Na₃B₄O₇X (X = Cl, Br): two new borate halides with a 1D Na-X (X = Cl, Br) chain formed by the face-sharing XNa₆ octahedra

Abstract

Two new sodium borate halides, namely, Na₃B₄O₇X (X = Cl, Br), have been successfully synthesized through high-temperature solid state reactions. Single-crystal X-ray diffraction analysis reveals that both of them crystallize in the same space group P6₅22 of the hexagonal system. In their crystal structures, the B₄O₉ groups link with each other by oxygen-sharing to form a three dimensional (3D) framework containing channels viewing along the [001] direction, in which the 1D helical [Na₃X] chain formed by the face-sharing XNa₆ octahedra resides. To the best of our knowledge, the 1D helical [Na₃X] chain has not been found in other alkali metal borate halides. From another perspective, the two compounds can also be regarded as NaX salt inclusion borates. In addition, infrared (IR) spectroscopy and bond valence sum (BVS) calculations are used to verify the validity of the structures. Differential scanning calorimetry (DSC) and powder X-ray diffraction (XRD) results prove that both of them are incongruently melting compounds. The electronic structures have been calculated by the first-principle method based on density-functional theory.

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Introduction

Borates,^1–8 owing to the possible 3- or 4-fold coordination of boron atoms, form a great number of compounds having diverse structures. The diverse structures give them interesting and excellent properties for technical applications, which has attracted the attention of material scientists. After much research effort, many excellent borate crystals have been obtained, such as α-BaB₂O₄ (α-BBO),⁹ β-BaB₂O₄ (β-BBO),¹⁰ KBe₂BO₃F₂ (KBBF).¹¹

Studies of alkali metal borates have also produced a large family of compounds with outstanding physical properties, such as LiB₃O₅ (LBO),¹² CsB₃O₅ (CBO),¹³ CsLiB₆O₁₀ (CLBO),¹⁴ Li₄Cs₃B₇O₂₄ ¹⁵ and Li₆Rb₅B₁₁O₂₂,¹⁶ the crystals of which have wide range transparency. In addition, it is well-known that combination of halogen with borate can widen band gaps.¹⁷ In our group, by introducing halogen atoms into borate, a series of new crystals consisting of similar XM₆ (X = Cl, Br; M = alkali metals) octahedra have been successfully synthesized, such as K₃B₆O₁₀X,¹⁸ Na₃B₆O₁₀X,¹⁹ K_3−xNa_xB₆O₁₀Br (x = 0.13, 0.67, 1.30, 2.20)²⁰ and RbNa₂B₆O₁₀X.^19a Interestingly, although these crystals have similar stoichiometry and contain the B₆O₁₃ groups and XM₆ (X = Cl, Br; M = alkali metals) octahedra, their properties have great difference owing to the different XM₆ (X = Cl, Br; M = alkali metals) octahedra. Inspired by the above work, we believe that it is significant to continue an investigation of alkali-metal borate halides containing XM₆ octahedra. After extensive efforts, we successfully synthesized two new alkali metal borate halides Na₃B₄O₇X (X = Cl, Br) containing 1D helical [Na₃X] chains of face-sharing XNa₆ octahedra, which have not been found in other alkali metal borate halides. From another perspective, the two compounds can also be regarded as NaX salt inclusion borates. This paper describes the syntheses, crystal and electronic structures and optical properties of Na₃B₄O₇X (X = Cl, Br). These results give a hint of the very extensive crystal chemistry of alkali-metal borate halides containing XM₆ (X = Cl, Br; M = alkali metals) octahedra system, which continues to provide a rich source of new compounds.

Experimental

Reagents

NaCl (Tianjin BaiShi Chemical Reagent Co., 99.0%), NaBr (Tianjin HongYan Chemical Reagent Co., Ltd., 99.0%), Na₂CO₃ (Tianjin HongYan Chemical Co., Ltd., 99.5%) and H₃BO₃ (Tianjin HongYan Chemical Co., Ltd., 99.5%) were used as received.

Crystal growth

Single crystals of Na₃B₄O₇Cl (X = Cl, Br) were grown by spontaneous crystallization. A mixture of 0.424 g (4.00 mmol) of Na₂CO₃, 0.234 g (4.00 mmol) of NaCl (or 0.411 g (4.00 mmol) of NaBr), and 0.7420 g (12.00 mmol) of H₃BO₃ was thoroughly ground for Na₃B₄O₇Cl (X = Cl, Br). The mixture was then placed in a platinum crucible that was placed into a vertical, programmable-temperature furnace. The crucible was gradually heated to 750 °C in air, held for 12 h until the solution became transparent and clear. The homogenized solution was cooled rapidly to 700 °C and then slowly cooled to 600 °C at a rate of 2 °C h⁻¹, followed by rapid cooling to room temperature. The crystals were separated mechanically from the crucible for the further characterization by single-crystal X-ray measurements.

