Na3Ba2(B3O6)2F: Next Generation of Deep-Ultraviolet Birefringent

Dec 8, 2014 - Na3Ba2(B3O6)2F: Next Generation of Deep-Ultraviolet Birefringent Materials ... that NBBF is an attractive candidate for the next generat...
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Na3Ba2(B3O6)2F: Next Generation of Deep-Ultraviolet Birefringent Materials Hui Zhang,†,‡ Min Zhang,*,† Shilie Pan,*,† Zhihua Yang,† Zheng Wang,† Qiang Bian,†,‡ Xueling Hou,† Hongwei Yu,† Fangfang Zhang,† Kui Wu,† Feng Yang,§ Qinjun Peng,§ Zuyan Xu,§ Kelvin B. Chang,∥ and Kenneth R. Poeppelmeier*,∥ †

Key Laboratory of Functional Materials and Devices for Special Environments of CAS, Xinjiang Technical Institute of Physics & Chemistry of CAS, Xinjiang Key Laboratory of Electronic Information Materials and Devices, 40-1 South Beijing Road, Urumqi 830011, China ‡ University of Chinese Academy of Sciences, Beijing 100049, China § Technical Institute of Physics & Chemistry of CAS, Beijing 100190, China ∥ Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208-3113, United States S Supporting Information *

ABSTRACT: Birefringent materials are of great importance in optical communication and the laser industry, as they can modulate the polarization of light. Limited by their transparency range, few birefringent materials, except α-BaB2O4 (α-BBO), can be practically used in the deep ultraviolet (UV) region. However, α-BBO suffers from a phase transition and does not have enough transparency in the deep UV region. By introducing the relatively small alkali metal Na+ cation and the F− anion to keep the favorable structural features of α-BBO, we report a new birefringent crystal Na3Ba2(B3O6)2F (NBBF), which has the desirable optical properties. NBBF not only maintains the large birefringence (Δn = no − ne = 0.2554−0.0750 from 175 nm to 3.35 μm) and extends its UV cutoff edge to 175 nm (14 nm shorter than α-BBO) but also eliminates the phase transition and has the lowest growth temperature (820 °C) among birefringent materials. These results demonstrate that NBBF is an attractive candidate for the next generation of deep UV birefringent materials.

1. INTRODUCTION

interference ability of UV technology. High-temperature barium borate α-BBO,14−16 however, is the unique material to date that can be used as a deep UV birefringent material. Unfortunately, α-BBO has a phase transition at 925 °C and a markedly diverse thermal expansion coefficient between the a and c axes (αc/αa = 9), which causes it to easily crack during its growth and application. Additionally, the relatively high UV transmittance cutoff (189 nm) edge prevents its effective use in the deep UV range. Long-term stability is also a limiting factor for use of α-BBO owing to its slightly hydroscopic nature. Because of these obstacles, it is still urgent to develop a new generation of deep UV birefringent materials that satisfies the following essential property requirements: relatively large

Birefringent materials, which can modulate the polarization of light, are very important and interesting in optical communication and the laser industry as they are vital crystalline materials in producing optical devices such as polarization beam splitters, optical isolators, circulators, and Q switches.1−5 After continuous efforts in the past few decades, many birefringent materials have been found.6−9 YVO4,10 TiO2,11 LiNbO3,12 and CaCO313 are the most advanced birefringent materials, which have been widely used in the near-infrared (NIR) and visible range. Meanwhile, with the rapid development of ultraviolet (UV) technology for communication and sensing systems, birefringent crystals in the deep UV (λ < 200 nm) range are of current interest and are attracting considerable attention owing to the higher power density, higher capacity, faster data transmission rate, and superior anti-interception and anti© XXXX American Chemical Society

