Expanding Frontiers of Ultraviolet Nonlinear Optical Materials with

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Cite This: Chem. Mater. 2018, 30, 5397−5403

Expanding Frontiers of Ultraviolet Nonlinear Optical Materials with Fluorophosphates Bingbing Zhang,†,§ Guopeng Han,†,§,‡ Ying Wang,† Xinglong Chen,†,‡ Zhihua Yang,*,† and Shilie Pan*,† †

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CAS Key Laboratory of Functional Materials and Devices for Special Environments, 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 S Supporting Information *

ABSTRACT: If a bucket is to hold more water, its shortest plank must be made longer. This guideline also applies to the exploration of ultraviolet (UV) and deep-UV (DUV) nonlinear optical (NLO) materials that are limited by multiple criteria. Phosphates are one kind of promising candidate for new NLO materials. Unfortunately, the small birefringence, as the shortest plank, severely restricts the phase-matching of second harmonic generation (SHG) in the UV/DUV region. In this work, fluorophosphates are rationally proposed as substitutes for phosphates to break down the limitation of birefringence and simultaneously enhance SHG response and retain wide UV transmittance. The (PO3F)2− and (PO2F2)− groups are confirmed as superior material genomes to achieve the discussed combination properties. Accordingly, (NH4)2PO3F was screened out by density functional theory calculation, and single crystals with centimeter size have been grown. It possesses a powder SHG efficiency of 1 × KH2PO4 (KDP) and is phase-matchable with output of SHG wavelength at 266 nm. To the best of our knowledge, it is the first time that fluorophosphates are identified and developed as new and ideal candidates to new UV/DUV NLO materials by combining theories and experiments.

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INTRODUCTION The nonlinear optical (NLO) crystals are crucial materials to extend the limited wavelengths provided by normal laser sources and as a result expand the application of a laser,1−3 particularly in the ultraviolet (UV) and deep-UV (DUV) regions including semiconductor manufacturing, laser micromachining, and superhigh-resolution photoemission spectroscopy.4−7 The urgently needed functional materials are limited by multiple rigorous requirements, just like different planks of a bucket. The whole material cannot be applied if any one of the requirements fails to meet the criteria. To meet these requirements, borates and phosphates are considered as rich exploration systems for UV and DUV NLO materials attributable to their short UV absorption edge, the variety of NCS structures, and easy growth of large single crystals. A number of well-known commercial NLO crystals such as KDP (KH2PO4),8 KTP (KTiOPO4),9 β-BBO (β-BaB2O4),10,11 LBO (LiB3O5),12 and KBBF (KBe2BO3F2)13 are used industrially as frequency conversion devices from visible to UV. In addition to wide UV transmittance and large second harmonic generation (SHG) response, a large birefringence is a vital criterion for NLO materials to achieve phase-matching condition in UV/DUV region. In borates, coplanar aligned (BO3)3− triangles could produce large birefringence to meet © 2018 American Chemical Society

this criterion. However, phosphates exclusively contain (PO4)3− tetrahedron as building units. The polarizability anisotropy of the (PO4)3− tetrahedron is much smaller than that of the planar (BO3)3− triangles, which inherently results in small birefringence of phosphates. There is still no phosphate that can phase-match with output of SHG wavelength down to the DUV region, even though they can transmit as far as 150 nm in the UV spectral region.14 Recently, some phosphates with wide band gap and appropriate SHG have been synthesized.15−19 However, the small birefringence limits them to satisfy the phase-matching condition, which becomes the shortest plank of phosphates as UV/DUV NLO materials. Therefore, enhancing the birefringence and SHG response and meanwhile retaining the merit of wide UV transmittance in phosphates are crucial to achieving the UV/DUV wavelength output. Very recently, we proposed a design strategy for UV/DUV NLO materials by introducing the [BOxF4−x] (x = 1, 2, 3) groups in borate system as new functional building units (FBUs).20 The introduction of the [BOxF4−x] groups could cut Received: May 26, 2018 Revised: July 17, 2018 Published: July 17, 2018 5397

