Molecular Engineering as an Approach To Design a New Beryllium

Mar 23, 2016 - Molecular Engineering as an Approach To Design a New Beryllium-Free Fluoride Carbonate as a Deep-Ultraviolet Nonlinear Optical Material...
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Molecular Engineering as an Approach to Design a New BerylliumFree Fluoride Carbonate as Deep-Ultraviolet Nonlinear Optical Material Min Luo, Yunxia Song, ChenSheng Lin, Ning Ye, Wendan Cheng, and XiFa Long Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b00360 • Publication Date (Web): 23 Mar 2016 Downloaded from http://pubs.acs.org on March 26, 2016

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Chemistry of Materials

Molecular Engineering as an Approach to Design a New Beryllium-Free Fluoride Carbonate as Deep-Ultraviolet Nonlinear Optical Material † ‡

†‡

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†

†

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Min Luo ’ , Yunxia Song , , Chensheng Lin , Ning Ye ’*, Wendan Cheng , and XiFa Long †

Key Laboratory of Optoelectronic Materials Chemistry and Physics, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian, 350002, P. R. China ‡

University of Chinese Academy of Sciences, Beijing 100049, China

ABSTRACT: It is a great challenge to explore deep-ultraviolet (deep-UV) nonlinear optical (NLO) materials that can achieve a subtle balance between large nonlinear coefficients, moderate birefringence and deep-UV transparency. A new beryllium-free fluoride carbonate Ca2Na3(CO3)3F was successfully synthesized through molecular engineering design and large single crystal were grown by spontaneous crystallization with molten fluxes. The substitution of NLO-active [BO3] groups for [CO3] groups resulted in an optimal balance among SHG coefficient, birefringence, and UV transparency. Comparing these two iso-structural compounds, the second harmonic generation (SHG) coefficients and birefringence of Ca2Na3(CO3)3F have been greatly improved. Remarkably, Ca2Na3(CO3)3F exhibited a wide transparent region with a deep-UV absorption edge at 190nm. These results demonstrated Ca2Na3(CO3)3F is a promising NLO material in the UV or deep-UV region.

1. INTRODUCTION Deep-ultraviolet (deep-UV, λ 6.2 eV) nonlinear optical (NLO) materials have become increasingly important because they manifest many important applications in the fields of photonics, such as semiconductor photolithography, laser micromachining, photochemical synthesis, and material processing1-8. For deep-UV NLO applications, ideal NLO materials require a strict combination of properties that need to be satisfied simultaneously in a non-centrosymmetric (NCS) crystal: relatively large effective second-harmonic generation (SHG) coefficients, wide transparency window down to the deep-UV region, and sufficient birefringence to realize the phase-matching. Up to now, KBe2BO3F2 (KBBF)9 is the sole NLO crystal used for generating coherent light below 200 nm by direct SHG methods at room temperature. With KBBF, single crystal is very difficult to grow in thickness because of its strong layer tendency, which severely hinders their practical applications. Meanwhile, the constituent beryllium is highly toxic. Thus, it is really a challenge to design beryllium-free DUV NLO materials that can achieve a subtle balance between large nonlinear coefficients, moderate birefringence and deep-UV transparency. Owing to the restriction of acentric symmetry, only few crystalline materials can be used as NLO crystals. The first challenge in this area, therefore, is how to control anionic groups in the crystal lattice to make materials crystallize in NCS space groups. An effective approach for designing NCS compounds is to employ pyramidal ligands with a nonbonding, but stereochemically active, pair of electrons, such as I10, 11 or Se12, into crystalline materials. These anions have a tendency to undergo at least partial alignment in the solid state to create NCS structures that are often polar13. Nevertheless, this design strategy has a negative effect on the band gap of a material, which makes these crystals hardly used in deep UV

