A new non-centrosymmetry compound: Synthesis, crystal structure and electronic structure study of hydrated sodium borate Na2[BO2(OH)]·H2O

Accepted Manuscript A new non-centrosymmetry compound: Synthesis, crystal structure and electronic structure study of hydrated sodium borate Na2[BO2(OH)]·H2O Wenwu Zhao PII:

S0022-2860(16)30723-2

DOI:

10.1016/j.molstruc.2016.07.042

Reference:

MOLSTR 22745

To appear in:

Journal of Molecular Structure

Received Date: 10 May 2016 Revised Date:

7 July 2016

Accepted Date: 12 July 2016

Please cite this article as: W. Zhao, A new non-centrosymmetry compound: Synthesis, crystal structure and electronic structure study of hydrated sodium borate Na2[BO2(OH)]·H2O, Journal of Molecular Structure (2016), doi: 10.1016/j.molstruc.2016.07.042. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT Synopsis The material exhibits a three-dimensional structure consisting of interconnecting Na(1)O6 groups, Na(2)O7 units and isolated BO3 triangles. Two Na(1)O6 groups and two Na(2)O7 groups are connected by O atoms to form parallelogram-type structure

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viewed down the c-axis. And two Na(1)O6 groups are located on a pair of edges, which are parallel to each other, in parallelogram-type structure respectively and two

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Na(2)O7 groups are located in another pair of parallel edges severally.

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A new Non-centrosymmetry Compound: Synthesis, Crystal Structure and Electronic Structure Study of Hydrated Sodium Borate

Wenwu Zhaoa,*

Environmental and Chemical Engineering Department, Tangshan College, Hebei,

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a

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Na2[BO2(OH)]·H2O

063000, China; Key Laboratory of Micro-nano Materials Preparation and Application of

*

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Tangshan City, Tangshan 063000, China.

To whom correspondence should be addressed. E-mails: [email protected]. Phone: (86)-315-2010649.

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Abstract:

The new hydrated sodium borate Na2[BO2(OH)]·H2O with noncentrosymmetric (NCS) structure was synthesized through slow evaporation at room temperature utilizing NaOH,

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H3BO3, and LiF as reagents. Its crystal structure was determined from single-crystal X-ray diffraction data and further characterized by FT-IR. It crystallizes in the

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Orthorhombic space group Pca21 with unit cell of dimensions a = 10.697(3) Å, b= 5.2389(15) Å, c = 6.4568(18) Å, Volume = 361.84(18) Å3 and Z = 4. Calculation of magnitude of local dipole moment of BO3 groups indicates that the material maybe display a weak SHG efficiency. Its electronic structure and absorption spectrum for Na2[BO2(OH)]·H2O crystal have been calculated by DFT method. Keywords: Hydrated sodium borate, Na2[BO2(OH)]·H2O, Crystal structure, Slow

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evaporation method, Electronic structure 1. Introduction Borate compounds crystallizing in the NCS crystal classes have been of

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considerable interest because of their propensity to display a wide range of non-symmetry based properties like ferroelectricity, pyroelectricity, piezoelectricity, and second order nonlinear optical (NLO) behavior [1-3]. Meanwhile, Boron atoms coordinate with

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oxygen not only in 4-fold coordination (tetrahedral, BO4) but also in 3-fold coordination (triangular, BO3). The BO3 and BO4 groups may be further linked via common oxygen

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atoms to form isolated rings and cages or polymerize into infinite chains, sheets and networks, leading to rich structural chemistry [4, 5]. In the past several decades, much interest has been focused on studies of alkali metal borate compounds because some of these compounds show interesting physical properties, such as nonlinear optical behavior

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for LiB3O5, CsLiB6O10 CsB3O5, KB5O8·4H2O, Cs2[B7O9(OH)5] and so on [6-11]. About the sodium borate compound, there have been extensively studied [12-16], for example, Na4[B10O16(OH)2]·4H2O, NaB(OH)4·2H2O, Na2B4O12H10, Na2B5O8(OH)·2H2O,

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Na[B6O8(OH)3]·6H2O·H3BO3, Na2B4O7·10H2O, NaB5O8·5H2O, Na2[BO2(OH)] and so on. In our work, much effort has been made to search for new polar materials, our

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investigation of the alkali metal borate system resulted in the finding of NCS materials, Na2[BO2(OH)]·H2O. Single crystal X-ray diffraction reveals that it crystallizes in the monoclinic system, space group Pca21 (NO.29) with unit cell of dimensions a = 10.697(3) Å, b= 5.2389(15) Å, c = 6.4568(18) Å, Volume = 361.84(18) Å3 and Z = 4. Herein its synthesis, crystal structure, IR spectrum, electronic structure and absorption spectrum are reported.

