Rational Design via Synergistic Combination Leads to an Outstanding

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Rational Design via Synergistic Combination Leads to an Outstanding Deep-Ultraviolet Birefringent Li2Na2B2O5 Material with Unvalued B2O5 Functional Gene Min Zhang, Donghai An, Cong Hu, Xinglong Chen, Zhihua Yang, and Shilie Pan J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b13402 • Publication Date (Web): 18 Jan 2019 Downloaded from http://pubs.acs.org on January 18, 2019

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Journal of the American Chemical Society

Rational Design via Synergistic Combination Leads to an Outstanding Deep-Ultraviolet Birefringent Li2Na2B2O5 Material with Unvalued B2O5 Functional Gene Min Zhang,†,# Donghai An,†,§,# Cong Hu,†,‡ Xinglong Chen,† Zhihua Yang†,* and Shilie Pan†,*

† CAS Key Laboratory of Functional Materials and Devices for Special Environments, Xinjiang Technical Institute of Physics & Chemistry, CAS; Xinjiang Key Laboratory of Electronic Information Materials and Devices, 40-1 South Beijing Road, Urumqi 830011, China ‡ Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049, China. § Changji University, Changji 831100, China. # These authors contributed equally. * Corresponding authors, E-mails: [email protected]; [email protected].

Abstract Birefringent materials, the key component to modulate the polarization of light, are of great importance in optical communication and the laser industry. Limited by their transparency range, few birefringent materials, can be practically used in the deep ultraviolet (DUV, λ < 200 nm) region. Different from the traditional BO3- or B3O6-based DUV birefringent crystals, we propose a new functional gene, the B2O5 unit, for designing birefringent materials. Excitingly, the synergistic combination of Li4B2O5 and Na4B2O5 generates a new compound, Li2Na2B2O5, with enhanced optical properties. The Li2Na2B2O5 crystal with size up to 35 × 15 × 5 mm3 was grown by top seeded solution growth (TSSG) method, and its physicochemical properties were systematically characterized. Li2Na2B2O5 features large birefringence (0.095@532 nm), short DUV cut off edge (181 nm) with high laser-induced damage threshold (LDT, 7.5 GW/cm2 @1064 nm, 10 ns), favorable anisotropic thermal expansion (αa/αb = 5.6) and lowest crystal growth temperature (< 609 oC) among the commercial birefringent crystals. Moreover, the influences of the B2O5 structural configurations on the optical anisotropy were explored. The fascinating experimental results will provide a prominent DUV birefringent crystal and an effective synthesis strategy, which can facilitate the design of DUV birefringent materials.

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1. Introduction Birefringent materials have attracted continuously intensive studies as they can modulate the polarization of light and are critically important for optical communication and the laser industry.1 In recent decades, the DUV nonlinear optical (NLO) crystals, represented by typical KBe2BO3F2 (KBBF) families2 and newly developed fluorooxoborate by our group,3 have been vigorously updated and promotes the development of all-solid-state DUV laser technology.4-6 On the other hand, the DUV birefringent crystal is retarded,7 which drives us to develop the applicable DUV birefringent crystals. Currently, the commercial α-BaB2O4 (α-BBO) birefringent crystal has a large birefringence (△n = 0.122@532), whereas its DUV cut off edge is not short enough (189 nm).1 Also, the phase transition and high anisotropic thermal expansion limit its extensive applications.8 Moreover, MgF2 has a very low DUV transmittance cutoff edge (~110 nm), but the extremely small △n (0.012@532)9 hampers its efficient application. Therefore, it is of great significance to develop new DUV birefringent materials to promote the development of DUV laser technology. As a potential DUV birefringent material, the following requirements must be satisfied: transparent in practical DUV region, large birefringence (~0.1@visible) and easy to grow large crystals. Although the birefringence of borates, that is profited from planar B-O units, may not be superior to that of traditional vanadates,10 carbonates,11 telluromolybdates or tellurotungstates, etc.,12 the strong covalent B-O bonds can lead to short cut-off edges13 and promise them to be very good candidates for designing DUV birefringent materials.7,14 Specifically, planar B-O groups with coplanar alignment will produce large optical anisotropy along the directions that is parallel and perpendicular to the B-O planes. Recently, a series of birefringent materials with the BO3 and B3O6 units were continuously reported,7 which advance the practical applications of birefringent materials. However, the B2O5-based birefringent materials are rarely investigated.15 The abundant pyroborates drive us to investigate the feasibility of B2O5 as a functional gene for designing birefringent materials. 2