DOI: 10.1021/acs.chemmater.8b02223 Chem. Mater. 2018, 30, 5397−5403

Article

Chemistry of Materials

Powder XRD Measurement. The sample was carried out at room temperature with a Bruker D2 PHASER diffractometer (Cu Kα radiation with λ = 1.5418 Å). The 2θ range was 10−70° with a scan step width of 0.02° and a fixed counting time of 1 s per step. TG/DSC Analysis. Thermal gravimetric (TG) analysis and differential scanning calorimetry (DSC) of (NH4)2PO3F were carried out on a simultaneous NETZSCH STA 449 F3 thermal analyzer instrument in a flowing N2 atmosphere. The sample was enclosed in a platinum crucible and heated from room temperature to 450 °C at a rate of 5 °C min−1. Single-Crystal Structure Determination. The XRD data of preselected (NH4)2PO3F single crystals were collected on a Bruker SMART APEX II 4K CCD diffractometer using Mo Kα radiation (λ = 0.71073 Å) at 296(2) K. Data integration, cell refinement, and absorption corrections were carried out with the program SAINT.35 The crystal structure was solved by direct methods and refined on F2 by full-matrix least-squares techniques using the program suite SHELXTL.36 The structure was checked for missing symmetry elements using the program PLATON.37 Crystal data and structure refinement for (NH4)2PO3F are given in Table S1. Atomic coordinates with equivalent isotropic displacement parameters and bond lengths in (NH4)2PO3F are listed in Tables S2 and S3, respectively. Elemental Analysis. Elemental analysis was carried on clean single-crystal surfaces with the aid of a field-emission scanning electron microscope (SUPRA 55VP) equipped with an energydispersive X-ray spectroscope (EDX) (BRUKER X -flash-sdd-5010). UV−Vis Transmittance and IR Spectrum. The UV−vis transmittance spectrum was measured on single-crystal plates with a thickness of 1 mm. The IR spectrum was recorded on a Shimadzu IR Affinity-1 Fourier transform infrared spectrometer in the 400−4000 cm−1 range on a sample that was mixed thoroughly with dried KBr. Birefringence Measurement. The birefringence of (NH4)2PO3F crystal was measured on a single-crystal plate (110) with a thickness of 1 mm by immersion technique38,39 on GR-5 Gemological refractometer (Wuhan Zhongdixueyuan Gem Instrument Ltd., China). The light source was sodium yellow light (589.3 nm) with a measuring range of 1.35−1.85 and an accuracy of ±0.002. During the refractive index measurement, we turn the crystal, and four sets of refractive indices were observed and recorded, i.e., 1.450 and 1.480, 1.452 and 1.480, 1.457 and 1.480, and 1.454 and 1.475, respectively. Because 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 points in each set, i.e., 0.030, 0.028, 0.023, and 0.021, is not larger than the birefringence of (NH4)2PO3F. Therefore, it is expected that the birefringence of (NH4)2PO3F crystal is >0.030 at 589.3 nm. Powder SHG Measurement. Powder SHG measurement was performed on a Kurtz−Perry NLO system40 using Q-switched Nd:YVO4 solid-state lasers at wavelengths of 1064 and 532 nm for visible and ultraviolet SHG, respectively. As the powder SHG efficiency has been shown to depend strongly on a particle size of crystal, polycrystalline (NH4)2PO3F was ground and sieved into distinct particle size ranges (20−38, 38−55, 55−88, 88−105, 105− 150, and 150−200 μm) to investigate its phase-matching behavior. The KDP and BBO samples were also sieved into the same particle sizes for SHG efficiency comparison.

