Monofluorophosphates: A New Source of Deep-Ultraviolet Nonlinear

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Monofluorophosphates: A New Source of Deep-Ultraviolent Nonlinear Optical Materials Lin Xiong, Jie Chen, Jing Lu, Chun-Ya Pan, and Li-Ming Wu Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b03310 • Publication Date (Web): 19 Oct 2018 Downloaded from http://pubs.acs.org on October 21, 2018

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Chemistry of Materials

Monofluorophosphates: A New Source of DeepUltraviolent Nonlinear Optical Materials Lin Xiong, Jie Chen, Jing Lu, Chun-Ya Pan, Li-Ming Wu* Key Laboratory of Theoretical and Computational Photochemistry of Ministry of Education, College of Chemistry, Beijing Normal University, Beijing 100875, P. R. China ABSTRACT: Deep-ultraviolet (DUV) nonlinear optical (NLO) materials which have attracted considerable renewed interests are faced with urgent demands from relevant industries but at the same time are suffering from a severe shortage. The reason for being in such a dilemma may attribute to the extremely rigorous prerequisite for such materials, including a very wide energy gap (wider than 6.2 eV), which can be met merely by a few borates, phosphates and carbonates, among which only one, KBe2BO3F2 (KBBF), is practically usable. Herein, we theoretically identify PO3F2- as a novel DUV NLO building unit based on first-principle studies. Further, we thoroughly survey more than 100 known monofluorophosphates and identify three of them, (NH4)2PO3F, (C(NH2)3)2PO3F and NaNH4PO3F·H2O, as the promising DUV NLO candidates. To confirm our prediction, we experimentally synthesize all of them. Interestingly, these compounds exhibit remarkably strong second harmonic generations (0.9–1.1 × KH2PO4 (KDP) at 1064 nm and 0.2–0.3 × β-Ba2B2O4 (BBO) at 532 nm) and high damage thresholds (2.3–3.1 × KDP at 1064 nm). The significance lies in that this study offers a new functional building unit, PO3F, which opens a new avenue for designing and identifying DUV NLO materials, and thus new monofluorophosphates showing better performance are highly anticipated. INTRODUCTION Deep-ultraviolet (DUV) nonlinear optical (NLO) material, the key material of a solidstate laser, which has attracted renewed interest are pushed forward to meet the urgent demands of the never-ending race to manufacture semiconductors with smaller features and better imaging resolutions.1–7 However, DUV NLO materials are suffering from a severe shortage because of the extremely rigorous prerequisites, including a noncentrosymmetric (NCS) crystallographic structure with large second-order NLO susceptibility (χ2) and moderate birefringence (∆n) and a very wide band gap (Eg, wider than 6.2 eV, which corresponds to an absorption edge below 200 nm and ensures 1

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transparency in the DUV region). Consequently, only approximately 60 borates and a few phosphates and carbonates are known for their DUV NLO properties to date,5,8–17 and among them, only one borate fluoride, KBBF (KBe2BO3F2),16 is practically usable; however, KBBF is highly toxic because of the fatal beryllium component and often has a strong, undesirable, intrinsic layered growth preference that makes it extremely difficult to grow larger crystals, which strictly limits the output power of the coherent laser.1 On the other hand, many commercially available benchmark NLO materials are inadequate for the DUV region, for example, LiB3O5 (LBO), with an absorption edge at 155 nm, is unusable because its small birefringence makes phase matching impossible;8 β-Ba2B2O4 (BBO), with an absorption edge at 198 nm, suffers from the walk-off effect caused by the large birefringence;17 and KH2PO4 (KDP), which shows two-photon absorption that limits the generation of DUV lasers below 200 nm.17 Therefore, the identification of new DUV NLO materials is of great scientific significance and urgent industrial demand. A DUV NLO material requires an extremely wide Eg, which confines its minimum electronic transition to be a single jump between those states, where only the principal quantum numbers are involved. On the other hand, in the 1970s, Chen developed an anionic group theory6,7 that suggests that the NLO effects depend upon the anionic microscopic χ2 of the functional group (building unit), which successfully led to the discovery of BBO, LBO, KBBF, etc. Since then, the conventional research on DUV NLO materials has mainly focused on borates and a few carbonates that are constructed by BOx or CO3 building units, respectively.9–11,13,14 In 2014, we reported the first DUV NLO phosphate, Ba3P3O10Cl, showing impressive second harmonic generation (SHG) properties,12 which initiated a series of studies that revealed the great structural diversity of the PO4 group as a building unit.18,19 Some of these phosphates exhibit impressive DUV NLO properties with short absorption edges falling in the 163–180 nm range and high SHG intensities that are approximately 1–2 times that of KDP.20 Consequently, the development of a new NLO-active building unit is very important, which will lead to the discoveries of 2