region. Thus, a comprehensive strategy for designing deep-UV NLO materials is highly desirable. Based on the anionic group theory, the inorganic triangular groups that can produce the large birefringence and secondorder susceptibility, such as [BO3]3- and [CO3]2-, have been proved as ideal structural units for designing UV or deep-UV NLO materials. Numerous borates with [BO3]3- structural unit have been investigated: KBe2BO3F2 (KBBF)8, Sr2Be2(BO3)2O (SBBO)14, YCa4O(BO3)3 (YCOB)15, 16 and recently several carbonates, such as ABCO3F (A =K, Rb, Cs; B = Mg, Ca, Sr, 17 , Na8Lu2(CO3)6F2,18 CsNa5Ca5(CO3)819 and Ba)1, 20 Na3Re(CO3)3(Re=Y, Gd) were evidenced. Moreover, it is worth noting that the planar [CO3]2- group exhibits a high and anisotropic polarizability in calcite, in the plane α∥= 4.2×1024 cm-3, and perpendicular α ⊥ =3.18×10-24 cm-3. Therefore, [CO3]2- groups may have a distinct advantage in birefringence and SHG effects than [BO3]3- groups in NLO crystals. Given this, on analyzing the crystal structures of the NCS borates that contain [BO3]3- groups, we proposed that [CO3]2- groups substitute for [BO3]3- groups in the NCS borates lattice, which could possibly increase the non-linear coefficients and overcome the small birefringence limit in some borate NLO crystals. To validate the proposed method, YCa4O(BO3)3 was initially adopted in the structure design. YCa4O(BO3)3 has many extraordinary optical properties including a wide transparent range and moderate SHG coefficients, whereas its small birefringence limits the application in the UV region. Followed by the strategy mentioned above, [BO3]3- groups were replaced by [CO3]2- groups and other balance ions were replaced accordingly. The alkali metal cation Na+ is introduced to replace the rare-earth metal cation Y3+, not only because both cations have similar radius, but also alkali metal cation in the compound

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will not cause the UV absorption. Besides, we have substituted O2- with F- in the compound for the following reasons: firstly, the incorporation of the fluorine element can preserve the electro neutrality of its structure; secondly, the large electro negativity of F- enlarges the band gap; thirdly, alkali-alkaline earth fluoride carbonate may have lower melting point which contributes to crystal growth by conventional solid methods. Under the guidance of this idea, we successfully obtained a novel UV NLO crystal Ca2Na3(CO3)3F that holds an optimal balance among SHG coefficients, birefringence, and UV transparency. Remarkably, the large single crystal has been obtained to determine the UV cut-off edge. To our knowledge, this is first report of quantitative measurements of the UV cutoff edge in fluoride-carbonate system. Herein, we report a deep-UV NLO material through molecular engineering design, specifically Ca2Na3(CO3)3F, a phase-matching material with extraordinary NLO effects. 2. EXPERIMENTAL SECTION Reagents. All of the chemicals were of analytical grade from commercial sources and used without further purification. Na2CO3 (99.8%), CaCO3 (99.0%), NaF (99.0%), LiF (99.0%) were purchased from Sinopharm. Synthesis. Single crystals of Ca2Na3(CO3)3F were grown from a high temperature solution by using LiF as a flux. A mixture of Na2CO3, CaCO3, NaF and LiF was placed into a platinum crucible at a molar ratio of Na2CO3/CaCO3/NaF/LiF=1:2:1:2. Then the mixture was put into a temperature-programmable electric furnace and heated to 550 ℃ until the melt became clear and transparent. The homogenized melt solution was then slowly cooled at 5℃/h to 450℃ and then allowed to cooled down to room temperature at a rate of 50℃/h. Colorless block crystals of Ca2Na3(CO3)3F used for the determination of single crystal structure and structural characterization were obtained after dissolving the flux in water. Single Crystal X-ray Diffraction. Single crystal X-ray diffraction data were collected at room temperature on a Rigaku Mercury CCD diffractometer with graphite-monochromatic Mo Kα radiation (λ= 0.71073 Å). A transparent block of crystal was mounted on a glass fiber with epoxy for structure determination. A hemisphere of data was collected using a narrow-frame method with ω-scan mode. The data were integrated using the CrystalClear program, and the intensities were corrected for Lorentz polarization, air absorption, and absorption attributable to the variation in the path length through the detector faceplate. Absorption corrections based on the Multiscan technique were also applied. The structure was solved by the direct methods using SHELXS-9721. At the stage of refinement, all the Na atoms shared the same sites with Ca atoms, because abnormal U(eq) and large R value appeared when these positions were designated as Na atoms. Further, in consideration of charge balance, the occupancy ratios of Na/Ca was fixed to 3/1 in the disorder sites. The chemical composition of Na/Ca in Ca2Na3(CO3)3F was confirmed by the elemental analysis (see Table S3). Moreover, all nonhydrogen atoms were refined with anisotropic thermal parameters. The structure was verified using the ADDSYM algorithm from the program PLATON22, and no higher symmetries were found. Relevant crystallographic data and details of the experimental conditions for Ca2Na3(CO3)3F are summarized in Table 1. Atomic coordinates and isotropic displacement coefficients are

