Article pubs.acs.org/IC
Na2B6O9F2: A Fluoroborate with Short Cutoff Edge and DeepUltraviolet Birefringent Property Prepared by an Open HighTemperature Solution Method Guoqiang Shi,†,‡ Fangfang Zhang,*,† Bingbing Zhang,† Dianwei Hou,†,‡ Xinglong Chen,†,‡ Zhihua Yang,† and Shilie Pan*,† †
Key Laboratory of Functional Materials and Devices for Special Environments, Xinjiang Technical Institute of Physics & Chemistry, and Xinjiang Key Laboratory of Electronic Information Materials and Devices, Chinese Academy of Sciences, 40-1 South Beijing Road, Urumqi 830011, China ‡ University of the Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *
ABSTRACT: As important materials in modulating the polarization of light, birefringent crystals have attracted considerable attention and played crucial roles in the field of optical communication and the laser industry. Limited by the transparency range, few birefringent crystals can be used in the deep-ultraviolet (DUV) region, except for α-BaB2O4 (α-BBO). However, the application of α-BBO in the DUV range is restricted by the relatively high cutoff edge and low transmittance rate below 200 nm. In this paper, we design and synthesize a new fluoroborate, Na2B6O9F2, by introducing fluorine into borate system. It possesses a short cutoff edge of 169 nm and birefringence larger than 0.080 at 589.3 nm. The Na2B6O9F2 crystals with sizes up to 3.0 mm × 1.5 mm × 0.2 mm have been grown with good quality by a high-temperature solution method in the open system. Firstprinciples calculations were carried out to understand the optical properties.
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INTRODUCTION
In the design of DUV crystals, it has been found that the combination of alkali- or alkaline earth-metals with fluorine atoms results in large band gap making it suitable for light transmission in the DUV region.17−22 The main reason is that alkali- and alkaline earth-metals have no d−d or f−f electronic transitions that will red-shift the absorption edge,20b and the incorporation of fluorine atoms usually blue-shifts the absorption edge of a material attributable to its large electronegativity.20a For example, the reported fluorinecontaining alkali- and alkaline earth-metal borates KBe2BO3F2,23 Be2BO3F,24 Ba4B11O20F,25 Rb3Al3B3O10F,26 and Ca3Be6B5O16F,27 etc., possess short cutoff edges below 200 nm. In these structures, the fluorine ions connect metal cations forming M−F (M = alkali or alkaline earth metal) bonds, which are named as borate fluoride.28 It is noticed that there are another kind of fluorine-containing borates named as fluoroborates, where fluorine atoms coordinate with boron atoms forming B−F bonds.28 It is expected that the fluoroborates may increase the optical anisotropies, because the electronegativity of the F atom is larger than that of the O atom. For example, research has shown that the [BO3F]4− unit is more asymmetric than the [BO4]5− unit.29 Hence, the introduction of the F atoms into borates may increase the local polarization of the final materials.29 At the same time, the terminal F atoms may increase the probability of layer structure,
Deep-ultraviolet (DUV) birefringent crystals with cutoff edges below 200 nm have important applications in fields of 193 nm immersion lithography, photo fragment translational spectroscopy, DUV Raman spectrometers, and fluorescence spectrometers, etc.1−9 Limited by the transparency range, rare birefringent materials can be used in DUV region. For example, the commercial birefringent crystals CaCO3 (have a transparency range of 350−2300 nm),10 YVO4 (400−5000 nm),11 TiO2 (400−5000 nm),12 and LiNbO3 (420−5200 nm)13 are widely used in the near-infrared (NIR) and visible ranges. The famous birefringent crystal, α-BaB2O4 (α-BBO),14 has a short cutoff edge of 189 nm and large birefringence (0.116 at 1064 nm), therefore, it is the unique material to date that can be used as a DUV birefringent material. However, the application of αBBO in the DUV range is restricted by the relatively high cutoff edge and low transmittance rate below 200 nm. Hence, extensive effort has been paid to explore new birefringent crystals with even shorter cutoff edge. Recently, a new crystal Ba2Mg(B3O6)2 (BMBO) has been found and studied by Li et al.15 The cut off edge of BMBO is 178 nm, and the birefringence in the wavelength range from 180 to 3350 nm is from 0.2263 to 0.0677. In our previous work, the Na3Ba2(B3O6)2F (NBBF)16 crystal was developed. It is a promising birefringent crystal with large birefringence (Δn = n0 − ne = 0.2554−0.0750 from 175 nm to 3.35 μm) and short cutoff edge of 175 nm. © XXXX American Chemical Society