off the B−O network and increase the possibility to construct an appropriate structure that is beneficial to large birefringence with short cutoff edge. Following this proposition, a series of fluorooxoborates with excellent DUV NLO properties were synthesized, including NH 4 B 4 O 6 F (ABF), 21 CsB 4 O 6 F (CBF), 2 2 RbB 4 O 6 F (RBF), CsKB 8 O 1 2 F 2 (CKBF), CsRbB8O12F2 (CRBF),23 and MB5O7F3 (M = Ca, Sr).24,25 In this work, following the materials genome approach,26−28 the (PO3F)2− and (PO2F2)− groups with large polarizability anisotropy, wide highest occupied molecular orbital (HOMO)−lowest unoccupied molecular orbital (LUMO) gap, and high hyperpolarizability are identified as superior FBUs to substitute PO4 group to enhance birefringence and simultaneously generate large SHG effect and short cutoff edge as UV/DUV NLO materials. Fluorophosphates are identified as new and ideal candidates to replace phosphates to explore new UV/DUV NLO materials by combining theories and experiments. Accordingly, (NH4)2PO3F was screened out as a promising UV NLO material. Single crystals of (NH4)2PO3F with centimeter size have been grown using the solventevaporation method for the first time. It possesses an SHG efficiency of 1 × KDP and is phase-matchable with output of second harmonic wavelength at 266 nm. More importantly, fluorophosphates have small chromatic dispersion that significantly reduces the criteria of birefringence for DUV NLO materials from ∼0.07 of borates to ∼0.045. This work provides a feasible way to expand frontiers of UV/DUV NLO materials with fluorophosphates.

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METHODS

Computational Details and Methods. The electronic and band structures as well as linear optical property calculations were performed by employing CASTEP,29 a plane-wave pseudopotential density functional theory (DFT) package, with the norm-conserving pseudopotentials (NCPs).30−32 The exchange-correlation functionals were Perdew−Burke−Emzerhof (PBE) functional within the generalized gradient approximation (GGA)33 and the hybrid PBE0 exchange-correlation functional. The plane-wave energy cutoff was set at 850.0 eV. Self-consistent field (SCF) calculations were performed with a convergence criterion of 1 × 10−6 eV/atom on the total energy. The k-point separation for each material was set as 0.04 Å−1 in the Brillouin zone. The empty bands were set as 3 times the valence bands in the calculation to ensure the convergence of optical properties. Polycrystalline Synthesis and Crystal Growth of (NH4)2PO3F. CAUTION: Hot autoclave is extremely hazardous and should be handled with the utmost care. Also, the reaction may produce HF gas as a side product, which is toxic by inhalation or in contact with skin when the cooled autoclave is opened. All chemicals used were of analytical grade and were used as received without any further purification. Polycrystalline samples of (NH4)2PO3F were synthesized according to the method described by Schülke and Kayser.34 A mixture of NH4HF2, CO(NH2)2, and H3PO4 with a molar ratio of 1:2:2 was put into a 75 mL Teflon-lined autoclave, heated at 170 °C for 83 h, and slowly cooled to room temperature at a rate of 2 °C/h. The purity of the prepared sample was checked by powder X-ray diffraction (XRD). The product was then recrystallized from an acetone−water solution (volume ratio of 4:1). The solution was allowed to slowly evaporate at room temperature without air convection and dust-free. After the growth period of 30 days, centimeter-size (NH4)2PO3F crystals with a blocklike habit were harvested. The harvested crystals were recrystallized repeatedly to obtain high-quality single crystals. Powder XRD pattern of the ground bulk material is in good agreement with the diffraction pattern calculated from the singlecrystal data.

