Optimized arrangement of non-π-conjugated PO₃NH₃ units leads to enhanced ultraviolet optical nonlinearity in NaPO₃NH₃

Abstract

The key to achieving nonlinear optical (NLO) crystals with superior optical properties lies in the development of new high-performance functional units and their precise arrangement. In this study, we identify the polar covalent tetrahedron (PO₃NH₃)⁻, with its remarkable optical properties, as a novel deep-ultraviolet (DUV) NLO-active unit for the first time. To ensure the (PO₃NH₃)⁻ units are accurately aligned, we selected NaNH₄PO₃F·H₂O as a template. Subsequently, we successfully synthesized NaPO₃NH₃, wherein the (PO₃NH₃)⁻ units were arranged in an orderly manner. This excellent ordered arrangement of (PO₃NH₃)⁻ units enables NaPO₃NH₃ to exhibit not only a significant SHG effect (1.2 × KDP), but also the largest birefringence (0.062@546.1 nm) among DUV non-π-conjugated phosphate systems.

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Introduction

Deep-ultraviolet (DUV) nonlinear optical (NLO) crystals hold significant potential in extending and regulating laser wavelengths into the DUV region through the frequency-doubling effect. This capability is crucial for applications in semiconductor manufacturing, optical communication, and laser micromachining.^1–4 However, achieving a balance between a short cutoff edge (λ_cut-off), high second harmonic generation (SHG) efficiency, and moderate birefringence (Δn) remains a challenge for DUV crystals.^5–7 Previous research has indicated that obtaining suitable birefringence for phase matching in DUV crystals is difficult.^8,9 Among the few crystals capable of directly realizing DUV laser output, KBe₂BO₃F₂ (KBBF) stands out.¹⁰ Nonetheless, the practical application of KBBF is limited due to its severe layered growth habit and the presence of highly toxic beryllium oxide.

Non-π-conjugated systems have garnered significant interest due to their broader optical transmittance window, making phosphate systems a primary research focus. However, phosphate systems are often hindered by weak SHG and small birefringence, which can be ascribed to the low hyperpolarizability and minimal optical anisotropy resulting from the absence of π orbitals.^11,12 Numerous studies have been conducted to enhance SHG and birefringence in phosphates. One of the most effective approaches involves the use of NLO-active cations, such as Ti⁴⁺,¹³ Bi³⁺,¹⁴ Sn²⁺,¹⁵ and Pb²⁺,¹⁶ among others.¹⁷ Nevertheless, this method invariably induces a red shift of absorption edge, limiting its applicability in the DUV region. To ensure high UV transmission, the selection of cations must be restricted to metal ions with a full shell, including alkali metals, alkaline earth metals, and certain rare earth metal cations.^18–20 Therefore, to maintain the wide band gap of phosphates, an alternative strategy to enhance SHG and birefringence is the modification of [PO₄]³⁻ group.

Along this path, polar polyphosphates, such as P₂O₇ dimer,^21,22 P₃O₁₀ trimer,^23,24 and [PO₃]_∞ chain groups,^25,26 have been investigated. However, these crystals exhibit enhancement only in SHG performance. The birefringence of polyphosphates is rarely greater than 0.03@1064 nm, which is attributed to the steric hindrance between groups caused by co-vertex connections, preventing the ordered arrangement of polyphosphate groups. In contrast, isolated tetrahedral groups offer greater flexibility in adjusting their positions to maximize optical anisotropy. Consequently, identifying polar tetrahedral phosphate groups with exceptional properties is crucial. In light of this, Pan and Chen (2018) groups proposed introducing highly electronegative fluorine (F) into [PO₄]³⁻ groups to obtain polar (PO₃F)²⁻ with excellent DUV optical properties. Subsequent studies discovered crystals such as (NH₄)₂PO₃F ²⁷ and [C(NH₂)₃]₂PO₃F;²⁸ however, the arrangement of polar tetrahedral groups in these crystals did not favor maximizing enhanced birefringence. In 2019, Chen's group successfully synthesized and grew NaNH₄PO₃F·H₂O,²⁹ which binds fluorophosphate together through hydrogen bonds from NH₄⁺, achieving near-directional arrangement of non-π-conjugated DUV functional units of polar phosphate units for the first time. Ultimately, NaNH₄PO₃F·H₂O exhibited a large SHG and an extremely prominent birefringence of >0.053@589.3 nm while maintaining deep-ultraviolet transmittance. Given that NaNH₄PO₃F·H₂O has been demonstrated to be an outstanding DUV NLO crystal, it potentially serves as a novel parent compound to guide the synthesis and screening of new materials.