Received: September 19, 2016
A
DOI: 10.1021/acs.inorgchem.6b02269 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry which is beneficial to optical anisotropies. For these reasons, fluoroborates are superior candidates for exploration of birefringent materials. The greatest challenge with fluoroborates is synthesis and crystal growth, because the B−F bonds tend to be oxidized in air to release BF3 under high temperature in the open system.30−33 The known alkali metal fluoroborates include Na3B3O3F6,30 Li2B6O9F2,31 Li2B3O4F3,32 and LiB6O9F,33 which were synthesized through standard Schlenk techniques by Jansen et al. In their experiments, silver crucibles and glass ampules under the protection of argon atmosphere were used. In our previous work, attempts to obtain alkali metal fluoroborate under open system were made, and single crystals of K3B3O3F6 were synthesized via high-temperature method.34 In the present work, a new fluoroborate, Na2B6O9F2, has been synthesized, and single crystals have been grown via hightemperature solution method. It is the first fluoroborate that can be obtained in the open air in NaBF4−B2O3−NaBO2 system. By the incorporation of F atoms, Na2B6O9F2 possesses a new structure which consists of [B6O9F2]2− layers. It has a short cutoff edge of 169 nm, and the birefringence measurement shows that its birefringence is larger than 0.080 at 589.3 nm. The transmittance spectrum, infrared spectroscopy, thermal analysis, and birefringent property were fully characterized. In addition, the first-principles calculations on the band structure, density of states (DOS), and birefringence were carried out to understand the relationship between the electronic structure and optical properties.
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Figure 1. XRD curves of Na2B6O9F2: calculated one, annealed at 400, 520, 540, 560, 580, 600, and 620 °C for 5 h, respectively (+ the unknown phase).
EXPERIMENTAL SECTION
Reagents. NaBF4 (Tianjin BaiShi Chemical Reagent Co., 99.0%), NaBO2·4H2O (Tianjin BaiShi Chemical Reagent Co., 99.0%), and H3BO3 (Tianjin BaiShi Chemical Reagent Co., 99.5%) were used as received. Solid-State Synthesis. Polycrystalline powders of Na2B6O9F2 were synthesized via the standard solid-state reaction. A mixture of NaBF4 (30 mmol, 3.294 g), NaBO2·4H2O (30 mmol, 4.135 g), and H3BO3 (120 mmol, 7.420 g) was well ground and placed in the corundum crucible. Raw materials were heated to 200 °C and held at this temperature for about 2 h for releasing water, then gradually heated to 400 °C and held for 24 h. Polycrystalline powders of Na2B6O9F2 were obtained. The purity of Na2B6O9F2 was identified by powder X-ray diffraction pattern. Powder X-ray Diffraction. As shown in Figure 1, the experimental pattern is identical to the calculated one (the bottom two profiles), verifying the phase purity of the synthesized powder. Powder X-ray diffraction measurement on the thoroughly ground polycrystalline powders of Na2B6O9F2 was carried out with a Bruker D2 PHASER diffractometer equipped with an incident beam monochromator set for Cu Kα radiation (λ = 1.5418 Å). Diffraction patterns were taken from 10 to 70° with a scan step width of 0.02° and a fixed counting time of 1 s/step. Single-Crystal Growth. The single crystal of Na2B6O9F2 was grown in an open system by spontaneous crystallization method by using NaBO2 as the flux. A mixture of NaBF4 (30 mmol, 3.294 g), NaBO2·4H2O (60 mmol, 8.272 g) and H3BO3 (120 mmol, 7.420 g) was thoroughly ground. The mixture was then packed into a platinum crucible that was placed into a vertical, programmable temperature furnace. The crucible was gradually heated to 460 °C in air and held for 10 h until the solution became transparent and clear. Then the solution was cooled to 380 °C with a rate of 2 °C/h, followed by rapid cooling to room temperature. Crystals were separated mechanically from the crucible, and transparent and colorless single crystals were obtained repeatedly in a yield of ∼50% based on Na. The crystals were lamellar shaped with millimeter sizes as shown in Figure 2. The theoretical morphology of Na2B6O9F2 is established by using the