Received: November 19, 2014

A

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was determined by observing the growth or dissolution of crystal seeds on the melt surface. At the temperature 2 °C higher than the melting point, a selected seed attached to a platinum rod was dipped into the melt and held in place for 10 min to melt the outer surface of the seed. The temperature was then lowered to 1 °C below the melting point and held at that temperature until the end of the growth. The growing crystal was rotated at a rate of 8 rpm. After the growth, the crystal was cooled to room temperature at a rate of 2−5 °C h−1. We used seed crystals oriented in the [001] direction. 2.3. Structure Determination. A small crystallite with dimensions of 0.253 mm × 0.212 mm × 0.146 mm that was broken from the obtained crystal was selected for single crystal data collection. The crystal structure was determined by single crystal X-ray diffraction on a Bruker SMART APEX II CCD diffractometer using monochromatic Mo Kα radiation (λ = 0.71073 Å) at 296(2) K and integrated with the SAINT program.43 Numerical absorption corrections were carried out using the SCALE program for the area detector.43 All calculations were performed with programs from the SHELXTL crystallographic software package.44 All atoms were refined using full matrix least-squares techniques; final least-squares refinement is on Fo2 with data having Fo2 ≥ 2σ(Fo2). The final structure was checked for missing symmetry elements with PLATON.45 Crystal data and structure refinement information are summarized in Table S1 in the Supporting Information. The final refined atomic positions, isotropic thermal parameters, and bond valence sum are given in Table S2. The selected bond distances and angles are listed in Table S3. 2.4. Thermal Analysis. Thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) were carried out on a simultaneous NETZSCH STA 449 F3 thermal analyzer instrument in a flowing N2 atm. In order to investigate its melting behaviors, the sample was placed in Pt crucible, heated from 40 to 900 °C, and then cooled to 200 °C at a rate of 5 °C min−1. Polycrystalline samples of NBBF were prepared by grinding an as-grown NBBF single crystal. The samples were heated from 300 °C to the melting point. Samples were checked by powder X-ray diffraction (PXRD) every 10−50 °C from 300 °C to the melting point. The sample was held for 20 h at each temperature and then quenched in air. The PXRD data were collected at room temperature using an automated Bruker D2 X-ray diffractometer. Thermal expansion coefficients of NBBF along the a and c axes were measured by a NETZSCH DIL 402 PC dilatometer in the temperature range of 40−600 °C with a heating rate of 5 °C/min in air. The sample lengths along the a and c direction were 6.08 and 5.24 mm, respectively. 2.5. Transmittance Spectra. The transmittance spectra of a transparent NBBF crystal plate 1.5 mm thick were measured by a SolidSpec-3700DUV spectrophotometer for the range of 165−2600 nm in a nitrogen gas atmosphere and by a SHIMADZU IRAffinity-1 Fourier transform infrared spectrometer for the range of 2500 nm (4000 cm−1) to 4000 nm (2500 cm−1) in air. 2.6. Laser Damage Threshold. The laser-induced damage test of NBBF and α-BBO was performed on a Q-switched YAG laser (1064 nm, 30 ps, 10 Hz) using crystal samples (4 mm × 4 mm × 1 mm) without any pretreatment, respectively. Pulse energy was adjusted by changing the laser operating voltage. An optical concave lens was used to obtain the appropriate laser beam diameter (0.5 mm). Damage was detected by using a He−Ne laser beam focused on the irradiated sites. The damage was clearly visible since it caused most of the probe beam to scatter. The damage was confirmed afterward by observing the irradiated sites under a microscope. 2.7. Refractive Indices Determination. The refractive index dispersion of NBBF was characterized by the minimum deviation technique at 16 different monochromatic sources from 253.7 to 2325.4 nm (Trioptics Spectromaster HR, Germany). Since NBBF is a uniaxial crystal, it is possible to measure the values of refractive indices for both the ordinary (no) and extraordinary (ne) polarizations using one prism. The prism (10 mm × 5 mm × 4 mm, Figure 2f) was cut along the surface that composed the apex parallel to the crystallographic c-axis (apex angle is 30.0°). The incident polarized beam is perpendicular to the (100) incident surface; no and ne are the refractive indices of light