Structure determination

The single crystals of Na₃B₄O₇X (X = Cl, Br) with dimensions 0.245 mm × 0.126 mm × 0.109 mm and 0.200 mm × 0.172 mm × 0.091 mm were selected for the structure determination, respectively. Their crystal structures were determined by single-crystal XRD on an APEX II CCD diffractometer using monochromatic Mo Kα radiation (λ = 0.71073 Å) at 293(2) K and integrated with the SAINT program.²¹ Numerical absorption corrections were carried out using the SCALE program for area detector. All calculations were performed with programs from the SHELXTL crystallographic software package.²² All atoms were refined using full matrix least-squares techniques; final least-squares refinement is on F_o² with data having F_o² ≥ 2σ(F_o²) (F_o: observed structure factors, σ: standard uncertainty). The structures were checked for missing symmetry elements by the program PLATON.²³ Crystal data and structure refinement information are presented in Table 1. The final refined atomic positions, isotropic thermal parameters and bond valence sums (BVS)²⁴ of each atom are summarized in Table 2. Selected bond lengths (in Å) and angles (in degree) are listed in Tables S1 and S2 in the ESI.†

Table 1 Crystal data and structures refinement for Na₃B₄O₇X (X = Cl, Br)

Empirical formula	Na3B4O7Cl	Na3B4O7Br
a R1 = Σ‖Fo| − |Fc‖/Σ|Fo| and wR2 = [Σw(Fo^2 − Fc^2)^2/ΣwFo^4]^1/2 for Fo^2 > 2σ(Fo^2).
Formula weight	259.66	304.12
Crystal system	Hexagonal	Hexagonal
Space group, Z	P6522, 6	P6522, 6
Unit cell dimensions	a = 8.031(1) Å	a = 8.146(5) Å
	c = 20.369(3) Å	c = 20.24(3) Å
Volume	1137.7(2) Å^3	1163.1(18) Å^3
Density (calculated)	2.274 Mg m^−3	2.605 Mg m^−3
Theta range for data collection	2.93° to 24.96°	2.89° to 27.50°
Limiting indices	−9 ≤ h ≤ 8, −7 ≤ k ≤ 9, −24 ≤ l ≤ 20	−9 ≤ h ≤ 10, −7 ≤ k ≤ 10, −23 ≤ l ≤ 26
Reflections collected/unique	5755/671 [R(int) = 0.0408]	7065/896 [R(int) = 0.0502]
Completeness to theta	100%	99.8%
Goodness-of-fit on F^2	1.184	1.100
Final R indices [Fo^2 > 2σ(Fo^2)]^a	R1 = 0.0609, wR2 = 0.1809	R1 = 0.0482, wR2 = 0.1304
R indices (all data)^a	R1 = 0.0636, wR2 = 0.1822	R1 = 0.0548, wR2 = 0.1343
Absolute structure parameter	0.0(4)	0.06(3)
Extinction coefficient	0.031(10)	0.005(3)
Largest diff. peak and hole	0.819 and −0.356 e·Å^−3	0.938 and −0.883 e·Å^−3

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Table 2 Atomic coordinates, equivalent isotropic displacement parameters and bond valence analyses for Na₃B₄O₇X (X = Cl, Br)

Atom	wyck	x	y	z	U^a (eq.)	BVS^b^,^c
a U (eq.) is defined as the one-third of the trace of the orthogonalized Uij tensor.b Bond valences calculated with the program Bond Valence Calculator Version 2.00, C. Hormillosa, S. Healy, T. Stephen, McMaster University, 1993.c Valence sums calculated with the formula: Si = exp[(R0 − Ri)/B], where Si = valence of bond “i” and B = 0.37.
Na3B4O7Cl
Na(1)	12c	0.5438(5)	0.7980(5)	0.5576(2)	0.049(1)	0.838
Na(2)	6a	0	0.8478(6)	0.6667(0)	0.034(1)	0.979
B(1)	12c	0.5134(11)	0.3563(10)	0.5298(3)	0.017(2)	3.072
B(2)	12c	0.8631(10)	0.5462(11)	0.5479(4)	0.018(2)	3.088
O(1)	6b	0.5131(4)	0.4869(4)	0.5833(0)	0.017(1)	1.906
O(2)	12c	0.7133(7)	0.4383(7)	0.5050(2)	0.023(1)	2.048
O(3)	12c	0.4374(7)	0.1675(6)	0.5546(2)	0.022(1)	2.105
O(4)	12c	0.3994(8)	0.3529(7)	0.4734(2)	0.024(1)	2.057
Cl(1)	6b	0.9023(2)	0.0977(2)	0.5833(0)	0.049(1)	0.693
Na3B4O7Br
Na(1)	12c	0.4667(4)	0.2071(3)	0.0555(1)	0.034(1)	0.926
Na(2)	6a	0.8454(5)	0	0	0.035(1)	0.961
B(1)	12c	0.1428(7)	0.4548(8)	0.0477(3)	0.010(1)	3.058
B(2)	12c	0.4878(8)	0.6284(7)	0.0296(2)	0.008(1)	3.043
O(1)	6b	0.4896(3)	0.5104(3)	0.0833(0)	0.011(1)	1.933
O(2)	12c	0.2895(5)	0.5581(5)	0.0054(2)	0.013(1)	1.904
O(3)	12c	0.5635(6)	0.8252(5)	0.0536(2)	0.014(1)	2.032
O(4)	12c	0.5991(5)	0.6398(6)	0.9719(8)	0.016(1)	1.995
Br(1)	6b	0.1055(1)	0.8945(1)	0.0833(0)	0.042(1)	0.890