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RESULTS AND DISCUSSION The reported fluorophosphates could be classified as two categories, i.e., monofluorophosphates and difluorophosphates, according to their anionic groups ((PO3F)2− or (PO2F2)−). To investigate the possible structure-directing optical properties of fluorophosphates, we first carried out ab initio calculations at the molecular level. The calculation was carried out using density functional theory (DFT) implemented by the Gaussian09 package41 at 6-31G level. Figure 1 shows the calculated frontier molecular orbital diagrams of the (PO4)3−, (PO3F)2−, and (PO2F2)− anionic groups. One can find that the 5398

DOI: 10.1021/acs.chemmater.8b02223 Chem. Mater. 2018, 30, 5397−5403

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Chemistry of Materials

out crystals that contain the (PO3F)2− or (PO2F2)− groups. The cations are limited to (NH4)+ and alkali and alkaline-earth metal cations for large band gap. The final selected crystals are shown in Table 2. Structurally, (NH4)2PO3F34,43 and Na2PO3F44 are noncentrosymmetric (NCS) and expected to have SHG response. A2PO3F (A = K, Rb, Cs),45,46 BaPO3F,47 APO 2 F 2 (A = K, Cs, NH 4 ), 48−50 K 2 P 2 O 5 F 2 , 51 and NaK3(PO3F)252 are centrosymmetric. Among them, the A2PO3F (A = K, Rb, Cs) family, the same as APO2F2 (A = K, Cs, NH4), are isostructural. For simplicity, only the structures of K 2 PO 3 F and KPO 2 F 2 are described as representations. As shown in Figure 2, the crystal structures of A2PO3F (A = NH4, Na, K, Rb, Cs), BaPO3F, and NaK3(PO3F)2 are composed of isolated (PO3F)2− tetrahedra. APO2F2 (A = K, Cs, NH4) are composed of the (PO2F2)− tetrahedra, while in K2P2O5F2, two (PO3F)2− units are connected by sharing corner oxygen, forming the (P2O5F2)2− dimer. It illustrates that (PO3F)2− can polymerize and would result in a rich structural diversity of fluorophosphates, and there may be more new structures waiting to be discovered. Subsequently, we performed computational screening of fluorophosphates and afforded detailed prediction of their optical properties. We first predicted the band gaps, SHG coefficients (only for NCS structures), and birefringences of the selected crystals using DFT method. The hybrid exchangecorrelation functional PBE0 was used to calculate band gap due to its good accuracy.53 The SHG coefficients are calculated using the sum-overstates approach within length-gauge54−56 based on the ground-state wave functions obtained using GGA. As shown in Table 2, all the predicted band gaps of listed fluorophosphates using PBE0 functional are >6.7 eV, corresponding to a cutoff edge lower than 185 nm, which indicates the applicability of fluorophosphates in the DUV region. As to NLO properties, the independent SHG coefficients of (NH4)2PO3F are d15 = 0.14, d24 = 0.40, and d33 = −0.35 pm/V. The SHG coefficients are comparable with that of KDP (d36 = 0.39 pm/V), which is large enough for UV/ DUV light frequency conversion. (NH4)2PO3F also shows a large band gap of 7.59 eV (163.4 nm). Only one independent tensor of Na2PO3F is d14 = 0.08 pm/V. Moreover, the calculated birefringences of these fluorophosphates (at 1064 nm) are also listed in Table 2. As we mentioned earlier, the small birefringence of phosphates hinders phase-matching in the shorter-wavelength region. Here, we compared the calculated (or measured) birefringences of several typical NLO phosphates with those of fluorophosphates. As shown in Figure 3, fluorophosphates, especially difluorophosphates, exhibit much larger birefringences (∼0.04−0.05) than other phosphates (∼0.00−0.01). The results confirm that the (PO3F)2− and (PO2F2)− groups could induce larger optical anisotropy than (PO4)3−. Futhermore, three difluorophosphates APO2F2 (A = K, Cs, NH4) exhibit larger birefringences than monofluorophosphates. The results are in accordance with the calculated results that (PO2F2)− has larger polarizability anisotropies than (PO3F)2−. The shortest type I phase-matching wavelength (λs) of (NH4)2PO3F could be easily found by satisfying nz(λ) = nx(λ/2). As illustrated in Figure 4a, the predicted λs of (NH4)2PO3F is 264 nm. This suggests that (NH4)2PO3F could achieve 266 nm coherent light generation by direct second-harmonic generation. Our computational predictions suggest that (NH4)2PO3F not only has large SHG coefficients and wide band gap but also has relatively large birefringence. It has the highest combined

