitals. The conduction band is mainly madeup by B 2p, Al 3p, and Na 3s states (Figure 7a). ForNaBa4(B5O9)2F2Cl, the top of the valence band is mostly fromcontributions of O 2p states with a small amount of F 2p andCl 3p. The conduction band is mainly composed of B 2p and O2p unoccupied states (Figure 7b).

Optical Property. As described above, NaBa4(AlB4O9)2Br3is expected to possess a potential application for NLOmaterials. Therefore, its linear and NLO properties were alsocalculated based on the electronic structure. The curve of thelinear refractive indices versus wavelength is shown in Figure S4in the Supporting Information. It is clear that Na-Ba4(AlB4O9)2Br3 is a positive uniaxial crystal with birefringenceΔn = 0.0287 at 532 nm. The SHG coefficients ofNaBa4(AlB4O9)2Br3 were also obtained on the basis of abinitio calculations. NaBa4(AlB4O9)2Br3 belongs to 4mmsymmetry, and consequently has two independent SHGcoefficients under the restriction of Kleinman symmetry. Thecalculations reveal that the SHG coefficients for Na-Ba4(AlB4O9)2Br3 are d33 = 0.86 pm/V, d31 = d15 = 0.27 pm/V, respectively. The calculated SHG coefficient d33 is largerthan that of KDP (d36 = 0.39 pm/V), while d31 and d15 aresmaller than that of KDP. Therefore, the calculated values areconsistent with the experimental ones.

■ CONCLUSION

On the basis of the understanding of the effect of rigid units onthe symmetry of the framework of materials, two new boratehalides, NaBa4(AlB4O9)2Br3 and NaBa4(B5O9)2F2Cl, have beensuccessfully designed and synthesized. Although the twocompounds have similar stoichiometry, they exhibit obviouslydifferent structures. NaBa4(AlB4O9)2Br3 is NCS and polar,while NaBa4(B5O9)2F2Cl is centrosymmetric. Detailed struc-tural analyses show that the difference is mainly due to the rigidAlO4 tetrahedra reducing the degree of freedom of theconnection of the building units and increasing the distortionof B−O groups in framework. Powder SHG measurementsindicate that NaBa4(AlB4O9)2Br3 has a SHG efficiencycomparable to that of KDP, and is type I phase-matchable. Inthe future, we will continue to introduce other rigid units intoborates to design other novel NCS materials and investigatetheir effects on the symmetry of the framework of materials.

Figure 5. The diffuse reflectance spectra for NaBa4(AlB4O9)2Br3 (a)and NaBa4(B5O9)2F2Cl (b).

Figure 6. (a) SHG intensities of NaBa4(AlB4O9)2Br3 with commercial KDP as a reference: Oscilloscope traces for the powder of KDP andNaBa4(AlB4O9)2Br3. (b) Phase-matching, that is, particle size versus SHG intensity, data for NaBa4(AlB4O9)2Br3. The solid curve drawn is to guidethe eye and is not a fit to the data.

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dx.doi.org/10.1021/cg4004774 | Cryst. Growth Des. 2013, 13, 3514−35213519

■ ASSOCIATED CONTENT*S Supporting InformationCIF files, the final refined atomic positions and isotropicthermal parameters, bond distances and angles, figures ofinfrared spectroscopy, and calculated band structures. Thismaterial is available free of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: slpan@ms.xjb.ac.cn (S.P.); wuhp@ms.xjb.ac.cn(H.W.).NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work is supported by the National Key Basic ResearchProgram of China (Grant no. 2012CB626803), the “NationalNatural Science Foundation of China” (Grant nos. 21201176,U1129301, 51172277, 21101168, 11104344), Main DirectionProgram of Knowledge Innovation of Chinese Academy ofSciences (Grant no. KJCX2-EW-H03-03), the “One HundredTalents Project Foundation Program” of Chinese Academy ofSciences, Major Program of Xinjiang Uygur AutonomousRegion of China during the 12th Five-Year Plan Period (Grantno. 201130111), and the “High Technology Research andDevelopment Program” of Xinjiang Uygur Autonomous Regionof China (Grant nos. 201116143, 201315103).

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Figure 7. The total and partial densities of states of NaBa4(AlB4O9)2Br3 (a) and NaBa4(B5O9)2F2Cl (b).

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