Figure 2. Photograph of single crystals of Na2B6O9F2 (the minimum scale of lattice is 1 mm). Mercury program35 according to the Bravais−Friedel and Donnay− Harker (BFDH) theory36 (see Figure S1 in the Supporting Information). The simulated morphology is block which indicates Na2B6O9F2 intrinsically has no layered growth habit that is beneficial for crystal growth. Structure Determination. Crystal structure determination of Na2B6O9F2 was performed on a Bruker SMART APEX II 4K CCD diffractometer using Mo Kα radiation (λ = 0.71073 Å) at room temperature and integrated with the SAINT program.37 Numerical absorption corrections were carried out using the SCALE program37 for the area detector. Then the crystal structure was solved via direct methods and refined using the SHELXTL program package.38 The program XPREP was used for multiscan absorption corrections. The structure is checked with PLATON.39 Crystal data and structure refinement information are presented in Table 1. Anisotropic displacement parameters, atomic coordinates, and equivalent isotropic displacement parameters are listed in Tables S1 and S2 in the Supporting Information, respectively. Selected bond distances (in B
DOI: 10.1021/acs.inorgchem.6b02269 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
interactions were modeled by the norm-conserving pseudopotential (NCP)42 for all elements. The kinetic energy cutoff of 850.0 eV and Monkhorst−Pack k-point meshes of 3 × 2 × 3 in the Brillouin zone (BZ) are chosen. Geometry optimization was adopted with the energy, force, and displacement of 5.0 × 10−6 eV per atom, 0.01 eVÅ−1, and 5.0 × 10−4 Å, respectively. The exchange−correction functional was Perdew−Burke−Emzerhof (PBE) within the generalized gradient approximation (GGA).43 The valence electrons were set as 2s2 2p5 configuration for F, 2s2 2p1 for B, 2s2 2p4 for O, and 2s2 2p6 3s1 for Na. The optical properties were finally calculated on the basis of the electronic structure obtained. The linear optical property of Na2B6O9F2 was obtained through the dielectric function formula ε(ω) = ε1(ω) + iε2(ω). The imaginary part of the dielectric function, ε2, was calculated from the electronic transition between the occupied and the unoccupied states by the following formula:44
Table 1. Crystal Data and Structure Refinement for Na2B6O9F2 empirical formula formula weight (g/mol) crystal color crystal size (mm) measurement temperature (°C) crystal system space group a (Å) b (Å) c (Å) β (°) V (Å3) Z μ (mm−1) F (000) data/restraints/parameters R (int) GOF (F2) extinction coefficient final R indices [F02 > 2σ(Fc2)]a final R indices (all data)a a
Na2B6O9F2 292.84 colorless 0.081 × 0.161 × 0.167 22.85 (2) monoclinic P21/c 8.1964(12) 13.0005(19) 7.8955(11) 90.750(2) 841.3(2) 4 0.317 568 1924/0/173 0.0361 1.031 0.004(2) R1 = 0.0373, wR2 = 0.0808 R1 = 0.0570, wR2 = 0.0895
ε2(ℏω) =
2πe 2 Ωε0
∑ |⟨ψkc|u·̂ r |⃗ ψkv⟩|2 δ(Ekc − Ekv − ℏω) k ,v ,c
where Ω is the volume of the elementary cell, v and c portray the valence bands (VBs) and the conduction bands (CBs), respectively, ω is the frequency of the incident light, û is the vector of the polarization of the electric field of the incident light, and |⟨ψkc|û·r|⃗ ψkv⟩|, under supercell geometry (periodic boundary conditions), can be stated as the momentum matrix element between CB and VB at a given k-point in the first Brillouin zone. The real and the imaginary part of the dielectric constant is linked by the Kramers−Kronig transformation.45 Hence we can obtain the real part of the dielectric function ε1(ω) and then the refractive index n and birefringence Δn.