Figure 1. Frontier molecular orbitals of the (PO4)3−, (PO3F)2−, and (PO2F2)− anionic groups. The HOMO and LUMO are shown in the middle and lower panels, respectively. The red, yellow, and cyan balls represent the oxygen, phosphorus, and fluorine atoms, respectively.

nonbonding 2p orbitals of O occupied the highest occupied molecular orbital (HOMO) in all three anionic groups, while the F atoms are not or slightly involved in the HOMO due to its large electronegativity. The anti-σ P−O/F bonds are formally empty, making them the lowest unoccupied molecular orbital (LUMO). The presence of the F atoms results in the obvious anisotropy of HOMO and LUMO in both the (PO3F)2− and (PO2F2)− groups that will further lead to optical anisotropy. As shown in Table 1, the (PO3F)2− and (PO2F2)− Table 1. Calculated Properties of the (PO3F)2− and (PO2F2)− Anionic Groups and of (PO4)3− as Comparison (Dipole Moment (P), Polarizability Anisotropy (δ), Largest Hyperpolarizability Tensor (|βmax|), and HOMO−LUMO Gap (Eg)) groups 2−

(PO3F) (PO2F2)− (PO4)3−

Px, Py, Pz

δ

|βmax|

Eg (eV)

0.0, 0.0, 0.94 0.0, 0.0, −0.98 0.0, 0.0, 0.0

5.3 6.6 0.0

18.4 20.8 13.4

8.9 9.2 9.1

groups exhibit larger polarizability anisotropy than the isotropous (PO 4 )3− group, and even larger than the [BOxF4−x] groups.20 The calculated HOMO−LUMO gaps of the (PO3F)2− and (PO2F2)− groups are close to each other and correspond to an extremely short UV cutoff edge. In addition, the hyperpolarizabilities of the (PO3F)2− and (PO2F2)− groups are greater than that of the (PO4)3− group. The calculated optical properties imply that the (PO3F)2− and (PO2F2)− groups could improve the birefringence and SHG response and retain the merit of wide UV transmittance of phosphates. Our preliminary calculation presents micro origins of superior optical properties of (PO3F)2− and (PO2F2)− groups. We can expect that fluorophosphates with these groups may show excellent NLO properties according to the anionic group theory.42 The investigation of structure features and the arrangement of anionic groups in fluorophosphates crystals are important prior to further characterization. We comprehensively searched the Inorganic Crystal Structures Database (ICSD) to screen 5399

DOI: 10.1021/acs.chemmater.8b02223 Chem. Mater. 2018, 30, 5397−5403

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Chemistry of Materials

Table 2. Space Groups, Calculated Band Gaps at Level of GGA and PBE0 Approximations, Birefringence (Δn) at 1064 nm, and SHG Coefficients (dij) with a Correction of the Band Gap by Using Scissors Operator (Set as Difference between GGA and PBE0 Gaps) for the Selected Fluorophosphates band gap (eV) GGA

PBE0

Δn@ 1064 nm

(NH4)2PO3F

Pna21

5.04

7.59

0.027

Na2PO3F K2PO3F Rb2PO3F Cs2PO3F BaPO3F K2P2O5F2 NaK3(PO3F)2 KPO2F2 CsPO2F2 NH4PO2F2

P212121 Pnma Pnma Pnma P21/c C2/c P-3m Pnma Pnma Pnma

4.57 4.38 4.38 4.61 5.47 5.24 4.47 5.42 5.53 5.62

6.14 6.79 6.76 6.94 7.90 7.74 6.91 7.95 7.99 8.15

0.028 0.021 0.020 0.018 0.010 0.023 0.021 0.047 0.045 0.046

crystals

space group

dij (pm/V) d15 d24 d33 d14

= = = =

0.14 0.40 −0.35 0.08

/

Figure 2. Crystal structures of (a) (NH4)2PO3F, (b) Na2PO3F, (c) K2PO3F, (d) BaPO3F, (e) KPO2F2, (f) K2P2O5F2, and (g) NaK3(PO3F)2. The anionic groups in (a−d) and (g) are (PO3F)2−, in (e) are (PO2F2)−, and in (f) are (P2O5F2)2−.