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To develop novel birefringent materials, many strategies were employed. Therein, cation/anion substitutions or cosubstitutions are very effective,16 for example, the cosubstitution of Ba2+ in α-BBO by (Na3F)2+ leads to Na3Ba2(B3O6)2F with comparable birefringence and better optical properties. Recently, as an effective method to enhance the properties of maternal materials, synergistic combination17 is widely applied in the fields of chemical synthesis, biochemistry and photocatalysis, etc. For borates, the discovery of CsLiB6O10 (CLBO)18 can be considered as the synergistic combination of LiB3O519 (LBO) and CsB3O5 (CBO),20 which generates a novel NLO material with better phase matching property. Guided by this idea, we attempt to explore the structural regulations via synergistic combination and anticipate achieving novel B2O5-containing borates with improved birefringence. Hence, Li4B2O521 and Na4B2O5,22 the only two examples in B2O5-containning alkali metal borates, were selected as maternal compounds to achieve synergistic combination. Finally, a new Li2Na2B2O5 compound, featuring favorable coplanar B2O5 units, was successfully obtained. The benign structure configuration of B2O5 leads to an obviously improved birefringence of 0.095@532, which is much larger than those of Li4B2O5 and Na4B2O5 (~0.04-0.07@532) (Table S1 in the Supporting Information). The prominent merits of Li2Na2B2O5 are as follows: a short DUV cutoff edge and high laser damage threshold (LDT), a large birefringence, a favorable anisotropic thermal expansion and easy to grow bulk crystals at low temperature ensuring it to be an outstanding DUV birefringent crystal. 2. Experimental section 2.1 Synthesis and crystal growth All commercially available chemicals Li2CO3, Na2CO3 (Aladdin, 99.5%) and H3BO3 (Aladdin, 99.8%) are of reagent grade and used as received. The polycrystalline sample of Li2Na2B2O5 was synthesized by the solid-state reaction method. A stoichiometric mixture of Li2CO3, Na2CO3 and H3BO3 was mixed homogeneously and transferred to an aurum crucible. The mixture was preheated to 400 ºC to decompose the carbonate and water by reaction-driven process. Then the mixture was heated to 550 ºC and held for 72 h until the powder X-ray diffraction 3



(XRD) patterns did not change. Single crystal of Li2Na2B2O5 was grown by top seeded solution growth (TSSG) method. The mixture of Li2CO3, Na2CO3 and H3BO3 with a molar ratio of 1.5:0.5:2 was loaded into an aurum crucible and placed in the center of a programmable temperature electric furnace. The temperature was gradually heated to 650 ºC and held for 24 h to ensure the mixture solution completely. Then the furnace was quickly cooled to 546 ºC and held for 1 h, and then the crystal seed with selected direction was dipped into the solution. The cooling rate during the crystal growth process is 0.2 oC

d-1, and a colorless block crystal with size up to 35 × 15 × 5 mm3 was obtained

after 2 days. 2.2 Thermal analysis. The circular thermal gravimetric (TG) and differential scanning calorimetry (DSC) analyses of Li2Na2B2O5 were investigated using a simultaneous NETZSCH STA 449C thermal analyzer instrument, with a heating rate of 5 ºC min-1 in an atmosphere of flowing N2 from 40 to 800 ºC. 2.3 Structure determination The powder X-ray diffraction data were carried out on 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° (2θ) with a scan step width of 0.02° and a fixed counting time of 1 s step-1. Block crystal of Li2Na2B2O5 (0.089× 0.141× 0.147 mm3) was selected for structure determination. The crystal data were collected on a Bruker SMART APEX II CCD diffractometer using monochromatic Mo Ka radiation (λ= 0.71073 Å) at 296(2) K and integrated with the SAINT program.23 The numerical absorption corrections were carried out using the SADABS program for the area detector. All calculations were performed with programs from the SHELXTL24 crystallographic software package and the structure was solved by direct methods using SHELXS–97. All atoms were refined using full matrix least-squares techniques with anisotropic thermal parameters, final least-squares refinement is on Fo2 with data having Fo2>2σ(Fo2). The structure was checked for missing symmetry elements with PLATON.25 The crystallographic data 4