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a series of excellent NLO materials. In this work, based on our first-principle theoretical studies, we uncovered a novel DUV NLO-active building unit, the PO3F group, which is superior to the PO4 unit due to its wide HOMO–LUMO gap of 9.4 eV (corresponding to an absorption edge of 132 nm) and large hyperpolarizability of 25.6 (approximately 3.6 times larger than that of PO4) (Figure 2). Subsequently, we conducted a thorough screen of all available compounds (approximately 100) and concluded theoretically the DUV NLO property evaluation (Table S3). To confirm our predictions, we experimentally prepared three candidates, (NH4)2PO3F, (C(NH2)3)2PO3F and NaNH4PO3F·H2O, and observed their Eg wider than 6.2 eV ,being phase-matchable with strong SHG intensities (0.9–1.1 × KDP at 1064 nm and 0.2–0.3 × BBO at 532 nm) and exhibiting high laser-induced damage thresholds (2.3–3.1 × KDP at 1064 nm). Therefore, we identified that monofluorophosphate, as a safe and widely utilized industrial raw material, has been promised as a DUV NLO material. We also show that the growth of large crystals is feasible. This study provides a new functional building unit, PO3F, which opens a new avenue for the design and identification of high-performance DUV NLO materials. (Note: during our revising and resubmission, we notice a paper just accepted by Chem. Mater. reporting (NH4)2PO3F as the first DUV NLO monofluorophosphate. doi: DOI: 10.1021/acs. chemmater.8b02223) METHODS Syntheses: All syntheses are followed by the corresponding references with necessary modifications. 21–23 Polycrystalline (NH4)2PO3F was synthesized by a modified solid-state reaction.26 A total of 1.4621 g of H3PO4, 0.8960 g of CO(NH2)2 and 0.4255 g of NH4HF2 in a molar ratio of 2:2:1 was mixed in a polytetrafluoroethylene reaction vessel in a glovebox. The vessel was capped and heated over 45 min to 150 °C, held for 4000 min, and then cooled to 30 °C at 0.04 °C/min. Finally, 1.955 g of white product was obtained in a high yield (97.8%). (C(NH2)3)2PO3F22 was synthesized by an ion exchange procedure. A 3.0233 g sample of 3



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Na2PO3F and 3.7818 g of C(NH2)3Cl were dissolved into 11 ml of water, and a colourless solid was precipitated by acetone. After being dried at 85 °C for 3 h, 1.163 g of the colourless product was obtained. Single crystals were obtained by recrystallization from a methanol solution. NaNH4PO3F·H2O23 was synthesized by dissolving 0.4319 g of Na2PO3F and 0.4019 g of (NH4)2PO3F in 1.8 ml of water and precipitating the product using acetone (74.0% yield). The colourless product is air and moisture stable. Up to now, we were able to grow crystals with improved size, 4 × 7 × 1 mm3 to 14 × 9 × 2.3 mm3. The quality of the single crystals has also been improved with regard to the number of inclusions and cracks. Extensive work is ongoing in our lab now. Powder X-ray diffraction: The data were collected using a Bruker Model D8 Advance powder diffractometer equipped with Cu Kα radiation source (λ = 1.5418 Å) at room temperature in the range of 2θ = 5–90° with a scan step width of 0.02°. Single-crystal structure determination: Although the single crystal structure of all the three reported compounds are known,