method (Figure S1). Powder XRD patterns of the ground bulk material are in good agreement with the diffraction one calculated from the single-crystal data (Figure S2). (NH4)2PO3F has weak deliquescence in moist air. The thermodynamic analysis reveals that (NH4)2PO3F is stable up to ∼180 °C (Figure S3). The existence of F and the P−F bonds were confirmed by EDX and IR spectrum, respectively (Figures S4 and S5). Powder SHG measurements were performed by using the Kurtz and Perry method.57 The curves of the SHG signal are consistent with phase-matching behavior under incident laser at both 1064 and 532 nm (Figure 4b and c). The oscilloscope traces of SHG signals show that the SHG efficiency of (NH4)2PO3F is ∼1 time that of KDP and ∼0.2 times that of BBO in the same particle size range of 150−200 μm at 1064 and 532 nm, respectively. The UV−vis transmittance spectrum indicates that (NH4)2PO3F has a wide UV transparency window with a cutoff edge less than 177 nm (Figure 4d). The birefringence of (NH4)2PO3F was measured on a single-crystal plate (110) with a thickness of 1 mm by immersion technique. The results show that the birefringence of the (NH4)2PO3F crystal is >0.030 at 589.3 nm (see details in the Methods section). All of the experimental results, including the UV transparency window, SHG response, and improved birefringence, are in good agreement with

Figure 3. Calculated or measured birefringences at 1064 nm of typical NLO phosphates (green region), monofluorophosphates (gray region), and difluorophosphates (blue region). The values of BPO4 and K4Mg4(P2O7)3 are obtained from refs 18 and 19, respectively. The other values are calculated in this work.

figure of merit for NLO materials. Therefore, it has been selected to be a candidate to grow large size crystal for further experimental measurement. (NH4)2PO3F single crystals with centimeter size were grown using the solvent-evaporation 5400

DOI: 10.1021/acs.chemmater.8b02223 Chem. Mater. 2018, 30, 5397−5403

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Chemistry of Materials

Figure 4. (a) Calculated refractive index, dispersion curves, and predicted shortest SHG phase-matching wavelength for (NH4)2PO3F. The phasematching curves (i.e., SHG response vs particle size) of (NH4)2PO3F at (b) 1064 nm and (c) 532 nm, respectively. (d) UV−vis transmittance spectrum of (NH4)2PO3F (insets: photographs of (NH4)2PO3F single crystal and polished plate). (e) Partial density of states (PDOS) and bandresolved χ(2) of virtual-electron (VE) processes and virtual-hole (VH) processes. (f) Chromatic dispersion characteristics of (NH4)2PO3F compared with those of LBO and BBO.

Although NH4PO2F2 is centrosymmetric, the evaluation is rational because the birefringence is independent of the presence or absence of inversion symmetry. Specifically, the birefringence of NH4PO2F2 could satisfy a phase-matching wavelength of 196 nm. The results show the capability of fluorophosphates to produce sufficient birefringence to achieve phase-matching in the DUV region. It is worth noting that the calculated birefringence of NH4PO2F2 is 0.046 at 1064 nm, which is much less than the birefringence criterion for borates (∼0.07) to achieve DUV phase-matching. The lower birefringence criterion for fluorophosphates is mainly attributed to the large band gap and small chromatic dispersion. These calculations provide evidence that fluorophosphates have the potential to function as UV/DUV NLO materials.