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are listed in Table 1. The related crystal data, including selected bond lengths and atomic coordinates equivalent isotropic displacement parameters are listed in Tables 1, S2-S3 in the Supporting Information, respectively. 2.4. Refractive index dispersion measurements. The refractive indices of Li2Na2B2O5 were measured using a (001) crystal plate on the Metricon model 2010/M prism coupler (Metricon Co.) at five wavelengths (405.0, 514.0, 636.0, 964.8 and 1546.7 nm), and the accuracy of the measurements is estimated to be 2 × 10−4. The refractive indices along (100) and (010) (n100 and n010) were measured using TE mode (transverse electric mode, tests the refractive indices parallel to the crystal plane), while n001 was tested using TM mode (transverse magnetic mode, tests the refractive indices perpendicular to the crystal plane). 2.5. Physicochemical properties The major facets of obtained Li2Na2B2O5 crystal were examined by a Dandong YX-2 X-ray orientator. The UV-Vis-NIR transmission spectra (Shimadzu SolidSpec-3700DUV) were measured on single crystal plates that are cut and polished from as-grown crystal along with thickness of 1 mm. The laser-induced damage test of Li2Na2B2O5 and α-BBO were performed on polished crystal plates without coating by a Q-switched Nd:YAG laser (1064 nm, 10 ns, 10 Hz). An optical convex lens was used to obtain the appropriate laser beam diameter (1 mm). The damage was confirmed afterwards by observing the irradiated sites under a microscope. 2.6. Calculation details The optical properties (refractive index and birefringence) of these alkali-metal borates were calculated based on density functional theory (DFT) through CASTEP package.26 The Perdew-Burke-Ernzerhof (PBE) functional under generalized gradient approximation (GGA) was used,27 and the norm-conserving pseudopotentials (NCP)28 were also adopted with the energy cutoff of 830 eV. The Brillouin zone (BZ) Monkhorst-Pack grids29 were 11 × 11 × 2, 3 × 4 × 6, 3 × 7 × 2 and 3 × 7 × 4 for the calculation of Li2Na2B2O5, Na4B2O5, α-Li4B2O5 and β-Li4B2O5, respectively. The birefringence was calculated as the difference between the maximum and minimum 5



values of refractive indices along corresponding principal axes.

3. Results and discussion 3.1 Crystal structure description Li2Na2B2O5 crystallizes in orthorhombic system with the space group Cmcm (CCDC No. 1845561, Tables 1, S2-S3 in the Supporting Information). Therein, one unique lithium atom, two unique sodium atoms, one unique boron atom, and three unique oxygen atoms exist in its asymmetric unit. The coplanar B2O5 (fundamental 1

zigzag chains that consist of

2-

three dimensional (3D) network, then the

building blocks) FBBs connect with edge-sharing LiO4 to form

3

∞(Li2B2O5)

∞(LiO(1+3/2))

Na(1)O5 and Na(2)O8 polyhedra locate in the tunnels to form the whole structure (Figure 1). Table 1. Crystallographic data for Li2Na2B2O5.

a

Empirical formula Crystal system Space group, Z

Li2Na2B2O5 Orthorhombic Cmcm, 4

a (Å) b (Å) c (Å) Volume (Å3) Density (calcd) (g/cm3) Range for data collection (o) Crystal size (mm3) Limiting indices Reflections collected / unique Completeness to theta = 27.45 (%) Data / restraints / parameters GOF on F2 Final R indices [Fo2>2σ(Fo2)]a R indices (all data) a Extinction coefficient Largest diff peak and hole (e/Å3)

3.313(2) 9.985(6) 13.400(8) 443.2(5) 2.420 3.04 to 27.45 0.147 × 0.141 × 0.089 -4 ≤ h ≤ 4, -12 ≤ k ≤ 8, -17 ≤ l ≤ 16 1361 / 313 [R(int) = 0.0252] 99.7 313 / 0 / 38 1.138 R1 = 0.0285, wR2 = 0.0778 R1 = 0.0340, wR2 = 0.0808 0.017(5) 0.248 and -0.266

R1 = Fo - Fc/Fo and wR2 = [w(Fo2 – Fc2)2 / w Fo4]1/2 for Fo2 > 2( Fo2) .