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2. Experimental sections 2.1 Crystal synthesis Polycrystalline samples of the Na2[BO2(OH)]·H2O compound were obtained from

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slow evaporation method at room temperature. All reagents used in the synthesis were of analytical grade. NaOH, H3BO3 and LiF powders were mixed in a molar ratio of 1:1:1 and then dissolved in deionized water. The solution was thoroughly mixed under

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ultrasonic agitation at 50 °C for 60 min and then cooled down to room temperature. The obtained solution was allowed to evaporate slowly at room temperature. After about 3

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months, the crystalline products were obtained and washed with excess deionized water. 2.2 Single-crystal X-ray crystallography

The crystal structure of Na2[BO2(OH)]·H2O was determined by standard crystallographic methods: A clear light crystal sized with 0.152 × 0.113 × 0.067 mm3

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was mounted on a thin glass fiber with silicone oil. It was investigated by SMART APEX II Single-Crystal diffractometer using monochromatic Mo Kα radiation (λ = 0.71073 Å) at 296(2) K and integrated with the SAINT-Plus program [17]. All refinements were

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completed with programs from the SHELXTL crystallographic software package [18]. The structure was solved by direct methods. The crystal structure of Na2[BO2(OH)]·H2O

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was solved in space group Pca21(NO.29) which was recommended by the SHELXTL crystallographic software package and all the atoms were located by subsequent cycles of refinements and Fourier difference maps. Then, we obtained the formula of the compound. The final full-matrix least-squares refinement was on Fo2 with data having Fo2 ≥ 2σ(Fo2) and all atoms were refined with anisotropic thermal parameters. The final refinement was converged with R1 = 0.0303 and wR2 = 0.0707. The final difference

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Fourier synthesis may have shown maximum and minimum peaks at 0.318 and -0.290 e /Å3, respectively. Then the structure was checked for missing symmetry elements with PLATON [19].

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The crystallographic data, the details of X-ray data collections, and refinement parameters for the structure determination are presented in Table 1. The final atomic coordinates, equivalent isotropic displacements for Na2[BO2(OH)]·H2O are given in

2.3 Infrared spectroscopy

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are given in Table S2 in Supporting Information.

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Table S1 in Supporting Information. Selected bond distances (Å) for Na2[BO2(OH)]·H2O

The infrared spectroscopy was recorded on the BRUKER EQUINOX 55 Fourier transform infrared spectrometer. The sample was in pellet form in the KBr phase (5 mg of the sample and 500 mg of KBr). The spectrum was collected using the

of 2 cm-1.

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Na2[BO2(OH)]·H2O crystal powder in the range from 400 to 4000 cm-1 with a resolution

2.5 Theoretical calculations

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In all these calculations, the CASTEP module based on the total plane-wave pseudopotential method of Materials Studio package has been employed [20]. The

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exchange-correlation effects were treated with the local density approximation (LDA) with the Ceperley and Alder-Perdew-Zunger (CA-PZ) functional [21, 22]. The Monkhorst-Pack scheme k-points grid sampling was set at 2 × 3 × 2 for the Brillouin zone (BZ). The plane-wave basis set energy cutoff was set at 750.0 eV and norm-conserving pseudopotential was used for all chemical elements. The other calculation parameters and convergent criteria were the default values of the CASTEP

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code. 3. Results and discussions 3.1. Crystal structure description

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Na2[BO2(OH)]·H2O crystallizes in the NCS space group Pca21 with unit cell of dimensions a = 10.697(3) Å, b= 5.2389(15) Å, c = 6.4568(18) Å, Volume = 361.84(18) Å3 and Z = 4, and the crystal structure is illustrated in Fig. 1. There are two sodium atoms,

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one boron atom, four oxygen atoms and three hydrogen atoms in the NCS structure. The material exhibits a three-dimensional structure consisting of interconnecting Na1O6

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groups, Na2O7 units and isolated BO3 triangles.

From Fig. 1(a), each B atom connects three O atoms, with the length of the bond from 1.271(4) Å to 1.287(4) Å, and the mean distance for B-O bonds is 1.277 Å. The O-B-O angles are ranging from 119.7(3)° to 120.3(3)° with an average of 120°, which is

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corresponding to the sp2 hybridization of B atoms. As shown in Fig. 1(b), this structure offers two crystallographic positions for the Na+ cations differing in coordination number with Na-O bonds ranging from 2.320(3) to 2.927(3) Å. Na1 atom is coordinated to six O

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atoms, which are one O1, two O2, two O3 and one O4 atoms, with the bonds ranging between 2.320(3) Å to 2.565(3) Å (see Table S2 in the Supporting information) to form