21-23

we still collected the single-crystal X-ray

diffraction data for further confirmation. Data were collected on a Bruker PHOTON II CPAD detector with mirror-monochromatic INCOATEC IμS micro-focus radiation source (50 kV per 1.4 mA). (Table S4) (NH4)2PO3F: orthorhombic Pna21, Z = 4, a = 7.9407(7) Å, b = 11.3362(10) Å, c = 6.0308(4) Å, V = 542.88(8) Å3, Flack parameter = 0.5, GOOF = 1.146, R1 = 5.24%, wR2 = 8.78%, in agreement with those reported in ref. 21. (C(NH2)3)2PO3F: monoclinic Cm, Z = 4, a = 13.2025(14) Å, b = 7.2951(7) Å, c = 11.7574(12) Å, β = 119.710(4)o, V = 983.54(18) Å3, Flack parameter = 0.14(6), GOOF = 1.144, R1 =5.69%, wR2 = 9.88%, in agreement with those reported in ref. 22. NaNH4PO3F·H2O: monoclinic Pn, Z = 2, a = 4.9431(4) Å, b = 9.0200(7) Å, c = 6.0536(5) Å, β = 90.797(3)o, V = 269.88(4) Å3, Flack parameter = 0.03(4), GOOF = 1.294, R1 = 2.29%, wR2 = 5.74%, in agreement with those reported in ref. 23 where the Flack parameter 4


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is higher of 0.11. A B-level alert in our cif-check report indicates a Pmn21 space group which gives a considerably higher R1 = 8%, and thus do not consider. UV−vis−NIR diffuse reflectance, single-crystal transmission and infrared spectra: The diffuse spectra were measured from 176 to 2600 nm on a Shimadzu Solid Spec3700DUV spectrophotometer. The deep-ultraviolet transmission spectra were acquired on a spectrophotometer (VUVas2000, McPherson) in the range of 140–220 nm on a 4 × 7 × 1 mm3 single-crystal of NaNH4PO3F·H2O without polishing. The IR spectra were acquired on a Nicolet Magana 750 FT–IR spectrophotometer in the range of 2.5−25 μm. Second harmonic generation (SHG) and laser-induced damage threshold (LIDT): Powder SHG and phase matchability were measured on polycrystalline samples using the Kurtz and Perry method with Q–switched Nd:YAG lasers at wavelengths of 1064 and 532 nm. The powder LIDT measurements were carried out using a Nd:YAG nanosecond laser at 1064 nm with a pulse duration of 10 ns, the spot diameter is 3.62 mm. Polycrystalline samples were ground and sieved into a series of distinct size ranges, namely, 25–45, 45– 75, 75–109, 109–150, and 150–212 μm, and KDP and BBO sieved into the same size ranges were used as references. The corresponding content in this work has been patented: PCT/CN2017/119815. RESULTS AND DISCUSSION Accurate band gap evaluation: One of the prerequisites of a DUV NLO material is an extremely large Eg. Thus, the accurate Eg calculation is of great significance with respect to designing materials and predicting their properties. The plane-wave density functional theory (DFT) is good at predicting the optical properties including birefringence and the SHG coefficient,24 but using the generalized gradient approximation (GGA) as the exchange-correlation functional generally severely underestimates the Eg.25 As shown in Figure 1, the Eg values calculated by GGA are usually 0.96–2.57 eV lower than the experimental values, whereas the Heyd-Scuseria-Ernzerhof (HSE06) hybrid functional based on a screened Coulomb potential26,27 is able to predict the Eg with an appreciable 5



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improvement in accuracy (more computation details are listed in Supporting Information). Therefore, we first tested the HSE06 calculations on selected DUV NLO compounds (Table S2). The HSE06 results provided a much better description of the experimental Eg values (Figure 1). Since the experimental values are not available, hereafter, we applied HSE06 to predict the Eg values of monofluorophosphates. Predicting the electronic structures and properties of monofluorophosphates: Monofluorophosphates have been known for over 120 years28 and have been widely used as raw materials for the industrial production of toothpaste and chewing gum as food additives and corrosion inhibitors in concrete, but never as NLO materials. The monofluorophosphate anion PO3F2- is derived from PO43- via the substitution of one of the oxygen atoms with a fluorine atom. We calculated the electronic structure and NLO-related properties of the asymmetric tetrahedral PO3F unit with the aid of the Gaussian09 package29 at the 6-31G(d,p) level of theory. Isotropic PO4 (Figure 2, left) has nearly nonpolar Td symmetry, whereas the PO3F unit is anisotropic due to the unequable P–F bond. In addition, the large electronegativity of fluorine induces a large polarizability anisotropy and consequently generates a relatively large dipole moment. The first hyperpolarizability ( β ijk) of PO3F is approximately 3.6 times larger than that of PO4, which indicates the potential for strong SHG (Figure 2). The HUMO–LUMO gap of the PO3F unit (9.4 eV) suggests a considerably short absorption edge of approximately 132 nm. To get more insight, we conducted a thorough survey over more than 100 monofluorophosphates that are reported in the inorganic crystal structure database (Ver. 3.6.0, Germany) and identified only 10 NCS candidates (Table S3). After excluding the compounds involving transition metals that are incapable of having a cutoff edge in the DUV region due to the d−d or f−f electronic transitions, only 5 monofluorophosphates met the prerequisites of a DUV NLO material: (NH4)2PO3F,21 (C(NH2)3)2PO3F,22 NaNH4PO3F·H2O,23 KHPO3F24 and Na2PO3F30. We then calculated their NLO properties with the aid of DFT implemented in the Vienna Ab initio Simulation Package (VASP).31 6