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listed in Tables S1 and bond lengths in Tables S2 in the Supplement Information. Table 1. Crystal Data and Structure Refinement for Ca2Na3(CO3)3F Formula

Ca2Na3(CO3)3F

Formula Mass (amu)

730.94

Crystal System

Monoclinic

Space Group

Cm

a (Å)

8.0892(8)

b (Å)

15.9002(17)

c (Å)

3.5273(4)

β (°)

101.662(10)

3

V(Å )

444.32(8)

Z

2

ρ(calcd) (g/cm3)

2.602

Temperature (K)

293(2)

λ(Å)

0.71073

F(000)

366 -1

Μ (mm )

1.541

R/wR (I>2σ (I))

0.0243/0.0453

R/wR (all data) GOF on F2 Absolute Structure Parameter

0.0259/0.0463 1.087 0.09(6)

R(F)=Σ||Fo|– |Fc||/Σ|Fo|. wR(Fo2) = [Σw(Fo2 – Fc2)2/Σw(Fo2)2]1 Powder X-ray Diffraction. X-ray diffraction patterns of polycrystalline materials were obtained on a Rigaku Dmax2500 powder X-ray diffractometer by using Cu Kα radiation (λ=1.540598 Å) at room temperature in the angular range of 2θ = 5-65° with a scan step width of 0.05° and a fixed time of 0.2 s. The powder XRD patterns for the pure powder sample of Ca2Na3(CO3)3F showed good agreement with the calculated XRD patterns from the single-crystal models (see Figure S4). Element Analysis. Element analysis of the crystals was performed by using a Jobin Yvon Ultima2 inductively coupled plasma optical emission spectrometer (ICP-OES) with Sepex Certiprep standards. The crystal samples were dissolved in a mixture of nitric acid (3 mL) and hydrochloric acid (3 mL). Thermal Analysis. Thermogravimetric analyses (TGA) and differential scanning calorimetry (DSC) were measured on a NETZCH STA 449F3. Reference (Al2O3) and crystal samples (3-10 mg) were enclosed in Al2O3 crucibles and heated from room temperature to 900 °C at a rate of 10 °C/min under a constant flow of nitrogen gas. UV Transmittance Spectroscopy. The UV transmittance spectrum of the Ca2Na3(CO3)3F crystal was collected from 190 to 300nm using a PerkinElmer Lamda-950 UV/vis/NIR spectrophotometer. A crystal with dimensions of about 5×2×1 mm3 (Figure S2) was used for the measurement without polishing. Birefringence. The Birefringence of Ca2Na3(CO3)3F crystal was measured on a Nikon ECLIPSE LV100 POL polarizing microscope. The wavelength of the light source was 589.6 nm.