The Li–O bond lengths range from 1.885 Å to 1.991 Å with an average distance of 1.956 Å. The Na-O distances in five-coordinated and eight-coordinated Na-O 6


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polyhedra are located in the range of 2.303-2.398 Å and 2.497-2.677 Å, respectively. The bond valence sums (BVSs)30 of Li, Na, B, and O atoms using Brown's formula are calculated to be 1.07, 0.99-1.06, 2.96, and 1.93-2.06, respectively, which are very close to their ideal oxidation states and further prove that the structure mode is reasonable. Li-O chain

(a)

(c)

(b)

(e)

(d)

Figure 1. Crystal structure of Li2Na2B2O5. a) The 1∞(LiO(1+3/2)) zigzag chain. b) Coplanar arrangement of B2O5 in Li2Na2B2O5 (the Na-O bonds are removed for clarity). The coordinations of c) Na1, d) Na2 and e) B atoms. 3.2 Structural comparisons of B2O5-containing borates In order to explore the structure transformation, the structures of Li4B2O5 (α- or β-phase), Na4B2O5 and Li2Na2B2O5 are compared. Firstly, the coordination numbers of alkli metals are different. The coordination number of the Li or Na atom is 4, 4-5, and 5-6 in α-Li4B2O5, β- Li4B2O5 and Na4B2O5, respectively, while the lithium atom in Li2Na2B2O5 is four-coordinated, and the Na(1) and Na(2) atoms are five- and eight-coordinated, respectively. Furthermore, the configurations of isolated B2O5 unit in four compounds are distinct. In order to express the flexibility of B2O5, dihedral angle (DA, angle between two BO3 planes) and torsion angle (TA, angle of B-O-B between two connected BO3) were defined. As shown in Figures 2a-2d, four compounds have similar TAs (120.245 -134.295o) and distinct DAs (0.000-67.021o) with large differences, respectively, which may affect the birefringence of four compounds. 7



Since the B2O5 units are isolated, the cation frameworks were analyzed to explore the structure transformation. Specifically, 3D frameworks that are composed by the B2O5 units and cations with lowest coordination in β-Li4B2O5, Na4B2O5 and Li2Na2B2O5 are established (α-Li4B2O5 is not discussed because all Li atoms are four-coordinated). As shown in Figure S1 in the Supporting Information, the crystal structure of three compounds can be described as Li/Na-O layers or chains composed of LiO4 or NaO5 stacking along a (β-Li4B2O5) or b (Na4B2O5 and Li2Na2B2O5) direction, and the B2O5 units further connect the layers or chains and form the tunnels with different sizes, then the cations with higher coordinations locate in the tunnels to form the whole structures. Clearly, the interlayer spacing of Na4B2O5 is relatively small (4.0075 Å), which will “compress” the B2O5 units and lead to the largest DA (67.021o). While the larger interlayer spacing in β-Li4B2O5 and Li2Na2B2O5 (5.1135 and 4.9925 Å, respectively) may “stretch” the B2O5 units and obtain the small DAs (42.626 and 0.000o, respectively). It is worth to note that the barrier-like 1∞(LiO(1+3/2)) chains in Li2Na2B2O5 are different from other Li-O chains in borates, which combine with highly coordinated Na atoms facilitating the benign arrangement of B2O5. Accordingly, a plenty of examples could affirm that the borate structures contain low coordinated (construct the frameworks with B-O groups) and highly coordinated cations (fill in the gaps or tunnels) can facilitate the benign B-O arrangement and produce excellent optical properties, such as KBBF and SBBO families,2 A3M3Li2T4B6O20F (A = K, Rb; M =Ba, Sr; T = Al, Ga) series,31 etc. In order to further investigate the connection flexibilities of DAs and TAs in B2O5, all available B2O5-containing anhydrous borates in ICSD were analyzed. Based on the inorganic chemistry structural database (ICSD-4.1.0, the latest release of ICSD-2018/12), there are 96 borates with 131 sets of current data. The DAs present relatively unconstrained distribution in the range of 0-90o, while most of these TAs (97.7 %, 128/131) are restrained to the range of 110-145o (only three B2O5 units with DAs equal to zero are beyond). Moreover, there are only six examples owning zero DAs with wider angle distribution ranges from 126.8 to 180o. Interestingly, Li2Sr4B10O18(B2O5)32 and Sr2Sc2(BO3)2(B2O5),15b containing two kinds of B-O FBBs 8