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Solid-state synthesis

The polycrystalline powders of Na₃B₄O₇X (X = Cl, Br) were synthesized by high temperature solid-state reactions. The stoichiometric mixtures of Na₂CO₃, NaX (X = Cl, Br) and H₃BO₃ at a molar ratio 1 : 1 : 4 for Na₃B₄O₇X (X = Cl, Br) were ground well and packed into Pt crucibles, respectively. The raw materials were heated to 400 °C to decompose the carbonate and eliminate the water, then the compounds were gradually heated up to 650 °C for Na₃B₄O₇X (X = Cl, Br) and kept at this temperature for 72 h. During the sintering steps, the samples were cooled to room temperature and ground. The powders of Na₃B₄O₇X (X = Cl, Br) were obtained. The powder X-ray diffraction (XRD) data were collected at room temperature in the angular range of 2θ = 10–70° with a scan step width of 0.02° and a fixed counting time of 1 s per step by using an automated Bruker D2 X-ray diffractometer with Cu-Kα radiation (λ = 1.5418 Å). The diffraction patterns are well agreeable with the calculated ones (Fig. 1).

Fig. 1 Powder XRD patterns of calculated, before melting and after melting for (a) Na₃B₄O₇Cl, (b) Na₃B₄O₇Br.

IR spectroscopy

IR spectra of Na₃B₄O₇X (X = Cl, Br) were recorded on a Shimadzu IR Affinity-1 Fourier transform infrared spectrometer in the 400–4000 cm⁻¹ range. The samples were mixed thoroughly with dried KBr, and the characteristic absorption peaks were shown in Fig. S1 in the ESI.†

UV-Vis-NIR diffuse reflectance spectra

Optical diffuse reflectance spectra of Na₃B₄O₇X (X = Cl, Br) were measured at room temperature with a Shimadzu SolidSpec-3700DUV spectrophotometer. Data were collected in the wavelength range 190–2600 nm. Also, reflectance spectra were converted to absorbance with the Kubelka–Munk function.^25,26

Thermal analysis

The melting behaviors of Na₃B₄O₇X (X = Cl, Br) were carried out on NETZSCH STA 449C simultaneous analyzer instrument in an atmosphere of flowing N₂. The sample heated from 25 °C to 1000 °C at a rate of 10 °C min⁻¹.

Elemental analysis

Elemental analysis of single crystal was measured by a VISTA-PRO CCD simultaneous ICP-OES. The crystal samples were dissolved in nitric acid. The elemental analysis results are listed in Table S3.†

Theoretical calculations

The electronic structures of the title crystals, including the band structure and full/partial density of states (DOS/PDOS), were performed by a plane-wave pseudopotential package employed in CASTEP.²⁷ Norm-conserving pseudopotentials (NCP)²⁸ were used. The exchange–correlation functional was Perdew–Burke–Emzerhoff (PBE) functional within the generalized gradient approximation (GGA).²⁹ The plane-wave energy cutoff was set at 750 eV. The numerical integration of the Brillouin zone was performed using a 4 × 4 × 2 Monkhorst–Pack k-point sampling.

Results and discussion

Crystal structures

Na₃B₄O₇X (X = Cl, Br) have similar 3D framework, except that the ClNa₆ group is replaced by BrNa₆ group. Hence only the structure of Na₃B₄O₇Br will be discussed in detail as a representation. X-ray analysis reveals that Na₃B₄O₇Br crystallizes in the chiral space group P6₅22 of the hexagonal system. In the asymmetric unit of Na₃B₄O₇Br, there are two unique Na atoms, two unique B atoms, four unique O atoms and one unique Br atom (Table 2).