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The x-z plane of an oriented crystal (Figure S3a) was used for the measurement. The thickness of crystal was 0.025 mm (Figure S3b). The formula of calculated Birefringence can be expressed as follows: △R (Retardation) = △n × T In this formula, △R denotes optical path difference, △n represent birefringence, T denotes the thickness of crystal. In this work, Retardation we measured was about 2050 nm. The calculated birefringence (△n=|nz-nx|) was presented in Table 4. Second-Harmonic Generation. Polycrystalline secondharmonic generation (SHG) signals were measured using the method source by Kurtz and Perry23. Since SHG efficiencies are known to depend strongly on particle size, polycrystalline samples were ground and sieved into the following particle size ranges: 25-45, 45-62, 62-75, 75-109, 109-150, and 150212μm. The samples were pressed between glass microscope cover slides and secured with tape in 1-mm thick aluminum holders containing an 8-mm diameter hole. To make relevant comparisons with well-known SHG materials, crystalline KDP and BBO used as the references for visible and UV SHG were also ground and sieved into the same particle size ranges, respectively. The samples were then placed in a light-tight box and irradiated with a pulsed laser. The measurements were performed with a Q-switched Nd:YAG laser at 1064nm and a frequency doubling at 532nm, for visible and UV SHG. A cutoff filter was used to limit background flash-lamp light on the sample, and an interference filter (530±10nm) was used to select the second harmonic for detection with a photomultiplier tube attached to a RIGOL DS1052E 50-MHz oscilloscope. This procedure was then repeated using the standard nonlinear optical materials KDP and BBO, and the ratio of the secondharmonic intensity outputs was calculated. No index-matching fluid was used in any of the experiment. Electronic Structure Calculations. Band structure and density of states (DOS) were performed by using density functional theory (DFT) calculations provided by CASTEP code24 in the Material Studio package. The experimental crystal structure was used without further optimization. The exchange correlation interaction was treated by the generalized gradient approximation (GGA) of Perdew−Burke−Ernzerhof (PBE)25. The energy cut-off for the plane wave was chosen as 780 eV and the self-consistent convergence of the total energy was set to be 2.0×10−5 eV/atom. The Monkhorst–Pack26 k-point sampling of 2×2×4 in the Brillouin zone of unit cell was selected for calculation. The interactions between the core and valence electrons were represented by the norm-conserving pseudopotentials, and the following valence configurations were regarded as: Na: 2s22p63s1, Ca: 3s23p64s2, C: 2s22p2, O: 2s22p4, F: 2s22p5. SHG Coe cient Calculations. The classical anharmonic oscillator (AHO) model27 was adopted to calculate the SHG coeffcients. The formula of calculated second-order susceptibilities derived from AHO model can be expressed in terms of the first-order susceptibilities as follows:

χijk( 2) (−ω3 ;ω1,ω2 ) = F ( 2) χii(1) (ω3 )χ (jj1) (ω1 )χkk(1) (ω2 ) The first-order susceptibility χ(1)ii at low frequency region is given by χ(1)(ω)ii = [ε(ω)ii –1]/4π where the dielectric function:

ε 2ij (ω) =

pi (k) p j (k) 8π 2h2e 2 Σk Σcv ( f c − f v ) cv 2 vc δ [Ec (k) − Ev (k ) − hω] 2 m Veff Evc

In this formula, δ[Ec(k)-Ev(k)-ħω] denotes the energy difference between the conduction and valence bands at the k point with absorption of a quantum ħω. The fc and fv represent the Fermi distribution functions of the conduction and valence bands, respectively. It is well acknowledged that DFT-GGA calculations commonly underestimate the band gap of crystal. Consequently, in order to match with the experimental band gap, the scissors operation should be employed to shift up the conduction bands energy. 3. RESULTS AND DISCUSSION

Figure 1. Structure relationship between YCa4O(BO3)3 and Ca2Na3(CO3)3F.

Figure 2 (a) View of the structure of Ca2Na3(CO3)3F down the c-axis. (b) 1D double chains formed by [(NaCa)2O8] and [(NaCa)1O4F2] polyhedra and CaO4F2 polyhedra. (c) The arrangement of CO3 groups along the c-axis.

Crystal structure. Ca2Na3(CO3)3F is isostructural to YCa4O(BO3)3. It can be regarded as that the planar [BO3] groups are replaced by [CO3] groups, while Y3+ and half of Ca2+ are substituted by Na+, and O2− is substituted by F−, in the specified locations in YCa4O(BO3)3 structure (Figure 1). In titled structure, [(NaCa)1O4F2], [(NaCa)2O8] and [(NaCa)3O5F] polyhedra are connected to each other via sharing edges and corners into a one dimensional (1D) chain (Figure 2b). With respect to the carbon atoms in the structure, they have two different crystallographic positions, C1 and C2, with three-fold coordination. The C-O distances vary in the narrow range of 1.268(9) to 1.286(3) Å and O-C-O bond angles are between 119.4(3)° and 120.8(3)°, which correspond to a CO3 group. Note that all [C1O3] groups are in co-planar alignment, contributing significantly to the SHG effect. [C2O3] groups, however, are approximately parallel to (001) layers with small acute angles (Figure 2c), which have an additional cooperative contribution to the macroscopic SHG coefficients. All the 1D