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the Na1O6 group. Na2 atom is coordinated to seven O atoms to form the Na2O7 group (Table S2 in the Supporting information) with seven Na-O bond-lengths ranging between 2.298(4) and 2.927(3) Å. From Fig. 1(a), two Na1O6 groups and two Na2O7 groups are connected by O2,

O3and O4 atoms to form parallelogram-type structure viewed down the c-axis. And two Na1O6 groups are located on a pair of edges, which are parallel to each other, in

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parallelogram-type structure respectively and two Na2O7 groups are located in another pair of parallel edges severally. It is observed that each Na1O6 group and Na2O7 group are distorted, but when they were connected together to form parallelogram-type

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structure, the distortion is offset. Every parallelogram-type structure is connected extended along the b-axis by sharing the O1, O2 and O4 atoms to shape the infinite chain structure (chain b). And each chain b is axisymmetric configuration with its adjacent

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chains with b-axis as the symmetric axis. Along a-axis, Every parallelogram-type structure is also connected to form chain (chain a) by sharing the other O2, O3and O4

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atoms, but the direction of the parallelogram-type rings, which are interconnected in chain a, are opposite to each other. Viewed down the c-axis, there are two BO3 triangles, differing at small angle to each other with b-axis as the symmetric axis, at the center of gap formed by two chain b and two chain a. Viewed down the a-axis (see Fig.2(a)),

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Na1O6 group and Na2O7 group are connected by O atoms to form layer-structure extended along the (110), and then BO3 units are located between layer and layer. As shown in Fig. 2(b), each BO3 triangle plane, which is opposite to each other along b-axis,

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is at small angle to the (001) face viewed down a-axis. From the structure of the Na2[BO2(OH)]·H2O crystal (Fig. 1(a) and 2), it can be seen

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that the BO3 units are opposite to each other along b-axis and a-axis, and the BO3 triangle plane is at small angle to the (001) face. So, it is observed that the main SHG efficiency of the compound comes from the addition of the distortion of BO3 triangles in the c-axis according to the anion group theory [23]. To confirm this speculation, the direction and magnitude of the distortion in BO3 groups can be further quantified by determining the local dipole moments. The bond-valence theory [24, 25] was used to estimate the

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distribution of the electrons on every B atoms in the unit cell. As shown in Table 2, it is clear that the polarizations of all the BO3 groups along the a-axis and b-axis are cancelled and they are enhanced along the c-axis, but the local dipole moment is too small to result

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that the Na2[BO2(OH)]·H2O structure maybe display only a weak SHG efficiency. 3.2. Infrared spectroscopy analysis

To analyze the presence of functional groups in Na2[BO2(OH)]·H2O qualitatively. The

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infrared spectroscopy (see Fig. S1 in the Supporting Information) was recorded on the BRUKER EQUINOX 55 Fourier transform infrared spectrometer. The sample was in

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pellet form in the KBr phase. As shown in Fig. S1, the presence of the lattice water and OH groups in this structure are confirmed by the O-H stretching near 3361 cm-1and the H-O-H bending at about 1659 cm-1. The bands observed in the 1300-1500 cm-1 region in the FTIR spectrum, at 1352, 1429 and 1481 cm-1 are characteristics of B-O asymmetric

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stretching vibrations in BO3 and which is in agreement with other compounds containing BO3 anionic groups [10, 14, 26]. According to report in the literature [27], the bands at 1134, 1260 cm-1 might be assigned as the bending modes of B-O-H. The characteristic

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B-O symmetric stretching modes of BO3 groups appear at 947 cm-1 [10, 14, 28]. The strong bands observed at 710 and 825 cm-1, are assigned to the bending modes of B-O in

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BO3 groups.

3.3. Electronic structure

Fig. 3 shows the calculated band structures of Na2[BO2(OH)]·H2O in the primitive unit cell, which is plotted along symmetry by density functional theory (DFT) method. In the band structures, Na2[BO2(OH)]·H2O is a direct gap crystal and the direct gap is 4.15 eV at G point with the valence band (VB) maximum being -0.14 eV and the conduction band

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(CB) minimum being 4.01 eV. The total density of states（DOS）of Na2[BO2(OH)]·H2O and partial density of states (PDOS) of Na, B, O and H atoms obtained are shown in Fig. 4. The 2p orbital of the Na atom is strongly localized at about -21.72 eV, while the 2s

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orbital of the O atom is localized between -20.56eV and -15.74eV. Below the Fermi level, the VB is consists of O-p states and B-sp mixed states. It clearly shows that the dominant component is O orbital from BO3 anion groups. Above the Fermi level, it is clear that the

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CB is mainly composed of Na-sp mixed states, B-p states, O-p states and H-s states.