The structure of Na₃B₄O₇Br is illustrated in Fig. 2. The structure consists of the B₄O₉ groups and BrNa₆ polyhedra as structure building units (Fig. 2(a) and (b)). Interestingly, a notable feature in the structure is the 1D helical [Na₃Br] chain formed by the face-sharing BrNa₆ polyhedra, which has not been found in other alkali metal borate halides. In general, XM₆ octahedron (A = alkali metals; X = halogen) exists in the structure as the 3D framework, such as K₃B₆O₁₀X,¹⁸ Na₃B₆O₁₀X,¹⁹ K_3−xNa_xB₆O₁₀Br (x = 0.13, 0.67, 1.30, 2.20),²⁰ RbNa₂B₆O₁₀X.^19a The B₄O₉ group is built of two BO₄ tetrahedra (T) and two BO₃ triangles (Δ) through corner-sharing, which is the typical tetraborate block with notation 4:[2Δ + 2T] introduced by Christ and Clark.³⁰ Each B₄O₉ group is connected by sharing its four terminal oxygens to form a 3D network with channels pointing along the c axis (Fig. 2(c) and S3 in the ESI†), where the [Na₃Br] chains are filled. The tetraborate B₄O₉ group is also found in many anhydrous as well as hydrous borates. It can be found that the B₄O₉ groups can exist as isolated groups or further linked with each other to form 3D frameworks. For example, it can exist as, an isolated [B₄O₅(OH)₄]²⁻ group in Borax Na₂B₄O₅(OH)₄·8H₂O,³¹ as a [B₄O₇(OH)₂]²⁻ group with a [MnB₄O₇(OH₂)]²⁻ chain in Roweite Ca₂Mn₂(OH)₄B₄O₇(OH)₂,³² as an isolated B₄O₉ group in Na₃GaB₄O₉,³³ Cs₂GeB₄O₉ (ref. 34) and as a 3D [B₄O₉]²⁻ network in Li₂B₄O₇ (ref. 35) and LiNaB₄O₇.³⁶

Fig. 2 Crystal structure of Na₃B₄O₇Br: (a) the B₄O₉ group, (b) the BrNa₆ polyhedron, (c) the B₄O₉ network, (d) the [Na₃Br] chain, (e) the crystal structure of Na₃B₄O₇Br (the BO₃ triangles are shown in rose, the BO₄ tetrahedra are shown in green).

The Na atoms have one kind of coordinated environment: both Na(1) and Na(2) atoms are coordinated by four O atoms and two Br atoms (Fig. S2 in the ESI†). The Na–O bond distances have a wide region varying from 2.282(5) Å to 2.928(5) Å, and Na–Br distances vary from 2.822(3) Å to 3.346(4) Å. The B atoms have two kinds of coordinated environments: the B(1) atoms are coordinated by three O atoms to form BO₃ triangles while the B(2) atoms are coordinated to four O atoms to form BO₄ tetrahedra. The B–O distances range from 1.356(6) Å to 1.501(6) Å. All of the bond lengths are consistent with those observed in other compounds.^19,20 The results of bond valence calculations (Na, 0.926–0.961; B, 3.043–3058; Br, 0.890) indicate that the Na, B, and Br atoms are in oxidation states of +1, +3, and −1, respectively.^24,37

The difference between the two crystal structures is mainly in the size and the coordination environment of the Cl and Br atoms. In Na₃B₄O₇Cl, the Na–Cl bond lengths are range from 2.725(3) Å to 3.402(5) Å, while in Na₃B₄O₇Br, the Na–Br bond lengths are range from 2.822(3) Å to 3.346(4) Å.

It is of some interest to find that the Na(1) and Cl (Br) atoms have large thermal factors in the two crystals during the refinement (Table 2), either indicating diffuse positioned atoms or site deficiencies. Such phenomenon has been observed in some salt inclusion borates, such as Ba₄Ga₂B₈O₁₈Cl₂·NaCl,³⁸ Ba₄(BO₃)₃(SiO₄)·Ba₃X (X = Cl, Br).³⁹ From the bond length calculation, in Na₃B₄O₇Cl, the Na(1)–O distances are in the range of 2.406(6) Å to 2.940(6) Å with an average length of 2.558(2) Å. It is found that the Na(1)–O bond length of 2.558(2) Å is significantly longer than the sum of their ionic radii (2.40 Å), whereas that of Na(1)–Cl (2.725(3) Å) is much shorter than the sum (2.83 Å). Such observation may indicate that, Na(1) and Cl are more or less forms of the NaCl molecule, and because it is weakly bonded to the rest of the lattice with a bond valence sum for Na(1) of only +0.84 and that for Cl of −0.69. In the same way, the average bond length of the Na(1)–O bonds in Na₃B₄O₇Br is 2.553(7) Å, which is longer than the sum of their ionic radii (2.40 Å), whereas that of Na(1)–Br (2.822(3) Å) is much shorter than the sum (2.98 Å), which further indicate that weak interaction between NaBr and the rest of the lattice. Therefore, from another perspective, Na₃B₄O₇X (X = Cl, Br) also can be regarded as salt inclusion borates.⁴⁰