calculated ones, which strongly support our design philosophy of new UV/DUV NLO materials. To further establish the relationship between crystal structures and NLO properties of fluorophosphates, we provided in-depth electronic structure analysis by taking (NH4)2PO3F as an example. The band-resolved χ(2) and SHG density methods58 are used to extract the electronic states that contribute to the largest SHG tensor d24. As shown in Figure 4e (partial density of states (PDOS) and bandresolved χ(2)) and Figure S6 (SHG density), the nonbonding 2p orbitals of oxygen and fluorine atoms in the valence bands and anti-σ orbitals of the (PO3F)2− groups in the conduction bands are the major contributor orbitals. It is interesting that the SHG density of unoccupied states around the P atom has a strong anisotropy. The shapes show that anti-σ bond of P−F makes a larger contribution than that of P−O bond, which explains why (PO3F)2− has a larger hyperpolarizability than (PO4)3−. In addition, the (NH4)+ cations give some weak but non-negligible contribution to SHG. It is interesting that (NH4)2PO3F achieves a shorter λs than LBO even though the birefringence of (NH4)2PO3F (0.027 at 1064 nm) is much less than that of LBO (0.041 at 1064 nm). As we know, except large birefringence, small chromatic dispersion is also beneficial to extend the phase-matching region.56 To investigate the effect of chromatic dispersion on phase-matching, the dispersion index, which is defined as the gradients of refractive indices with respect to wavelength (dn/ dλ), is calculated. As shown in Figure 4f, (NH4)2PO3F has a smaller chromatic dispersion than LBO and BBO, which results in a shorter λs. To evaluate the phase-matching performance of the fluorophosphates system, the calculated refractive indices of NH4PO2F2 are chosen as a representation.

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CONCLUSIONS

In summary, fluorophosphates are first proposed as a preferred exploration system of UV/DUV NLO crystals. The (PO3F)2− and (PO2F2)− groups in fluorophosphates with large polarizability anisotropies, wide HOMO−LUMO gaps, and high hyperpolarizabilities are superior FBUs to substitute (PO4)3−. Those FBUs help to enhance birefringence and SHG response and retain the merit in wide UV transmittance of phosphates. (NH4)2PO3F was screened out by DFT method, and large single crystals were grown using the solvent-evaporation method. The experimental measurements indicate that (NH4)2PO3F has a wide UV transparency window with a cutoff edge less than 177 nm, a birefringence larger than 0.03 at 589.3 nm, an SHG efficiency of 1 × KDP, and phasematching output of SHG at 266 nm. Further analysis shows that the large band gap and small chromatic dispersion make fluorophosphates promising candidates for UV/DUV NLO 5401

DOI: 10.1021/acs.chemmater.8b02223 Chem. Mater. 2018, 30, 5397−5403

Article

Chemistry of Materials

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materials. Besides, this study provides a feasible way to design and synthesize new UV/DUV NLO materials.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.8b02223. Photographs of the as-grown single crystals of (NH4)2PO3F; X-ray diffraction (XRD) patterns, thermal gravimetric (TG) analysis, and differential scanning calorimetry (DSC), energy-dispersive X-ray spectroscopy (EDX), IR spectrum, and SHG density of (NH4)2PO3 (PDF) Crystal data and structure refinement for (NH4)2PO3F (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Bingbing Zhang: 0000-0002-1334-5812 Ying Wang: 0000-0001-6642-543X Zhihua Yang: 0000-0001-9214-3612 Shilie Pan: 0000-0003-4521-4507 Author Contributions §

B.Z. and G.H. contributed equally. B.Z. proposed the main idea and performed predictions. G.H. performed all experimental work. X.C. performed the refractive index measurements of (NH4)2PO3F. S.P., Z.Y., and Y.W. conceived and designed the experiments. B.Z. and G.H. wrote the draft paper. All the authors discussed the results and commented on the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the Western Light Foundation of CAS (Grant no. 2016-QNXZ-B-9), the National Basic Research Program of China (Grant no. 2014CB648400), the National Natural Science Foundation of China (Grant nos. 51702356, 11774414, 11474353, and 51602341), and Shanghai Cooperation Organization Science and Technology Partnership Program (Grant no. 2017E01013).

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REFERENCES

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