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The reliability of the calculations was confirmed by the birefringence (Δn), and the values of Eg calculated for the benchmark KDP were consistent with the represented experimental data17. (Table 1) Our calculations show that these 5 monofluorophosphates all have an Eg > 6.50 eV, suggesting an absorption edge of < 190 nm. For the five crystals, the ∆n values vary from 0.028 to 0.039 at 532 nm. These values are moderate and comparable to that of KDP (0.041)17, indicating that these compounds may reach phase matchability in the DUV region. Utilizing the length-gauge formalism,32,33 we calculated the SHG tensors as listed in Table 1. The maximum values of the static dij are 1.001, 0.692, 0.727, 0.095 and 0.066 pm/V for (NH4)2PO3F, (C(NH2)3)2PO3F, NaNH4PO3F·H2O, Na2PO3F, and KHPO3F, respectively. The dij values of (NH4)2PO3F, (C(NH2)3)2PO3F and NaNH4PO3F·H2O are impressive compared to that of KBBF (d11(cal.) = 0. 61 pm/V). In addition, the refractive indices, absorption coefficients and other related optical properties of these compounds were also studied (Figure S3–S6). Based on the calculated dij and ∆n values, we concluded that (NH4)2PO3F, (C(NH2)3)2PO3F and NaNH4PO3F·H2O may be excellent candidates for DUV NLO materials. In addition, we probe the origin of the nonlinearity with the aid of the total and partial densities of states (DOS) and cutoff-energy-dependent static dij analyses.35,36 (Figure 3, S2) In (NH4)2PO3F, the O-2p and F-2p nonbonding states of the PO3F unit dominate the VB-1 region, and the P-3p and O-2p states dominate the CB-1 region. As a result, the O-2p, F2p and P-3p states mainly contribute to the SHG response (Figure 3a, b). Similar situations are observed in NaNH4PO3F·H2O (Figure 3e and f), Na2PO3F and KHPO3F (Figure S2). These results illustrate that the PO3F unit serves as an NLO-active functional building unit. The (C(NH2)3)2PO3F compound is special in that both the PO3F building unit and the guanidine cation contribute to the SHG (Figure 3c, d). The VB-1 is mainly composed of the F-2p and O-2p states of the PO3F unit and the N-2p states of the guanidine cation, whereas CB-1 is predominately derived from the N-2p and C-2p states. The top of the VB 7



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(VBM) and the bottom of the CB (CBM) are determined by the N-2p and C-2p states of the guanidine; in addition, the HOMO–LUMO gap of the free C(NH2)3+ cation is narrower than that of the free PO3F2- anion (9.0 vs 9.4 eV). As a consequence, the guanidine cation narrows the Eg of (C(NH2)3)2PO3F with respect to those of (NH4)2PO3F and NaNH4PO3F·H2O (6.55 vs 6.89 and 7.02 eV, respectively, Table 1). Syntheses, crystal growth and SHG properties of (NH4)2PO3F, (C(NH2)3)2PO3F and NaNH4PO3F·H2O: To confirm our predictions, we experimentally synthesized (NH4)2PO3F, (C(NH2)3)2PO3F and NaNH4PO3F·H2O by newly established methods. The crystallographic data (Table S4–6) are in good agreement with the previously reported data.21-23 The phase purity was confirmed by powder X-ray diffraction data (Figure 5a, c, e). Figure 4 shows that the acentric structures of the compounds featuring zero-dimensional arrays of the isolated PO3F tetrahedron are well separated by the counter ions. The ellipsoid drawings of the asymmetric unit are shown in Figure S7. Different counter ions have a slight impact on the distortion of the tetrahedron by altering the bond lengths of (NH4)2PO3F, (C(NH2)3)2PO3F and NaNH4PO3F·H2O (P–O(avg.): 1.504, 1.514, and 1.504 Å and P–F: 1.588, 1.535, and 1.597 Å) and the bond angles (O–P–O(avg.): 114.56, 110.37 and 114.36° and O–P–F(avg.): 103.71, 107.89 and 103.97°) relative to those of an ideal PO3F tetrahedron (1.538, 1.714 Å, and 115.83, 101.95°), respectively. (Table S6) However, as indicated by the dipole moment, the counter species significantly influence the alignment and orientation of the PO3F tetrahedron (Table S7). In the orthorhombic (NH4)2PO3F, which belongs to the Pna21 space group, the symmetry operation of the 21 axis sitting at (0, 0, 1/2) leads to the complete cancellation of the x- and y-components of the dipole moments of each PO3F unit; thus, the net dipole moment is along the z direction. Since the mirror plane of Cm symmetric (C(NH2)3)2PO3F at (x, 0, z) cuts the PO3F unit through the P, F, and O2/O4 atoms, the net dipole moment is parallel to the ac plane. In the monoclinic NaNH4PO3F·H2O, the net dipole moment also lies in the ab plane, and all the P–F bonds are roughly directed along the c axis. 8