IR spectra

Fig. S1 in the ESI† presents the IR spectra of Na₃B₄O₇X (X = Cl, Br) and they are similar. The assignments of IR spectra of the two compounds are listed in Table S4 in the ESI.† Referring to the literatures,^41,42 the peaks at 1346–1338 cm⁻¹ and 950 cm⁻¹ can be assigned to the asymmetric stretching and symmetric stretching vibrations of BO₃, respectively. The peaks located at 1140 and 862–753 cm⁻¹ arise from the asymmetric stretching and symmetric stretching vibrations of BO₄, respectively. The peaks observed in the regions of 663–654 cm⁻¹ are attributed to the out-of-plane bending of BO₃. The peaks at 520 cm⁻¹ characterize the bending of BO₃ and BO₄. The peaks observed in the regions of 475–465 cm⁻¹ are attributed to the bending of BO₄. The IR spectra further confirm the existence of the BO₃ triangles and BO₄ tetrahedra, which is consistent with the results obtained from the single crystal X-ray structural analyses.

UV-Vis-NIR diffuse reflectance spectroscopy

The UV-Vis-NIR diffuse reflectance spectra of Na₃B₄O₇X (X = Cl, Br) in the region 190–2600 nm are shown in Fig. S4 in the ESI.† It is clear that the two compounds have no obvious absorption from 340 nm to 2600 nm, but the absorption sharply increases below 340 nm and the cut-off edges for the two compounds are below 200 nm.

Thermal analysis

The DSC/TG curves of the two crystals are shown in Fig. 3. It can be seen that there is one endothermic peak at 745 and 766 °C on the DSC curves for Na₃B₄O₇Cl, and Na₃B₄O₇Br, respectively, along with weight loss on the TG curves. Analysis of the powder XRD pattern of the solidified melt reveal that the entire solid product exhibits a diffraction pattern different from that of the initial pure powder for the two compounds, as shown in Fig. 1. It demonstrates that the two compounds are incongruently melting compounds.

Fig. 3 TG/DSC curves for (a) Na₃B₄O₇Cl, (b) Na₃B₄O₇Br.

Theoretical calculations

The electronic band structure of Na₃B₄O₇Cl calculated by GGA method is plotted along high symmetry k-points in Fig. 4 (the electronic band structure of Na₃B₄O₇Br is in Fig. S5 in the ESI†). All the two crystals are direct band gap materials with the calculated band gap of 4.53 (Na₃B₄O₇Cl) and 4.16 eV (Na₃B₄O₇Br), which are relatively smaller than experimental optical gaps (>6.2 eV) due to a typical disadvantage in density functional theory (DFT) calculations.

Fig. 4 Calculated band structure of Na₃B₄O₇Cl.

The PDOS of Na₃B₄O₇Cl can be divided into four major distinct regions, as can be seen from Fig. 5. The lowest part located from −23.0 eV to −16.0 eV contains contributions mainly from the O2p orbital, with a small amount of B2s and B2p orbitals. The next region, around −12.5 eV, is contributed by Cl3s orbital with very sharp and narrow shape. The third region extends in a wide range from −9.5 eV to VBM, that is, the Fermi level. This band mostly originates from the O2p, B2p and Cl3p states, with some mixing of B2s state. For the lowest conduction band, the main components are the Na2p, Nas, and B2p states. The PDOS of Na₃B₄O₇Br (Fig. S6 in the ESI†) is similar to that of Na₃B₆O₁₀Cl due to the two crystals having the similar building units and chemical environments.

Fig. 5 The total and partial densities of states of Na₃B₄O₇Cl.

Conclusion

This study is a continuation of systematic investigation of the alkali-metal borate halides containing XM₆ (X = Cl, Br; M = alkali metals) octahedra system. In this system, two new alkali metal borate halides Na₃B₄O₇X (X = Cl, Br) have been reported for the first time. The compounds exhibit a 3D structure consisting of XNa₆ (X = Cl, Br) polyhedra and B₄O₉ groups. The B₄O₉ group is connected by sharing its four terminal oxygens to form a 3D network with channels pointing along the c axis, where the 1D helical [Na₃X] (X = Cl, Br) chains are filled. In addition, the two compounds also can be regarded as NaX salt inclusion borates. The DSC/TG curves show that Na₃B₄O₇X (X = Cl, Br) melt incongruently. The formation of Na₃B₄O₇X (X = Cl, Br) provides another example that alkali-metal borate halides containing XM₆ (X = Cl, Br; M = alkali metals) octahedra system possess very extensive crystal chemistry, which continues to provide a rich source of new compounds. In the future, the research on the alkali-metal borate halides containing XM₆ (X = Cl, Br; M = alkali metals) octahedra system will be further expanded.