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in one compound, have exceptional zero DAs and 180o TAs configuration. It is worth to note that, only Li2Na2B2O5, Er2Cl2B2O533 and MgUO2(B2O5)34 contain completely coplanar B2O5 groups, which may generate large optical anisotropy. However, the d-d and f-f transition-free system for Li2Na2B2O5 makes it a unique DUV birefringent material among the B2O5-containing borates. DA = 42.874o DA’ = 42.672o

TA = 124.762o TA’ = 124.527o

DA = 42.626o

(e)

TA = 123.212o

(b)

(a)

DA = 67.021 o

TA = 120.245o

97.7 % (128/131)

DA = 0.000o

TA = 134.295o

(d)

(c)

Figure 2. Comparisons of dihedral and torsion angles for B2O5 in selected compounds. a) α-Li4B2O5. b) β-Li4B2O5. c) Na4B2O5. d) Li2Na2B2O5. The DA and TA indicate the dihedral and torsion angles, respectively. e) The statistical chart of dihedral and torsion angles for all the available anhydrous pyroborates in ICSD. 3.3 Synthesis and crystal growth The polycrystalline powder of Li2Na2B2O5 was prepared by conventional high temperature solid state reaction method according to the stoichiometric ratio of reactant. It is worth to note that the pure phase of Li2Na2B2O5 also can be obtained using Li4B2O5 and Na4B2O5 as precursors in a molar ratio of 1:1, which can verify the reliability of synergistic combination. The TG-DSC curves (Figure 3a) show one strong endothermic peak around 609 oC in the heating process, and two obvious exothermic peaks at about 469 and 439 oC, respectively, in the cooling process. The powder XRD patterns of samples before and after melting (Figure S2 in the Supporting Information) indicate that Li2Na2B2O5 melts incongruently and appropriate fluxes system should be adopted to grow bulk crystal, the used fluxes in his system are listed in Table S4 in the Supporting Information. The Li2Na2B2O5 crystal with size up to 35×15×5 mm3 (Figure 3b) was grown at 9



546 oC in 2 days via TSSG method in a molar ratio of Li2CO3: Na2CO3: H3BO3 = 1.5: 0.5: 2. The experiments show that the dominant crystal facets using c direction seed are (002) and (020), which agrees well with the theoretical one according to the Bravais−Friedel and Donnay−Harker (BFDH) theory35 (Figure 3c).

Figure 3. Thermal and optical properties of Li2Na2B2O5. a) TG-DSC curves. b) The photograph of as-grown Li2Na2B2O5 crystal. c) The calculated crystal morphology of Li2Na2B2O5. d) Thermal expansion measurements. e) Transmittance spectrum of Li2Na2B2O5 crystal (Inset is the enlarged drawing of transmittance in DUV region). 3.4 Thermal expansion coefficients Thermal expansion coefficients of three principal axes were characterized to evaluate the possibilities of cracking caused by thermal expansion during crystal growth, processing and applications. As can be seen in Figure 3d, the Li2Na2B2O5 crystal exhibits only positive thermal expansion when heated and the calculated average thermal expansion coefficients are α100 = 47.72×10-6 K-1, α010 = 8.54×10-6 K-1 and α001 =9.01×10-6 K-1, respectively. The expansion differences α100/α010 is about 5.6, which is obviously smaller than that of benchmark α-BBO (9.0). Hence, Li2Na2B2O5 10