birefringence (>0.1), deep UV transparency range, high damage threshold, good chemical and mechanical stability, and ease of growth. To develop superior birefringent materials that not only possess a birefringence comparable to α-BBO, but also have extended UV cutoff edges and no phase transitions and are easier to grow, the following strategies have been considered: (1) the (Ba2B6O12)∞ double layer in α-BBO is ideal to obtain a large birefringence owing to its enhanced anisotropic polarizability;17−20 (2) small alkali or alkaline earth metals will be beneficial to reinforce the electrostatic force between cations and anionic frameworks, which may in turn eliminate the phase transition exhibited by α-BBO;21−28 (3) small alkali or alkaline earth metals and F atoms with large electronegativity and small radii are beneficial to shift the cutoff edge to the deep UV region.29−38 Thus, the relatively small alkali metal Na+ cation and the F− anion are introduced in the BaO−B2O3 system to keep the favorable structural features of α-BBO. Through our systematic investigation, we synthesized a new birefringent material, Na3Ba2(B3O6)2F (NBBF), which not only maintains the large birefringence (Δn = no − ne = 0.2554−0.0750 from 175 nm to 3.35 μm) and extends its UV cutoff edge to 175 nm (14 nm shorter than α-BBO) but also eliminates the phase transition and has the lowest growth temperature (820 °C) among birefringent materials. These results demonstrate that NBBF is an attractive candidate for the next generation of deep UV birefringent materials. Although NBBF was first found in the growth of single crystals of the low-temperature barium borate modification βBaB2O4 in the BaB2O4−NaF system by Kokh et al. in 2009,39 its full potential as a new birefingent material has not been reported. Single crystals of NBBF have been previously grown with UV transmittance and absorption edges measured to be 186 and 200 nm, respectively.40,41 We investigate its further potential value in the field of birefringent materials, and report a procedure to grow large, high quality single crystals,42 with an even deeper UV transmittance edge than previously reported, down to 175 nm. Furthermore, we provide a description of chemical design of how NBBF eliminates the α−β phase transition, report low anisotropic thermal expansion, perform DFT calculations to explain the origins of the deeper UVtransmittance cutoff edge, and report the laser damage threshold. These detailed property characterizations are required to advance NBBF to be a competitive next generation birefringent material.

2. EXPERIMENTAL SECTION 2.1. Reagents. NaF (Tianjin YaoHua Chemical Reagent Co., Ltd., 99.0%), Na2CO3 (Tianjin HongYan Chemical Reagent Co., Ltd., 99.0%), BaCO3 (Tianjin Bodi Chemical Co., Ltd., 99.0%), and H3BO3 (Tianjin HongYan Chemical Co., Ltd., 99.5%) were used as received. 2.2. Crystal Growth. Large single crystals of NBBF were grown by the Kyropoulos method from its stoichiometric ratio composition melt without a flux (because it melts congruently, as seen in the Thermal Analysis section). The pure crystalline powder samples were melted at 845 °C in a platinum crucible that was placed into a vertical, programmable temperature furnace, kept at this temperature for 24 h to ensure complete melting and homogeneity of the raw materials, and then cooled to 820 °C. To obtain a seed crystal, a platinum rod was dipped into the melt rapidly, and the temperature was reduced at a rate of 1 °C h−1. The obtained crystals were branchy with inclusions, but parts of them were usable as seeds. After several growth experiments, suitable seed crystals were obtained, and the melting point of NBBF B

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Figure 1. Crystal structural features of (a) β-BBO, (b) α-BBO, and (c) NBBF. polarized perpendicular and parallel to the optical axis (crystallographic c-axis), respectively. 2.8. Computational Methods. To investigate the relationship between microscopic and macroscopic properties, the electronic structures of NBBF and α-BBO were obtained using density functional theory (DFT) based ab initio calculations implemented in the CASTEP package.46 The generalized gradient approximation (GGA) with the Perdew−Burke−Ernzerhof (PBE) functional was employed for the exchange-correlation potential. A plane-wave basis set energy cutoff was 900 eV within the Norm-conserving pseudopotential, and the Monkhorst−Pack47 scheme was given by (6 × 6 × 4 for NBBF and 6 × 6 × 2 for α-BBO) in the irreducible Brillouin zone. Our tests reveal that the above computational setups are sufficiently accurate for present purposes. On the basis of the obtained electronic structures, the linear properties for NBBF were calculated. The linear optical refractive indices and birefringence can be obtained from the electronic transition between the occupied and unoccupied states caused by the interaction with photons.48