Acknowledgements

This work is supported by the “National Natural Science Foundation of China” (Grant nos 51302307, U1303193 51425206, U1129301, 51172277), the Western Light of Chinese Academy of Sciences (Grant no. XBBS201220), the High-level Professional and Technical Personnel of Autonomous region, the National Key Basic Research Program of China (Grant no. 2014CB648400), Major Program of Xinjiang Uygur Autonomous Region of China during the 12th Five-Year Plan Period (Grant no. 201130111).

Notes and references

1. (a) P. Becker, Adv. Mater., 1998, 10, 979; (b) P. Becker and R. Z. Frohlich, Zeitschrift für Naturforschung B., 2004, 59, 256; (c) C. T. Chen, B. C. Wu, A. D. Jiang and G. M. You, Sci. Sin., Ser. B, 1985, 28, 235; (d) H. W. Huang, J. Y. Yao, Z. S. Lin, X. Y. Wang, R. He, W. J Yao, N. X. Zhai and C. T. Chen, Angew. Chem., Int. Ed., 2011, 50, 9141.
2. (a) P. S. Halasyamani and K. R. Poeppelmeier, Chem. Mater., 1998, 10, 2753; (b) D. A. Keszler, A. Akella, K. I. Schaoers and T. Alekel III, Mater. Res. Soc. Symp. Proc., 1994, 329, 15; (c) S. C. Wang, N. Ye, W. Li and D. Zhao, J. Am. Chem. Soc., 2010, 132, 8779; (d) S. C. Wang and N. Ye, J. Am. Chem. Soc., 2011, 133, 11458.
3. (a) N. Ye, W. R. Zeng, J. Jiang, B. C. Wu, C. T. Chen, B. H. Feng and X. L. Zhang, J. Opt. Soc. Am. B, 2000, 17, 764; (b) H. W. Yu, S. L. Pan, H. P. Wu, W. W. Zhao, F. F. Zhang, H. Y. Li and Z. H. Yang, J. Mater. Chem., 2012, 22, 2105; (c) M. Zhang, S. L. Pan, J. Han, Y. Yang, L. Cui and Z. X. Zhou, J. Alloys Compd., 2011, 509, 6696; (d) H. W. Huang, J. Y. Yao, Z. S. Lin, X. Y. Wang, R. He, W. J. Yao, N. X. Zhai and C. Y. Chen, Chem. Mater., 2011, 23, 5457.
4. (a) H. X. Zhang, J. Zhang, S. T. Zheng and G. Y. Yang, Cryst. Growth Des., 2005, 5, 157; (b) G. Z. Liu, S. T. Zheng and G. Y. Yang, Inorg. Chem. Commun., 2007, 10, 84; (c) B. Petermüller, L. L. Petschnig, K. Wurst, G. Heymann and H. Huppertz, Inorg. Chem., 2014, 53, 9722; (d) H. Yang, C. L. Hu, J. L. Song and J. G. Mao, RSC Adv., 2014, 4, 45258.
5. (a) S. L. Pan, J. P. Smit, B. Watkins, M. R. Marvel, C. L. Stern and K. R. Poeppelmeier, J. Am. Chem. Soc., 2006, 128, 11631; (b) A. Q. Jiao, H. P. Wu, S. L. Pan, H. W. Yu and C. Lei, J. Alloys Compd., 2014, 588, 514; (c) H. P. Wu, H. W. Yu, S. L. Pan, Z. J. Huang, Z. H. Yang, X. Su and K. R. Poeppelmeier, Angew. Chem., Int. Ed., 2013, 52, 3406; (d) L. L. Liu, X. Su, Y. Yang, S. L. Pan, X. Y. Dong, S. J. Han, M. Zhang, J. Kang and Z. H. Yang, Dalton Trans., 2014, 8905.
6. (a) T. Pilz and M. Jansen, Z. Anorg. Allg. Chem., 2011, 637, 1; (b) T. Sasaki, Y. Mori, M. Yoshimura, Y. K. Yap and T. Kamimura, Mater. Sci. Eng., R, 2000, 30, 1; (c) T. S. Ortner, K. Wurst, L. Perfler, M. Tribus and H. Huppertz, J. Solid State Chem., 2015, 221, 66; (d) Z. S. Lin, L. Bai, J. Liu, M. H. Lee, J. Xu, X. Y. Wang and C. T. Chen, J. Appl. Phys., 2011, 107, 073721; (e) Z. S. Lin, L. F. Xu, L. J. Liu, J. Xu, M. H. Lee and C. T. Chen, Phys. Rev. B: Condens. Matter Mater. Phys., 2010, 82, 035124.