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The IR spectra of (NH4)2PO3F, (C(NH2)3)2PO3F and NaNH4PO3F·H2O (Figure S8) show the characteristic vibrations of the PO3F group with the O–P–O in-plane bending, asymmetric, antisymmetric and symmetric stretching vibrations observed at 500–650 cm1,

1050–1100 cm-1 and 940–970 cm-1.12,18,19 The corresponding P–F stretching vibrations

were observed at 769, 818 and 764 cm−1,36 respectively. UV cutoff edges shorter than 200 nm were observed (Figure 5b, d, f); in particular, the 4 × 7 × 1 mm3 single-crystal of NaNH4PO3F·H2O showed a cutoff edge of 176 nm (Figure 5f), which confirmed our theoretical Eg calculations. Remarkably

strong

SHG

intensities

of

(NH4)2PO3F,

(C(NH2)3)2PO3F

and

NaNH4PO3F·H2O were measured to be 0.9, 1.0, and 1.1 × KDP at 1064 nm and approximately 0.2, 0.2, and 0.3 × BBO at 532 nm, respectively (Figure 6). The curves of the SHG intensity-particle size revealed that the three crystals are phase matchable at both 1064 and 532 nm. These observations confirmed our theoretical prediction. The laserinduced damage threshold (LIDT) values were measured to be 122.3, 91.5 and 118.1 MW/cm2, respectively, and these values are approximately 3.1, 2.3 and 3.0 × KDP (39.8 MW/cm2), respectively. Since the calculated d11 of KBBF is 0.61 pm/V, the SHG intensities of (C(NH2)3)2PO3F, NaNH4PO3F·H2O and (NH4)2PO3F are therefore estimated to be 1.1–1.6 × KBBF, which indicates their promise as DUV NLO materials with remarkably high performances. CONCLUSIONS Monofluorophosphates known for over 120 years and widely used as safe raw materials for toothpaste and chewing gum are identified as a new source of DUV NLO compounds and exhibit comparable properties to the borate fluoride, KBBF. Our first-principle calculations on a thorough survey of all available monofluorophosphates illustrate their ability to meet the rigorous prerequisites for being a DUV NLO material. Such predictions are confirmed by experimental preparations of (NH4)2PO3F, (C(NH2)3)2PO3F and NaNH4PO3F·H2O that all have UV cutoff edges shorter than 200 nm and remarkably strong 9



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SHG intensities (0.9–1.1× KDP at 1064 nm and 0.2–0.3 × BBO at 532 nm) that are estimated to be 1.1–1.6 × KBBF, and have large laser-induced damage thresholds (2.3–3.1 × KDP at 1064 nm). Additionally, the three compounds are phase matchable at both 1064 and 532 nm. These promising compounds also show feasible growth of large crystals and appreciable mechanical stability. Our results provide a new building unit, PO3F, which opens a new avenue for design of, search for and selection of high-performance DUV NLO materials, and new NCS fluorophosphates with interesting properties and chemistry are highly in anticipation.