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chains are connected to each other by sharing corners and edges into a 3D framework with tunnels where C atoms are located (Figure 2a) Crystal Growth and Thermal Behavior. Up to now, due to carbonates easily decompose at high temperature, most fluoride carbonates that have been reported were synthesized by the subcritical or supercritical hydrothermal method28. Therefore, it is a great challenge to grow large single crystals from a high temperature solution. Compared with hydrothermal method, the crystal growth via conventional solid methods is more conducive to industrial applications. Remarkably, in our study, a new alkaline-alkaline earth fluoride carbonate Ca2Na3(CO3)3F have been successfully grown by spontaneous crystallization with molten fluxes (see Figure S5). As shown in Figure S4, Ca2Na3(CO3)3F is thermally stable up to about 600°C based on TGA curves, DSC diagram of Ca2Na3(CO3)3F exhibited two endothermic peaks at 556°C and 592°C respectively, corresponding to the two steps of the incongruent melting process associated with the decomposition of Ca2Na3(CO3)3F finally into Na2CO3, CaO and NaF (see Figure S6). Therefore, large crystals of Ca2Na3(CO3)3F should be grown using flux method below the decomposition temperature (~560℃).

Table 2. Optical properties for UV NLO crystals Materials

Absorption Edge

SHG coefficients (×KDP)

Birefringence (△n)

KBe2BO3F2 (KBBF)8 LiB3O5 (LBO)8 BaAlBO3F2 (BABF)31 Li4Sr(BO3)2 (LSBO)2 YCa4O(BO3)3 (YCOB)15 Ca2Na3(CO3)3Fa (CNCF) a: this work

147nm9

19

0.0779

150nm8 165nm31

229 231

0.04330 0.04531

186nm2

22

0.0562

205nm32

2

0.04116

190nm

3

0.082

NLO properties. The powder SHG signal of Ca2Na3(CO3)3F and YCa4O(BO3)4 crystals measured with an incident laser at 1064 and 532 nm are shown in Figure 4. A KDP and BBO samples were used as the reference for visible and UV SHG measurement, respectively. The curves of the SHG signal revealed that Ca2Na3(CO3)3F was type-I phase-matching in both visible and UV region according to the rule proposed by Kurtz and Perry. Considering the NLO coefficient of BBO is about 6.6 times as large as deff(KDP) which is 0.26 pm/V33, the relative SHG intensities in the visible region and the UV region coincide with each other, and the derived deff for the Ca2Na3(CO3)3F is approximately 0.78pm/V, which is larger than that of YCa4O(BO3)3 (0.52pm/V) we have measured in this work. Table 3. Contribution of Diff fferent Geometrical Factors (g) to Structure Factors (C) Crystal(n)

Figure 3 UV Transmittance Spectroscopy of Ca2Na3(CO3)3F

UV Transmittance Spectroscopy. The transmittance spectrum of Ca2Na3(CO3)3F from 190 to 300nm is shown in Figure 3. As shown in Figure 3, the UV absorption edge of Ca2Na3(CO3)3F is as short as 190 nm (corresponding to a band gap of 6.52 eV). Such an absorption edge is close to that of Li4Sr(BO3)2 (186 nm), but much shorter than that of YCa4O(BO3)4 (205 nm). Therefore, compared with the UV cut-off edge between Ca2Na3(CO3)3F and YCa4O(BO3)4, Ca2Na3(CO3)3F can be used in the deep UV region as a NLO material. Birefringence The Birefringence of Ca2Na3(CO3)3F crystal was measured on a Nikon ECLIPSE LV100 POL polarizing microscope.and the birefringence was presented in Table 2. As shown in Table 2, the birefringence of Ca2Na3(CO3)3F(0.082) was significantly larger than that of YCa4O(BO3)3 (0.040). Additionally, the birefringence of Ca2Na3(CO3)3F is similar with that of KBe2BO3F2 (KBBF, 0.077), but larger than most of the UV NLO borates, such as LiB3O5 (LBO, 0.043), BaAlBO3F2 (BABF, 0.042) and Li4Sr(BO3)2 (LSBO, 0.056). It's thus concluded that the Ca2Na3(CO3)3F crystal can easily realize the phase-matching in the UV region as supported by the UV SHG experiments (Figure 2).