The different coordination environments of Ba1 and Ba2 can also be viewed as an imbalance in the Lewis acidity within the cationic framework. It has recently been described how the cationic framework Lewis acidity can play a role in polymorphism.51 The Lewis acidity of a cation is defined as the formal valence divided by the average coordination number.52 A Ba cation in oxides has an average Lewis acidity of 0.20 valence units (vu), which corresponds to an average coordination number of 10. The Ba1 cations of α-BBO (CN = 9) within the (Ba2B6O12)∞ double layers exhibit a Lewis acidity of 0.22 vu, while the Ba2 cations between the (Ba2B6O12)∞ double layers exhibit a Lewis acidity of 0.33 vu. This large difference between Ba1 and Ba2 Lewis acidity values and relatively large Lewis acidity value of Ba2 leads to an imbalance of Lewis acidity within the cationic framework. Thus, we deem that the six-coordinated Ba2 in α-BBO is unstable, which gives rise to the α−β phase transition through a twist of the (B3O6)∞ layer. Additionally, substitution of Ba22+ in α-BBO with the [Na3F]2+ groups in NBBF stabilizes the desired structure of the unique (Ba2B6O12)∞ double layers (Figure 1(b), (c)), which is the birefringence active group. The [Na3F]2+ group has three Na cations and one F anion to bridge the adjacent (Ba2B6O12)∞ double layers through 12 Na−O bonds and two Ba−F bonds (Figure S2), which will strongly reinforce the connection of the adjacent (Ba2B6O12)∞ double layers.53 Additionally, the Na cations are five coordinated, which corresponds to a Lewis acidity of 0.20 vu. The average Lewis acidity of Na in oxides is 0.16 vu.52 Replacement of the Ba22+ cation with a [Na3F]2+ cation consequently decreases the imbalance of Lewis acidity within the cationic framework. The Lewis acidity of Ba2+ and Na+ in NBBF are compared to the average Lewis acidity values in Table 1. Therefore, through the molecular engineering modification by [Na3F]2+ and Ba22+ substitution, NBBF can eliminate the phase transition in αBBO.

3. RESULTS AND DISCUSSION NBBF crystallizes in the centrosymmetric hexagonal space group of P63/m. As illustrated in Figure 1c (Na−O bonds are not shown for clarity), the fundamental building units of NBBF are isolated planar B3O6 anionic groups, which are distributed perfectly parallel in the ab plane to form a (B3O6) layer. One (B3O6) layer links with another adjacent layer through 10 coordinate Ba atoms to generate a unique (Ba2B6O12)∞ double layer (Figure S1 in the Supporting Information). Three Na+ cations coordinate to one F− anion to form an [Na3F]2+ group (Figure S2) that resides between the (Ba2B6O12)∞ double layers. The neighboring (Ba2B6O12)∞ double layers are further bridged together by [Na3F]2+ groups through Na−O and Ba−F connections. To investigate the optical properties and the BBO α−β phase transition previously introduced, we compared the structure of α-BBO (Figure 1b) and β-BBO (Figure 1a). As depicted in Figure 1, the basic building frameworks of α-BBO and β-BBO are both parallel planar B3O6 anionic groups, which enhance the anisotropic polarizability to produce a large birefringence. As seen in Figure 1 and Figure S3, the difference between αBBO and β-BBO is the coordination environment of the Ba atoms. In α-BBO, there are two kinds of Ba atoms. Ba1 coordinates to nine O atoms, while Ba2 coordinates to six O atoms. According to Pauling’s fifth rule,49 Ba1 and Ba2 in αBBO tend to adopt similar coordination environments as the Ba atoms in β-BBO, which all coordinate to seven O atoms. Furthermore, the different Ba1 and Ba2 coordination environments lead to unbalanced electrostatic interactions between the inner and inter (Ba2B6O12)∞ double layers according to Coulomb’s law.50

Table 1. Comparison of the Average Lewis Acidity Values (in Valence Units) of Ba and Na in Oxides to the Lewis Acidity Values of Ba and Na Adopted in α-BBO and NBBFa cation

average Lewis (v.u.) acidity in oxides

Lewis acidity (v.u.) in α-BBO

Lewis acidity (v.u.) in NBBF

Ba1 Ba2 Na

0.20 0.20 0.16

0.22 0.33 n/a

0.20 n/a 0.20

Replacement of Ba2 in α-BBO with Na in NBBF relieves the Lewis acidity imbalance adopted by Ba2 in α-BBO.

a

C

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Figure 2. Crystal growth analysis of NBBF: (a) Theoretical morphology of NBBF; (b) the as-grown (001) NBBF crystal. Thermal properties and optical properties of NBBF: (c) XRD, TG, and DSC analysis; (d) thermal expansion measurements; (e) UV−vis-NIR transmission spectrum; (f) experimental refractive indices and fitted refractive index dispersion curves.