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exhibits a small anisotropic thermal expansion and can provide sufficient reliability for the crystal growth, processing and applications. 3.5 Transmittance spectrum and LDT measurement Transmittance spectrum of the Li2Na2B2O5 crystal (Figure 3e) was characterized from DUV to mid-infrared at room temperature. Li2Na2B2O5 has a high transparency in the range of 0.187-3.5 μm (> 60 %) with the DUV cut off edge of 181 nm, which has 8 nm blue shifts compared with commercial α-BBO. The short cutoff edge is benifical to obtain high LDT, therefore, LDT measurement was carried out using a pulsed nanosecond laser (1064 nm, 10 ns, 10 Hz). A well-polished high-quality (001) wafer of Li2Na2B2O5 has a LDT of ∼7.5 GW/cm2, which is superior to purchased α-BBO crystals (∼ 7.0 GW/cm2) at the same experimental conditions. As the resistance ability to laser damage for crytal is significantly dependent on the quaility of crystal, the higher LDT can be anticipated when the crystal quality is improved in the future. Hence, the short UV cut off edge and high LDT of Li2Na2B2O5 can expand its application wavelength to deeper DUV region, e.g. 193 nm photolithography. 3.6 Refractive indices As a very important optical parameter for birefringent crystal, the refractive index dispersion curves (Figure 4a) were characterized using (001) plate via the prism coupling method. The values of refractive indices along principal axes at 405.0, 514.0, 636.0, 964.8 and 1546.7 nm are summarized in Table S5 in the Supporting Information. The correlations of crystallographic and crystallophysical axes for Li2Na2B2O5 are Z∥c, Y∥b, and X∥a according to the rule nz > ny > nx and it is a negative biaxial crystal as nz − ny < ny − nx. The experimental data were fitted to the following Sellmeier equations: 𝑛2𝑥 = 2.18713 +

𝑛2𝑦 = 2.43384 +

𝑛2𝑧 = 2.43745 +

0.01373 𝜆2 + 0.00540 0.03248 𝜆2 + 0.07772 0.03664 𝜆2 + 0.09030

11



Where λ is the wavelength expressed in micrometers. It is worth noting that, the minimum refractive indices of materials with coplanar B-O units mainly lie in the direction perpendicular to B-O planes, and the refractive indices parallel to B-O plane are usually large. However, the thermal expansion coefficients are opposite. The results are also proved by other typical birefringent material with coplanar units, eg. BO3, B3O6 and CO3, etc.1,5,7,11 The birefringence of Li2Na2B2O5 is pretty large (n001n010 = 0.095@532) and comparable to that of commercial borate birefringent crystals (Table S6 in the Supporting Information), which indicates that Li2Na2B2O5 can be fabricated as birefringent components, such as, Glan prism, etc., from DUV to near-infrared region. 3.6 Origin of the large birefringence

(c)

(b)

(a) 1.64 Refractive Index

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(010) (100)

1.60

(001)

nz

1.56

ny

nx ny nz E F G

△n = 0.095@532

1.52 1.48

nx

0.4

0.8

1.2

1.6

2.0

2.4

2.8

3.2

3.6

Wavelength (m)

Figure 4. Refractive index dispersion curves of Li2Na2B2O5. a) Experimental refractive indices and fitted refractive index dispersion curves. Different arrangement of the distorted B-O groups in b) α-BBO and c) Li2Na2B2O5. The directions of the visibly smaller B-O bonds are chosen to represent the distortions in green arrows.

Since the birefringences originating from BO3, B2O5 or B3O6 are similar, it is feasible to analyze the influence of B2O5 structural configurations on the birefringence via resolving B2O5 to two individual B-O triangles. (1) As the maximum optical anisotropy lies in the directions parallel and perpendicular to the BO3 triangles, respectively, the smaller DAs (especially for the zero DAs) of B2O5 can produce the larger birefringence. Taking Na4B2O5, β-Li4B2O5 and Li2Na2B2O5 as 12