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Ca2Na3(CO3)3F (n=6)

g11 1/n 0.33 7

g12 2/n 0.52 1

g13 3/n 0.18 3

g31 1/n 0.36 6

g32 2/n 0.37 7

g333/ n 0.002

YCa4O(BO3)3 (n=6)

0.37 4

0.57 8

0.19 2

0.38 3

0.38 1

0.004

Table 4. The calculation of anionic group theory for Ca2Na3(CO3)3F and YCa4O(BO3)3 Crystals

Structural criterion C

densities of the [CO3] or [BO3] (n/V) (Å-3)

(n/V) C(Å-3)

Ca2Na3(CO3)3F

0.521

0.135

0.070

YCa4O(BO3)3

0.578

0.134

0.077

×

According to the anionic group theory34-36, the macroscopic SHG coefficients of Ca2Na3(CO3)3F and YCa4O(BO3)3 originated from a geometrical addition of the microscopic second-order susceptibility of the planar [CO3]2- or [BO3]3groups, respectively. To further elucidate the relationship between the NLO properties and structure, the calculation methods reported in the ref 37 were performed. The calculation methods were described in detail in the Supporting Information, and the results of calculation were displayed in Table

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3-4. Table 3 presented the contribution of different geometrical factors (g) to structure factors (C). The calculation results

Figure 4. (a) SHG measurements of ground Ca2Na3(CO3)3F crystals (red circle) and KDP (black square) and YCa4O(BO3)3 (green rhombus) as the reference with the laser at 1064 nm wavelength; (b) SHG measurements of ground Ca2Na3(CO3)3F crystals (red circle) and BBO (black square) as the reference with the laser at 532nm.

indicated that the SHG coefficients of Ca2Na3(CO3)3F and YCa4O(BO3)3 all come from the contribution of g122, which conforms with the arrangement of NLO-active groups in both crystals. In both compounds, the one-third of the NLO-active groups were aligned coplanar and parallel to the ab plane, giving major contribution to g122. Besides, the arrangement of the other two-thirds of [C2O3] groups were almost parallel to the ab plane as well, which gave an additional contribution to g122. According to Eq. (2) and (4) in the Supporting Information, when assuming that the localized field (F) is equal based on the similar refractive indices, the NLO coefficient χ(2)ijk is proportional to the structural criterion (C) and density of the groups (n/V). By comparing Ca2Na3(CO3)3F with isostructural YCa4O(BO3)3, we could clearly see that the structure factor and the density of NLO-active groups are nearly the same (see Table 4). However, the macroscopic SHG effect of Ca2Na3(CO3)3F was greater than that of YCa4O(BO3)3. It was shown that microscopic second-order susceptibility tensors of [CO3]2- groups should be greater than that of [BO3]3- groups. Meanwhile, it also demonstrated that substitution of NLOactive [BO3] groups for [CO3] groups in NLO borate crystals was conductive to the improvement SHG coefficients. Theoretical Analyses of the Optical Properties. To further elucidate the microscopic origin of the optical properties of Ca2Na3(CO3)3F, density functional theory (DFT) calculation was employed here. The calculated band structure of Ca2Na3(CO3)3F was presented in Figure S7, giving a direct gap of 4.864 eV. The calculated result was underestimated in comparison with the experimental values on account of the restriction of the DFT calculation. Since the optical properties depend on the experimental gap, a scissor of 1.656 eV was applied to the subsequent calculations of optical properties. The total and partial densities of states (DOS and PDOS) for Ca2Na3(CO3)3F were plotted in the Figure S8.The Na and Ca orbitals were mainly distributed in the deep region of the va-

lence band (VB) (