the thermal expansion is almost linear in the temperature range of 100−600 °C, which also indicates that NBBF is stable between 50 and 600 °C. Hence, we can confirm that NBBF is thermally stable without any phase transitions, which is beneficial for single crystal growth. Analysis of DSC curves and PXRD patterns before and after melting confirms that NBBF is a congruently melting compound (Figure 2c). Consequently, we report the growth of a typical hexagonal (001) NBBF single crystal with dimensions of 35 mm × 35 mm × 9 mm (Figure 2b) from a stoichiometric melt by the kyropoulos method at 820 °C in only 2 days. To the best of our knowledge, the growth temperature of 820 °C is the lowest among birefringent materials, which means that no harsh growth conditions (e.g., atmosphere protection, expensive iridium crucible, high power furnace etc.) are needed. Thus, the growth of NBBF from its stoichiometric melt is highly efficient with respect to time, cost, and energy. According to the Bravais−Friedel and Donnay−Harker (BFDH) theory,57 the theoretical morphology of NBBF (Figure 2a) was established by the Mercury program.58 The

NBBF has previously been reported to be a congruently melting material.54−56 Three different methods were used to confirm the lack of a phase transition in NBBF. Thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) curves of polycrystalline samples of NBBF are shown in Figure 2c. The results show that NBBF exhibits negligible weight loss up to 900 °C. Only one endothermic (820 °C) and one exothermic (711 °C) peak are observed from the heating curve and cooling curve, respectively, which indicates that NBBF does not show any evidence of a phase transition between room temperature and the melting point. In addition, the polycrystalline sample was checked by powder Xray diffraction (PXRD) from 300 °C to the melting point every 10−50 °C. As seen in Figure S4, the experimental PXRD patterns of NBBF from 300 °C to the melting point are in agreement with the calculated pattern based on single crystal data, further confirming that the NBBF phase has no change from 300 °C to the melting point. Thermal expansion measurements were also performed on NBBF single crystalline plates from 50 to 600 °C. Figure 2d shows the thermal expansion curves along the three crystallographic axes, in which D

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orientation determination by the XRD method was performed on the (001) crystal. While the largest distinguishable facet is (001), the (100) and (101) facets are also found (Figure 2b), which is consistent with the theoretical morphology. A small crystal of NBBF (3.835 g) was exposed in the air for 60 days at room temperature. The crystal was still transparent and the weight did not change, which indicates that NBBF is not hygroscopic and is stable in air, an improvement to α-BBO. The measured Mohs’ hardness is about 4−5. The chemical stability and the good mechanical properties make NBBF easy to cut and polish. Thermal expansion coefficients are important parameters for crystal growth and application in devices.59 As can be seen in Figure 2d, the NBBF crystal exhibits only a positive thermal expansion when heated. The calculated average thermal expansion coefficients are αa = α(110) = 12.1 × 10−6 K−1, and αc =27.5 × 10−6 K−1 from 50 to 600 °C, respectively. The value of αc/αa for NBBF is about 2.27. Compared with that of α-BBO (9.0), the NBBF crystal exhibits a weak anisotropic thermal expansion, which will effectively protect the crystal from cracking caused by thermal expansion during crystal growth, processing and applied use in devices. The optical transmission range and the UV cutoff edge are very important factors for single crystals used in optical applications. UV−vis-NIR transmission spectra of NBBF were measured on a single crystal plate 1.5 mm thick that was cut and polished from one as-grown crystal. As shown in Figure 2e, the NBBF crystal presents a broad transmission of 175−3350 nm, which indicates that the application of NBBF can cover a range from the UV to NIR. In particular, the UV cutoff edge (175 nm) is 14 nm shorter in comparison to α-BBO (189 nm), indicating that the NBBF crystal may be more effective for applications in the deep UV, especially in deep UV spectroscopy and the lithography industry. The difference in the UV cutoff edge between α-BBO and NBBF can be elucidated by the electronic structure obtained from the first-principles calculations of the plane-wave pseudopotential method. Figure 3 gives the highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO) of α-BBO and NBBF. It is clearly illustrated that both Ba sites (particularly Ba1) in α-BBO make up the LUMO. The limitation of the band gap caused by Ba1 is decreased in NBBF. The Na and F atoms, however, provide a wider transparent region as shown in Figure S5. These factors consequently offer a 14 nm violet shift of the absorption band. To the best of our best knowledge, NBBF possesses the lowest UV cutoff edge among the birefringent crystals that have been commercially applied or that have thus far been proposed. The laser-induced damage measurements (1064 nm, 30 ps, 10 Hz) of as-grown crystals (without any pretreatment) show that the laser damage threshold (LDT) of the NBBF and αBBO crystal are all about 108 GWcm−2. Although the mechanism for laser damage of a crystal is not yet fully clear, it has been normally accepted that strong optical absorption will cause thermal and electronic effects and eventually lead to laser damage. To investigate the optical absorption, band structures of NBBF and α-BBO were calculated (Figure S5). Along the high symmetry points of the first Brillouin zone, the valence band (VB) maximum is located at the T point, while the conduction band (CB) minimum is located at the G point, which results in an indirect energy gap. The band gap of NBBF (4.676 eV) is slightly larger than that of α-BBO (4.606 eV). It is known that wider band gaps indicate a shorter absorption edge