7. (a) Z. Wang, M. Zhang, S. L. Pan, Y. Wang, H. Zhang and Z. H. Chen, Dalton Trans., 2014, 2828; (b) S. L. Pan, Y. C. Wu, P. Z. Fu, G. C. Zhang, Z. H. Li, C. X. Du and C. T. Chen, Chem. Mater., 2003, 15, 2218; (c) Z. Wang, M. Zhang, X. Su, S. L. Pan, Z. H. Yang, H. Zhang and L. Liu, Chem.–Eur. J., 2015, 21, 1414; (d) X. W. Zhang, H. W. Yu, H. P. Wu, S. L. Pan, A. Q. Jiao, B. B. Zhang and Z. H. Yang, RSC Adv., 2014, 1, 13195.
8. (a) X. A. Chen, J. L. Zuo, X. A. Chang, Y. H. Zhao, H. G. Zang and W. Q. Xiao, J. Solid State Chem., 2006, 179, 3191; (b) X. A. Chen, F. P. Song, X. A. Chang, H. G. Zang and W. Q. Xiao, J. Solid State Chem., 2009, 182, 3091; (c) W. J. Yao, T. Xu, X. X. Jiang, H. W. Huang, X. Y. Wang, Z. S. Lin and C. T. Chen, Dalton Trans., 2014, 9998; (d) S. L. Pan, B. Watkins, J. P. Smit, M. R. Marvel, I. Saratovsky and K. R. Poeppelmeier, Inorg. Chem., 2007, 46, 3851.
9. Y. Z. Huang, L. M. Wu, X. T. Wu, L. H. Li, L. Chen and Y. F. Zhang, J. Am. Chem. Soc., 2010, 132, 12788.
10. C. T. Chen and G. Z. Liu, Annu. Rev. Mater. Sci., 1986, 16, 136.
11. (a) C. T. Chen, J. H. Lu, T. Togashi, T. Suganuma, T. Sekikawa, S. Watanabe, Z. Y. Xu and J. Y. Wang, Opt. Lett., 2002, 27, 637; (b) B. C. Wu, D. Y. Tang, N. Ye and C. T. Chen, Opt. Mater., 1996, 5, 105; (c) C. T. Chen, G. L. Wang, X. Y. Wang and Z. Y. Xu, Appl. Phys. B, 2009, 97, 9.
12. C. Chen, Y. Wu, A. Jiang, G. You, R. Li and S. Lin, J. Opt. Soc. Am. B, 1989, 6, 616.
13. Y. Wu, T. Sasaki, S. Nakai, A. Yokotani, H. Tang and C. Chen, Appl. Phys. Lett., 1993, 62, 2614.
14. Y. Mori, I. Kuroda, S. Nakajima, T. Sasaki and S. Nakai, Appl. Phys. Lett., 1995, 67, 1818.
15. Y. Yang, S. L. Pan, H. Y. Li, J. Han, Z. H. Chen, W. W. Zhao and Z. X. Zhou, Inorg. Chem., 2011, 50, 2415.
16. Y. Yang, S. L. Pan, J. Han, X. L. Hou, Z. X. Zhou, W. W. Zhao, Z. H. Chen and M. Zhang, Cryst. Growth Des., 2011, 11, 3912.
17. J. G. M. Jesudurai, K. Prabha, P. D. Christy, J. Madhavan and P. Sagayaraj, Spectrochim. Acta, Part A, 2008, 71, 1371.
18. (a) H. P. Wu, S. L. Pan, K. R. Poeppelmeier, H. Y. Li, D. Z. Jia, Z. H. Chen, X. Y. Fan, Y. Yang, J. M. Rondinelli and H. S. Luo, J. Am. Chem. Soc., 2011, 133, 7786; (b) H. P. Wu, S. L. Pan, H. W. Yu, D. Z. Jia, A. M. Chang, H. Y. Li and X. Huang, CrystEngComm, 2012, 14, 799; (c) M. Zhang, S. L. Pan, X. Y. Fan, Z. X. Zhou, K. R. Poeppelmeier and Y. Yang, CrystEngComm, 2011, 13, 2899.
19. (a) C. Y. Bai, H. W. Yu, S. J. Han, S. L. Pan, B. B. Zhang, Y. Wang, H. P. Wu and Z. H. Yang, Inorg. Chem., 2014, 53, 11213; (b) Z. H. Chen, S. L. Pan, X. Y. Dong, Z. H. Yang, M. Zhang and X. Su, Inorg. Chim. Acta, 2013, 406, 205.
20. S. J. Han, Y. Wang, S. L. Pan, X. Y. Dong, H. P. Wu, J. Han, Y. Yang, H. W. Yu and C. Y. Bai, Cryst. Growth Des., 2014, 14, 1794.
21. SAINT, Version 7.60 A, Bruker Analytical X-ray Instruments, Inc., Madison, WI, 2008.
22. G. M. Sheldrick, SHELXTL, Version 6.14, Bruker Analytical X-ray Instruments, Inc., Madison, WI, 2003.
23. A. L. Spek, J. Appl. Crystallogr., 2003, 36, 7.
24. N. E. Brese and M. O'Keeffe, Acta Crystallogr., Sect. B: Struct. Sci., 1991, 47, 192.