ASSOCIATED CONTENT Supplementary information is available in the online version of the paper. Reprints and permissions information is available online at http://pubs.acs.org. Correspondence and requests for materials should be addressed to L. M. Wu.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]. (L.-M.W.) ORCID Li-Ming Wu: 0000-0001-8390-2138

Author contributions L.M. Wu initiated and directed the study; L.X. designed and performed the computational studies; J.C. and L. M. Wu conceived of and carried out the main experiments; and J. C., J.L., and C.Y. P. synthesized the samples, grew single crystals, and carried out X-ray diffraction and nonlinear optical experiments. All authors discussed and commented on the manuscript.

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Notes The authors declare no competing financial interests.

ACKNOWLEDGEMENTS This research was supported by the National Natural Science Foundation of China under Projects 91422303, 21571020, and 21671023.

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(KBBF) crystal. Opt. Mater. 1996, 5, 105–109. (17) Dmitriev, V. G.; Gurzadyan, G. G.; Nikogosyan, D. N. Handbook of Nonlinear Optical Crystals; Springer: Berlin, 1999. (18) Li, L.; Wang, Y.; Lei, B. H.; Han, S.; Yang, Z.; Poeppelmeier, K. R.; Pan, S. A new deep-ultraviolet transparent orthophosphate LiCs2PO4 with large second harmonic generation response. J. Am. Chem. Soc. 2016, 138, 9101–9104. (19) Zhao, S.; Gong, P.; Luo, S.; Bai, L.; Lin, Z.; Ji, C.; Chen, T.; Hong, M.; Luo, J. Deep-ultraviolet transparent phosphates RbBa2(PO3)5 and Rb2Ba3(P2O7)2 show nonlinear optical activity from condensation of [PO4]3– units. J. Am. Chem. Soc. 2014, 136, 8560–8563. (20) Chen, J.; Ali, K. M.; Xiao, C. X.; Yan, Y. X.; Dai, Y.; Chen, L. Recent advances in nonlinear optical phosphate materials. Chinese J. Struct. Chem. 2017, 11, 1837–1858. (21) Krupková, R.; Fábry, J.; Císaová, I.; Vanĕk, P. Bis(ammonium) fluorophosphate at room temperature. Acta Cryst. 2002, 58, i66–i68. (22) Prescott, H. A. Diplom-Chemikerin. Thesis, Humboldt-Universität zu Berlin, 2001. (23) Fábry, J.; Dušek, M.; Krupková, R. Ammonium sodium fluorotrioxophosphate monohydrate. Acta Cryst. 2007, 63, 92–94. (24) Chen, C.; Lin, Z.; Wang, Z. The development of new borate-based UV nonlinear optical crystals. Appl. Phys. B 2005, 80, 1–25. (25) Perdew, J. P.; Wang, Y. Accurate and simple analytic representation of the electron-gas correlation energy. Phys. Rev. B 1992, 45, 13244–13249. (26) Heyd, J.; Scuseria, G. E.; Ernzerhof, M. Hybrid functionals based on a screened coulomb potential. J. Chem. Phys. 2003, 118, 8207–8215. (27) Krukau, A. V.; Vydrov, O. A.; Izmaylov, A. F.; Scuseria, G. E. Influence of the exchange screening parameter on the performance of screened hybrid functionals. J. Chem. Phys. 2006, 125, 224106. (28) Uchida, M.; Brown, N.; Ho, D. H. Enzymatic conversion of 5-fluoro-2'-deoxyuridine to 5-fluorouracil or 5-fluoro-2'-deoxyuridine 5'-monophosphate in human tissues. Anticancer Res. 1900, 10, 779–783. (29) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; et al. Gaussian09, RevisionD.01.Gaussian, Inc., Wallingford CT,. Gaussian, Inc., Wallingford CT. 2009. (30) Durand, P. J.; Cot, L.; Galigné, J. L. Etudes structurales de composés oxyfluorés du PV. II. structure cristalline de Na2PO3F β. Acta Cryst. 1974, 30, 1565–1569. (31) Kresse, G.; Furthmüller, J. Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set. Phys. Rev. B 1996, 54, 11169–11186.

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(36) Weil, M.; Puchberger, M.; Fueglein, E.; Baran, E. J.; Vannahme, J.; Jakobsen, H. J.; Skibsted, J. Single-crystal growth and characterization of disilver(I) monofluorophosphate(V), Ag2PO3F: crystal structure, thermal behavior, vibrational spectroscopy, and solid-state 19F, 31P, and 109Ag MAS NMR spectroscopy. Inorg. Chem. 2007, 46, 801–808.