R1 = ∑∥F0| − |Fc∥/∑| F0| and wR2 = [∑w(F02 − Fc2)2/∑wFo4]1/2.
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angstroms) and angles (in degrees) for Na2B6O9F2 are given in Table S3 in the Supporting Information. Infrared Spectroscopy. IR spectrum of Na2B6O9F2 was measured on a Shimadzu IR Affinity-1 Fourier transform infrared spectrometer in the 400−4000 cm−1 range. A mixture of Na2B6O9F2 powder (5 mg) and KBr (500 mg) powders was well-mixed and pressed into a thin disk. The disk was dried in the oven at 100 °C for 30 min before the IR spectrum test. UV−vis-NIR Diffuse-Reflectance. Polycrystalline powders of Na2B6O9F2 synthesized by the standard solid-state reaction were used for the UV−vis-NIR diffuse-reflectance measurement. Optical diffuse reflectance spectrum of Na2B6O9F2 was measured at room temperature with a Shimadzu SolidSpec-3700DUV spectrophotometer. Data were collected in the wavelength range from 190 to 2600 nm. UV-DUV Transmittance Spectroscopy. The crystal plate without polishing (see Figure S2 in the Supporting Information) was used for the transmittance spectrum measurement. The measurement was performed in the wavelength range from 165 to 400 nm on the SolidSpec-3700 DUV spectrophotometer. Birefringence. The birefringence of Na2B6O9F2 was measured by immersion technique40 on GR-5 Gemological Refractometer (Wuhan Zhongdixueyuan Gem Instrument Ltd., China), with the measuring range of 1.35 to 1.85 and the accuracy of ±0.002. The light source was sodium yellow light with the wavelength of 589.3 nm. A crystal plate with (200) incident surface was used for this measurement. Thermal Analysis. Thermal gravimetric (TG) analysis and differential scanning calorimetry (DSC) were carried out on a NETZSCH STA 449F3 thermal analyzer instrument. The sample of Na2B6O9F2 was prepared by grinding an as-grown single crystal. The sample was enclosed in a platinum crucible and heated from 40 to 800 °C with a heating rate of 5 °C min−1 in an atmosphere of flowing N2. In order to further investigate the thermal behavior of Na2B6O9F2, the sample was heated from 520 to 620 °C and checked by powder X-ray diffraction per 20 °C. The sample was held for 5 h at each temperature and then quenched in air. The powder X-ray diffraction data were collected at room temperature on the Bruker D2 PHASER diffractometer. Theoretical Calculations. The electronic structure and optical properties were calculated using the plane-wave pseudopotential method implemented in the CASTEP package.41 The ion−electron
RESULTS AND DISSCUSSION Crystal Structure. Na2B6O9F2 crystallizes in the centrosymmetric monoclinic space group P21/c. The asymmetric unit of Na2B6O9F2 comprises a total of 19 independent atoms: two sodium, six boron, nine oxygen, and two fluorine atoms (see Figure 3). The fundamental building block (FBB) of
Figure 3. Asymmetric unit and selected symmetry-equivalent atoms in Na2B6O9F2, showing the linkages and coordination spheres with thermal ellipsoids and atom labels. Thermal ellipsoids are drawn at the 50% probability level. Symmetry codes: #1 −x, −y, −z; #2 −x + 1, −y, −z + 1; #3 x, −y + 1/2, z + 1/2; #4 −x, −y, −z + 1; #5 −x, y + 1/2, −z + 1/2; #6 x + 1, y, z; #7 −x + 1, y − 1/2, −z + 1/2; #8 x, −y + 1/2, z − 1/2.