25. J. Tauc, Mater. Res. Bull., 1970, 5, 721.
26. (a) H. W. Yu, S. L. Pan, H. P. Wu, Z. H. Yang, L. Y. Dong, X. Su, B. B. Zhang and H. Y. Li, Cryst. Growth Des., 2013, 13, 3514; (b) Y. J. Shi, S. L. Pan, X. Y. Dong, Y. Wang, M. Zhang, F. F. Zhang and Z. X. Zhou, Inorg. Chem., 2012, 51, 10870; (c) L. Zhou, S. L. Pan, X. Y. Dong, H. W. Yu, H. P. Wu, F. F. Zhang and Z. X. Zhou, CrystEngComm, 2013, 15, 3412.
27. S. J. Clark, M. D. Segall, C. J. Pickard, P. J. Hasnip, M. J. Probert, K. Rrfson and M. C. Payne, Z. Kristallogr., 2005, 220, 567.
28. (a) M.-H. Lee, Ph.D. Thesis, The University of Cambridge, 1996; (b) J. Lin, A. Qteish, M. Payne and V. Heine, Phys. Rev. B: Condens. Matter Mater. Phys., 1993, 47, 4174; (c) A. M. Rappe, K. M. Rabe, E. Kaxiras and J. D. Joannopoulos, Phys. Rev. B: Condens. Matter Mater. Phys., 1990, 41, 1227.
29. J. P. Perdew, K. Burke and M. Ernzerhof, Phys. Rev. Lett., 1996, 77, 3865.
30. (a) J. D. Grice, P. C. Burns and F. C. Hawthorne, Can. Mineral., 1999, 37, 731; (b) P. C. Burns, J. D. Grice and F. C. Hawthorne, Can. Mineral., 1995, 33, 1131.
31. H. A. Levy and G. C. Lisensky, Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem., 1978, 34, 3502.
32. P. B. Moore and T. Araki, Am. Mineral., 1974, 59, 60.
33. P. Becker and R. Froehlich, Z. Kristallogr., 2001, 216, 31.
34. X. Xu, C. L. Hu, F. Kong, J. L. Zhang, J. G. Mao and J. L. Sun, Inorg. Chem., 2013, 52, 5831.
35. J. Krogh-Moe, Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem., 1968, 24, 179.
36. M. Maczka, A. Waśkowska, A. Majchrowskib, J. Żmija, J. Hanuza, G. A. Petersond and D. A. Keszler, J. Solid State Chem., 2007, 180, 410.
37. I. D. Brown and D. Altermatt, Acta Crystallogr., Sect. B: Struct. Sci., 1985, 41, 244.
38. X. X. Lin, F. F. Zhang, S. L. Pan, H. W. Yu, F. Y. Zhang, X. Y. Dong, S. J. Han, L. Y. Dong, C. Y. Bai and Z. Wang, J. Mater. Chem. C, 2014, 2, 4257.
39. R. K. Li and Y. Yu, Inorg. Chem., 2006, 45, 6840.
40. (a) J. P. West and S.-J. Hwu, J. Solid State Chem., 2012, 195, 101; (b) X. Mo and S.-J. Hwu, Inorg. Chem., 2003, 42, 3978; (c) Q. Huang and S.-J. Hwu, Inorg. Chem., 2003, 42, 655.
41. W. W. Zhao, S. L. Pan, J. Han, J. Y. Yao, Y. Yang, J. J. Li, M. Zhang, L. H. Zhang and Y. Hang, J. Solid State Chem., 2011, 184, 2849.
42. Y. J. Wang, S. L. Pan, X. L. Tian, Z. X. Zhou, G. Liu, J. Wang and D. Z. Jia, Inorg. Chem., 2009, 48, 7800.

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Footnote

† Electronic supplementary information (ESI) available: Selected bond lengths (Å) and angles (degree); the element analysis of Na₃B₄O₇X (X = Cl, Br); the assignment of infrared spectra; Infrared spectra; the coordination environments of the cations in Na₃B₄O₇Br; the boron–oxygen framework in the crystal structure of Na₃B₄O₇Br in bc plane; UV-Vis-NIR diffuse-reflectance spectra; calculated band structure of Na₃B₄O₇Br; the total and partial densities of states of Na₃B₄O₇Br. CCDC 1039806 and 1039807. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4ra16639f

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