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Chemistry of Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

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Table1: Selected calculated and experimental properties of monofluorophosphates and commercial KH2PO4 (KDP) Compound

(NH4)2PO3F

Point group

mm2

HSE06-calculated Eg

6.89

(C(NH2)3)2PO3F m 6.55

7.02

d11 = -0.173, Static SHG tensors (pm/V)

d24 = 1.001,

d12 = 0.361,

d15 = 0.197,

d15 = -0.692,

d33 = -0.918

d33 = 0.404, d13 = 0.028

Δn at 532 nm

0.035

0.039

2

-42m

3m

7.30

6.76

6.53

6.92

d16 = 0.056

d12 = 0.451, d15 = 0.534, d24 = 0.169,

d14 = 0.095

d14 = 0.068

d36 =

d23 = 0.022

0.76

d22 = 3.046

d22 = 0.066

d33 = -0.333, d13 = 0.060 1.1 × KDP

/

/

0.3 × BBO

/

/

PM

/

/

PM

BBO

d11 = -0.727,

1.0 × KDP

PM

a measured

222

0.2 × BBO

0.9 × KDP

c

KDP

0.028

0.2 × BBO

Phase matchability

KHPO3F

0.036

Iobv/𝑰𝒐𝒃𝒔 b 𝑩𝑩𝑶

Na2PO3F

0.035

Iobv/𝑰𝒐𝒃𝒔 a 𝑲𝑫𝑷

NaNH4PO3F·H2O

0.037

0.111

PM

PM

at 1064 nm. b measured at 532 nm. c measured at 1064 (KDP as a reference) and 532 nm (BBO as a reference).

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Figure 1. Comparison of the experimental and calculated energy Eg values by GGA and HSE06 functional for both classic and newly developed DUV compounds, such as KBBF, LBO, BBO, BPOC and MNCO3F series. The experimental values of these crystals come from references listed in Table S2. Using the least squares method, the red line shows that the HSE06 results provide a much better description of the band gaps compared to the experimental results.

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Figure 2. Energy diagram, geometry and calculated NLO-related properties of isolated ideal PO43- and PO3F2- tetrahedra. The calculated HOMO–LUMO gaps of PO43- and PO3F2- are 9.6 and 9.4 eV, respectively. The HOMO of both are nonbonding O-2px orbitals; the LUMO of both are O-2s, and Psp3 hybrid orbitals. The F-2p orbitals appear in the energy region, which is slightly lower than the HOMO level. Isotropic PO4 has nearly nonpolar Td symmetry, whereas the PO3F unit is anisotropic; it has a large polarizability anisotropy, which consequently generates a dipole moment. The first hyperpolarizability (βijk) of PO3F is approximately 3.6 times larger than that of PO4, which indicates the great potential for a strong SHG.

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Figure 3. Total and partial DOS of and cutoff-energy-dependent static SHG coefficients (NH4)2PO3F (a, b), (C(NH2)3)2PO3F (c, d) and NaNH4PO3F·H2O (e, f); numbers represent different band regions.

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Figure 4. Single-crystal structures of (NH4)2PO3F (a), (C(NH2)3)2PO3F (b) and NaNH4PO3F·H2O (c). The tetrahedron: PO3F unit, yellow ball: O; red: F. (d) Photos of the as-synthesized NaNH4PO3F·H2O crystals that are manually polished by us with sand paper. The crystal shown in the up-left panel is of a thickness of 2.1 mm that was polished from sizes of roughly 12 × 6 × 2.4 mm3.

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Figure 5. The calculated and experimental XRD patterns and ultraviolet-visible-near-infrared diffuse reflection spectra of polycrystalline samples of (NH4)2PO3F, (a, b); (C(NH2)3)2PO3F (c, d) and NaNH4PO3F·H2O, (e, f). The insert of panel f is the deep-ultraviolet transmission spectrum measured on the single-crystal as shown in the photo.

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Figure 6. Phase-matching curves, i.e., particle size vs SHG intensity, at the incident wavelength of 1064 nm with KDP as reference, and of 532 nm with BBO as reference of polycrystalline samples of (NH4)2PO3F, (a, b); (C(NH2)3)2PO3F (c, d) and NaNH4PO3F·H2O, (e, f).

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TOC

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Monofluorophosphates: A New Source of Deep-Ultraviolet Nonlinear

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