Based on the results of electronic structure, the absorption spectrum for this

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Na2[BO2(OH)]·H2O crystal can be calculated. And the calculation indicates that the absorption edge of Na2[BO2(OH)]·H2O is about 193 nm (see Fig.S2 in the Supporting Information). This is because B-O covalent bond with large covalent bond energy in BO3 unit, and Na-O bonds with no d-d electron transitions in UV range also have no

4. Conclusions

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absorption in the UV region.

In the current work, the new hydrated sodium borate Na2[BO2(OH)]·H2O with NCS

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structure has been synthesized and characterized. It is exhibits a three-dimensional structure consisting of interconnecting Na1O6 groups, Na2O7 units and isolated BO3

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triangles. It is also proved that the BO2(OH) and H2O groups exist in Na2[BO2(OH)]·H2O by the IR spectrum. The calculation of magnitude of local dipole moment of BO3 groups indicates that the main SHG efficiency of the material comes from the addition of the distortion of BO3 triangles in the c-axis. The calculated band structures and the density of states of Na2[BO2(OH)]·H2O suggest that its direct gap is 4.15 eV, and the calculation indicates that the absorption edge of Na2[BO2(OH)]·H2O is about 193 nm.

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Acknowledgment: We gratefully acknowledge the support from the Universities Science and Technology Research Program of Hebei (Grant Nos. QN20132015, QN2014332), Science and Technology Program of Tangshan (Grant No.14130285a), the Dr. Innovation

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fund of Tangshan College (Grant No. tsbc201303).

Supplementary material

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CCDC 431085 contains the supplementary crystallographic data of this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via

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www.ccdc.cam.ac.uk/data_request/cif. IR spectrum, absorption spectrum, selected bond distances (Å), atomic coordinates and equivalent isotropic displacement parameters for

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Na2[BO2(OH)]·H2O are shown in the supporting information.

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Empirical formula

Na2[BO2(OH)]·H2O

Formula weight

123.81

Temperature

296(2) K

Crystal system

Orthorhombic

Space group

Pca21

Unit cell dimensions

a = 10.697(3) Å b= 5.2389(15) Å c = 6.4568(18) Å

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Table 1. Crystal data and structure refinement for Na2[BO2(OH)]·H2O

4, 361.84(18) Å 3

Density (calculated)

2.273 g/cm3

Absorption coefficient

0.410 /mm

F(000)

248

Crystal size

0.152 × 0.113 × 0.067 mm

4.33 to 25.00 deg

-12 ≤ h ≤ 12, -6≤ k ≤ 6, -5 ≤ l ≤ 7

Index ranges Reflections collected

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Independent reflections

513[R(int) = 0.0296]

Completeness to theta=25.49 Max. and min. transmission Data / restraints / parameters

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Goodness-of-fit on F Final R indices

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Theta range for data collection

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Z, Volume

[F02>2σ(

R indices (all data)

F02)]

97.7%

0.7459 and 0.5978

513 / 4 / 68 0.967

R1 = 0.0303, wR2 = 0.0707 R1 = 0.0358, wR2 = 0.0745 -0.4(8)

Largest diff. peak and hole

0.318 and -0.290 e.Å-3

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Absolute structure parameter

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species

x(a)

y(b)

z(c)

dipole moment

Å

Å

Å

1-x, 2-y, 0.5+z

0.0076

-0.0752

-0.0416

-0.5+x, 2-y, z

-0.0076

-0.0752

-0.0416

1.5-x, -1+y, 0.5+z

0.0076

0.0752

-0.0416

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Table 2. Direction and Magnitude of the Distortion of BO3 in Na2[BO2(OH)]·H2O.

x, -1+y, z

-0.0076

0.0752

0

0

Symmetric Code

-0.0416

-0.1664

0.1664

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Unit Cell

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BO3

debye

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Figure Captions: Figure 1. Drawing of the structure of Na2[BO2(OH)]·H2O viewed along the c-axis, the

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green polyhedra is the BO3 triangle groups. Figure 2. Drawing of the structure of in Na2[BO2(OH)]·H2O viewed down the a-axis. Figure 3. Band structures of Na2[BO2(OH)]·H2O.

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Figure 4. The projected density of states (PDOS) of Na2[BO2(OH)]·H2O.

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Figure 1

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Figure 2

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Figure 3.

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Figure 4.

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ACCEPTED MANUSCRIPT New Na2[BO2(OH)]·H2O with NCS structure was synthesized by slow evaporation method. It crystallizes in the Orthorhombic space group Pca21.

Its direct gap is 4.15 eV at G point.

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Its absorption edge is about 198.5 nm.

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Local dipole moment calculation indicates that it display a weak SHG efficiency.