Figure 3. Molecule orbitals of α-BBO and NBBF: (a) HOMO and (b) LUMO of NBBF; (c) HOMO and (d) LUMO of α-BBO.

in the UV region owing to the mechanism of photon energy determined by electron transitions from the VB to the CB. Thus, the wider gap of NBBF is beneficial for enhancing the LDT, larger LDT of NBBF can be expected when high purity reagent used in the progress of crystal grown. Hence, NBBF can be widely applied in high power laser systems as well as αBBO. The refractive index dispersion of NBBF was characterized by the minimum deviation technique at 16 different monochromatic sources from 253.7 to 2325.4 nm on a prism (10 mm × 5 mm × 4 mm, Figure 2f). The values of refractive indices for no and ne at specific wavelengths are summarized in Table S4, which shows that NBBF is a negative uniaxial optical crystal since no > ne. The experimental data were fitted to the following Sellmeier equations: no2 = 2.6222(6) +

0.0163(7) − 0.0131(1)λ 2 λ 2 − 0.0170(3)

(1)

ne2 = 2.2767(6) +

0.0117(0) − 0.0028(2)λ 2 λ − 0.0114(1)

(2)

2

where λ is the wavelength expressed in micrometers. The calculated values agree well with experimental ones to the fourth decimal place, which indicates that the fitted Sellmeier equations are reliable. Figure 2f shows the measured and fitted refractive index data for both no and ne. The results show that NBBF has a large birefringence (Δn = 0.13−0.0937 from 253.7 nm to 2.325 μm) that is comparable to α-BBO. When applied in the UV and deep UV range, an even larger birefringence can be expected, e.g., Δn = 0.1991 at 193 nm as calculated from the Sellmeier equations. Equation 3 deduces the walk-off angle (ρ), which is the inclined angle of ordinary light and extraordinary light in a uniaxial crystal, where θ represents the inclined angle of the optical axis and the interface. The larger the value of ρ, the larger the light beam displacement will be, which E

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Table 2. Properties of Main Industrial Birefringent Crystals and NBBF space group birefringence ρ (walk-off angle) transparency range (nm) αc/αa Mohs hardness hygroscopy phase transition growth temperature (°C)

YVO4

TiO2

CaCO3

LiNbO3

α-BBO

NBBF

I41/amd 0.225@633 nm 6.042 400−5000

P42/mnm 0.256@1530 nm 5.648 400−5000

R3c̅ 0.171@633 nm 6.205 350−2300

R3c 0.0836@633 nm 2.135 420−5200

R3c̅ 0.1222@532 nm 4.345 189−3500

P63/m 0.1149@532 nm 4.120 175−3350

2.59 5 no no 1810

1.30 6.5 no no ∼1300

4.87 3 slightly no mineral

2.39 5 no no 1300

9 4.5 slightly @925 ± 5 °C 1100

2.27 4.5 no no 820

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consequently reduces the size that an optical device could be. It is not hard to find that ρ will reach a maximum value when θ reaches 45°. Thus, the deduced ρ of NBBF at 193 nm is as large as 6.46°, which means that NBBF possesses excellent birefringent properties in the deep UV range. tan ρ =