Na2B6O9F2 is the [B6O11F2]8− units which are composed of two crystallographic independent [B3O6F]4− rings. Each [B3O6F]4− ring is composed of one asymmetric BO3F tetrahedron and two BO3 triangles via sharing O atoms. As indicated in Figure 4a, the FBBs connect with each other by sharing O1 and O6 atoms in the ab-plane forming a twoC
DOI: 10.1021/acs.inorgchem.6b02269 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
Figure 4. (a) [B6O9F2]2− layer in the ab-plane; (b) [B6O9F2]2− layer of Na2B6O9F2 viewed along the a axis (Na atoms are omitted for clarity).
dimensional (2D) zigzag [B6O9F2]2− layer. Viewed along the c axis, there are 12-membered tunnels which consist of four [BO3F]4− tetrahedra and eight [BO3]3− triangles. The O···O distances range from 6.095 to 10.774 Å. The Na1 atoms are 8coordinates with six oxygen atoms and two fluorine atoms, and the Na2 atoms are 7-coordinates with five oxygen atoms and two fluorine atoms, respectively (see Figure 3), filling in the 12membered tunnels. The layers extend along the c axis via sharing Na atoms forming the three-dimensional (3D) framework of the title compound. The Na−O (Na−F) bond lengths vary from 2.3706(17) to 2.666(2) Å (2.2719(16) to 2.8907(16) Å). The B−O bond lengths and B−F bond lengths vary from 1.338(3) to 1.472(3) Å and 1.427(3) to 1.431(3) Å, respectively. Bond valence sum (BVS) calculations46,47 were performed for all atoms (Na, 0.80−0.96; B, 3.05−3.13; F, 0.87−0.88; O, 2.00−2.13), which are consistent with the expected valences (see Table S2 in the Supporting Information). The introduction of fluorine atoms can embrace more diversity in borate structures. Figure S3a−d in the Supporting Information shows the examples of anion groups of the reported alkali metal fluoroborates: the isolated [B3O3F6]3− groups of Na3B3O3F630 and K3B3O3F6,34 the [B3O4F3]2− chain of Li2B3O4F3,32 the [B6O9F]− layer of LiB6O9F,33 and the [B6O9F2]2− network of Li2B6O9F2.31 The solo fluoroborate that contains stereoactive lone pair cation, BiB2O4F,29 possesses a one-dimensional [B 2 O 4 F] 3− chain (Figure S3e in the Supporting Information). To our best knowledge, the [B6O9F2]2− layer in the title compound represents a new structure type of fluoroborate. Infrared Spectroscopy. In the IR spectrum of Na2B6O9F2 (Figure S4 in the Supporting Information), the peak at 1332 cm−1 is attributable to asymmetric stretching of the BO3 groups, whereas the peaks at 742 and 706 cm−1 are the out of plane bending of BO3, and the bending of BO3 occurs at 560 cm−1. Specifically, the peaks at 783 and 624 cm−1 are consistent with the B−F stretching vibration of the BO3F groups and symmetric pulse vibration of triborate anion, confirming the existence of the B3O6F ring. These results are in consistent with literature48−51 and identify the correctness of the crystal structure. Diffuse Reflectance and UV-DUV Transmittance Spectroscopy. The diffuse reflectance spectrum in Figure 5
indicates that Na2B6O9F2 possesses a wide transparency range from UV to NIR region without obvious absorption.
Figure 5. UV−vis-NIR diffuse reflectance spectrum of Na2B6O9F2 showing a transparency range from UV to NIR. The inset is the transmittance spectrum of Na2B6O9F2 which indicates the cutoff edge is 169 nm.
The cutoff edge of Na2B6O9F2 is identified by the transmittance spectrum in the DUV-UV region, as shown in the inset of Figure 5. It is clearly shown that the cutoff edge of Na2B6O9F2 is 169 nm (corresponding to a large band gap of 7.33 eV), which is the shortest one compared with the known UV-DUV birefringent crystals, α-BBO, BMBO, and NBBF. This property makes it applicable in the DUV region. Birefringence. During the refractive index measurement, three sets of refractive indices were observed and recorded under the radiation of the wavelength of 589.3 nm, i.e., 1.510 and 1.560; 1.490 and 1.550; 1.450 and 1.530, respectively. Since the direction of the incident beam may not be perpendicular or parallel to the optical axis of the crystal, the difference between the two data in each set, i.e., 0.050, 0.060, and 0.080, is not larger than the birefringence, which is the difference between the largest and smallest refractive indices. Therefore, it is D
DOI: 10.1021/acs.inorgchem.6b02269 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry expected that the birefringence of the crystal is not