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examples, the B2O5 units in three compounds have a similar TAs (123.212, 120.245 and 134.295o, respectively) and B2O5 arrangements, while the gradually decreased DAs (67.021, 42.626 and 0.000o, respectively) lead to the increased birefringences from 0.042 to 0.095@532. (2) The TAs of the B2O5 units have less influence on the birefringences because the changes of optical anisotropy caused by the varieties of TAs are small. The deduction can be indirectly verified by the α- and β-BBO: the rotation angles between adjacent B3O6 units in two compounds are about 180 and 12o, respectivley, while the birefringences are highly consistent. Similarly, the compounds in KBBF family with centro- or noncentrosymtric structures (which can be considered as zero DAs with different TAs) feature similar birefringences about 0.07. (3) Obviously, the structural arrangements of B2O5 (eg. coplanar or not) will seriously affect the degree of birefringences. The birefringence of α-Li4B2O5 is much smaller than that of β-Li4B2O5 (0.048 and 0.067, respectively) due to the unbenign arrangement of B2O5 in α-Li4B2O5 (Figure S3 in the Supporting Information). Excitingly, most of B2O5 in the pyroborates with small DAs usually have coplanar or nearly coplanar arrangements, which can help us to screen the potential birefringent or NLO materials. Moreover, through the response electron distribution anisotropy (REDA) approximation,36 we have compared the optical anisotropy of Li2Na2B2O5 with that of the famous birefringent crystal α-BBO. In general, the electron distribution is obviously different along the directions parallel and perpendicular to the plane of BO3 unit, which is the main source of birefringences in a series of borates, just like the KBBF families. Besides, as shown in Figures 4b and 4c, there is a visible difference of the B-O bond lengths in the BO3 units, which would form asymmetrical electron distribution and contribute to the optical anisotropy in the BO3 planes. However, the discordant arrangement of the three distorted BO3 in B3O6 leads to a counteraction of anisotropy, while the parallel arrangement in B2O5 can enhance the optical anisotropy. Here, the calculated bonding electron density difference (Δρ) of the covalent bonds along the optical principal axes in Li2Na2B2O5 is 0.0297 Å-1, similar with 0.0306 Å-1 of α-BBO, which is in accordance with their birefringences. Hence, the comparable 13



△n and Δρ indicate that the B2O5 unit is another advantageous functional gene for developing large birefringence. 4. Conclusion A new pyroborate, Li2Na2B2O5, with coplanar B2O5 unit has been successfully obtained via the synergistic combination of Li4B2O5 and Na4B2O5. The experimental results suggest that Li2Na2B2O5 has a large birefringence (0.095@532 nm), a short UV cutoff edge (181 nm) and high LDT (~7.5 GW/cm2 @ 1064 nm, 10 ns), favorable anisotropic thermal expansion (αa/αb = 5.6) and low crystal growth temperature (< 609 oC).

The prominent optical and physicochemical properties promise it to be an

excellent DUV birefringent crystal. Also, the extremely small birefringence in bc plane endows Li2Na2B2O5 with competitive power to fabricate high performance retardation plates. The attractive results indicate that B2O5 could serve as an advantageous functional gene. The influences of B2O5 configuration on the birefringence were investigated for facilitating the birefringent crystals design. The synergistic combination strategy may propose a new structure-driven approach to explore novel inorganic functional materials. Further investigations of growing large crystals and fabricating optical devices of Li2Na2B2O5 are underway. Supporting Information The calculated refractive indices of α-Li4B2O5, β-Li4B2O5, Na4B2O5, and Li2Na2B2O5; atomic coordinates, equivalent isotropic displacement parameters and bond valence sum; selected bond lengths and angles; experimental refractive indices along three principal axes; the layer or chain structures, B2O5 arrangements in four compounds; powder X-ray diffraction patterns. Those materials are available free of charge via the Internet at http://pubs.acs.org.

Corresponding Author *[email protected]; *[email protected].

Author Contributions 14


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#Min

Zhang and Donghai An contributed equally.

Notes The authors declare no competing financial interest. Acknowledgment This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 51872323, 61835014, 51425206), Xinjiang Key Research and Development Program (Grant No. 2016B02021), Shanghai Cooperation Organization Science and Technology Partnership Program (Grant No. 2017E01013), and Foundation of Director of XTIPC, CAS (Grant

No. 2016PY004).

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