|no2 − ne2| 1 sin θ 2 no2 sin 2 θ + ne2 cos2 θ

ASSOCIATED CONTENT

S Supporting Information *

View of (Ba2B6O12)∞ double layers in NBBF, [Na3F]2+ group and Na−O coordination in NBBF, coordination of Ba atoms in α-BBO and β-BBO, experimental PXRD patterns of NBBF from 300 to 800 °C, electronic band structure, total and partial density of states (PDOS) of NBBF, electronic band structure, total and partial density of states (PDOS) of α-BBO, birefringence of experimental, calculated and calculated B3O6 of NBBF, charge-density map of B3O6 group and Na, Ba, F in NBBF, crystal data and structure refinement for NBBF, atomic coordinates, equivalent isotropic displacement parameters (A2) and bond valence sum for NBBF, selected bond lengths (Å) and bond angles (degree) for NBBF, experimental and fitted refractive indices based on Sellmeier equations in UV to IR, and atom-cutting analysis and calculated birefringence of NBBF at 532 nm. This material is available free of charge via the Internet at http://pubs.acs.org.

(3)

On the basis of the electronic band structures calculated via the ab initio method implemented in CASTEP package,46 the dispersions of the linear refractive indices for NBBF were calculated. As seen in Figure S6, the derived birefringence is consistent with the experimental value. To further analyze the contribution of an ion (or an anionic group) to the birefringence, a real-space atom-cutting technique60 was adopted. As shown in Table S5, the anionic B−O groups (B3O6) make the dominant contributions to the birefringence, while the contributions of the alkali, alkaline earth cations (Na and Ba) and F anion are negligibly small. These optical properties can be elucidated from the microscopic structural features of an NBBF crystal as follows: According to the anionic group theory,61 in the alkali and alkaline earth borate crystals the B−O groups are the dominating active microscopic units owing to their anisotropic polarizabilities (Figure S7), which determine the birefringence. In NBBF, all of the planar B3O6 groups are arranged in parallel, which will enhance the anisotropic polarizabilities to obtain a large birefringence.62

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AUTHOR INFORMATION

Corresponding Authors

*(M.Z. ) E-mail: [email protected]. *(S.P.) E-mail: [email protected]. *(K.R.P.) Phone: 847-491-3505. E-mail: krp@northwestern. edu. Notes

The authors declare no competing financial interest.

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4. CONCLUSIONS In summary, we have fully characterized a new deep UV birefringent crystal of NBBF. A single crystal of NBBF with dimensions of 35 mm × 35 mm × 9 mm was grown from its stoichiometric melt for the first time in 2 days, a greatly improved crystal growth method. These NBBF crystals achieve desirable optical properties with a large birefringence (Δn = 0.0750−0.2554) from the IR (3.35 μm) to the deep UV (175 nm) range. NBBF melts congruently with no phase transitions and possesses the lowest growth temperature among birefringent materials, which is beneficial for the growth of large, high quality optical single crystals. The good chemical stability and mechanical properties make it easy to process by cutting and polishing. In particular, by comparing its optical, thermal, and mechanical properties with the industrial birefringent materials (Table 2), we believe that NBBF is an attractive candidate for the next generation of deep UV birefringent materials.

ACKNOWLEDGMENTS

This work is supported by the National Natural Science Foundation of China (Grant Nos. U1129301, 51172277), the Western Light of Chinese Academy of Sciences (Grant No. XBBS201217), 973 Program of China (Grant Nos. 2012CB626803, 2014CB648400), Main Direction Program of Knowledge Innovation of CAS (Grant No. KJCX2-EW-H0303), The Funds for Creative Cross & Cooperation Teams of CAS, Xinjiang International Science & Technology Cooperation Program (20146001), K.R.P. acknowledges support from the National Science Foundation (Solid State Chemistry Award No. DMR-1307698). We thank Yun Yang (from Key Laboratory of Functional Materials and Devices for Special Environments of CAS, for refractive indices determination) and Hongping Wu (from Key Laboratory of Functional Materials and Devices for Special Environments of CAS, for the help of structure determination). F

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dx.doi.org/10.1021/cg5016912 | Cryst. Growth Des. XXXX, XXX, XXX−XXX