In the investigation of the sulfonate system, our research group has discovered that polar tetrahedra, obtained by employing substituents such as amino and methyl groups, exhibit exceptional DUV optical properties.^30,31 Consequently, we have identified a series of DUV non-π-conjugated NLO crystals with outstanding characteristics, including M(SO₃NH₂)₂ (M = Ba, Sr),³² sulfamide,³³ and KNO₃SO₃NH₃.³⁴ Compared to (SO₄)²⁻, (NH₂SO₃)⁻ and SO₂(NH₂)₂ demonstrate superior performance in three key aspects: polarizability anisotropy, hyper-polarizability, and the highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) gap. Moreover, the incorporation of hydrogen-containing anionic groups facilitates the regulation of their arrangement. For instance, sulfamide employs the hydrogen bond of SO₂(NH₂)₂ to align itself unidirectionally within a layer. Since the geometric structure and optical properties of PO₄ closely resemble those of SO₄, we anticipate that modifying the phosphate group with amino or methyl groups could yield a novel polar tetrahedron containing hydrogen bonds and possessing exceptional optical properties.

In this study, we employed NaNH₄PO₃F·H₂O as a template. By replacing F⁻ with an NH₃ group, we formed the new polar tetrahedron (PO₃NH₃)⁻, with the expectation that it would display excellent DUV optical properties and achieve a consistent arrangement through its inherent hydrogen bond. Furthermore, our calculations revealed that the polar (PO₃NH₃)⁻ unit exhibits improved performance in the aforementioned three key aspects compared to (PO₄)³⁻. Encouraged by these computational findings, we successfully synthesized an amino-phosphate DUV NLO material, NaPO₃NH₃ (NPNH). Notably, the polar [PO₃NH₃]⁻ groups in NPNH achieved optimal alignment, which significantly enhanced the (010) in-plane anisotropy and resulted in the largest Δn (0.062) reported to date among DUV non-π-conjugated phosphate systems. Physical measurements demonstrated that NPNH exhibits a substantial second-harmonic generation (SHG) effect (1.2 × KH₂PO₄ (KDP)) and short cutoff edges (<190 nm).

Experimental section

Reagents

Diphenyl Phosphoramidate (C₁₂H₁₂NO₃P, 97%, Adamas), NaOH (≧98%, Titan), Gelatin (Aladdin), Acetic acid glacial (≧99.5%, Titan).

Synthesis

Polycrystalline samples of NaPO₃NH₃ were synthesized according to the method described by E. Hobbs, D. E. C. Corbridge, and B. Raistrick.³⁵ A mixture of Diphenyl Phosphoramidate (8 mmol, 1.994 g), NaOH (32 mmol, 1.280 g), and H₂O (10 ml) was heated until the Diphenyl Phosphoramidate was completely hydrolyzed. Then acetic acid glacial was slowly dripped into the solution and the insoluble monosodium salt (NaPO₃NH₃) was produced according to the following scheme:

However, the insoluble nature of NaPO₃NH₃ prevented the growth of the required crystals from a simple solution. Thus, we adopted the method of crystallizing within the gelatin gels in a U-tube to obtain large crystals for subsequent measurements. For this purpose, the gelatin (1 g) was soaked in 15 ml H₂O for 5 minutes and then heated until the gelatin was dissolved. Next, the gelatin solution was poured into a U-tube for 3 hours to solidify. Finally, the solution of (NaO)₂PONH₂ (10 ml, previously prepared) and acetic acid glacial (10 ml) were filled into the respective arms of the U-tube. After one week, a small number of transparent colorless polycrystalline NaPO₃NH₃ crystals were obtained. These crystals were stable in the air and hardly water-soluble.

Single-crystal X-ray diffraction

For single-crystal X-ray diffraction measurement, a premium NaPO₃NH₃ signal crystal was chosen. The graphite-monochromatic Cu-Kα radiation (λ = 1.54184 Å) was used to acquire the diffraction data by using a Rigaku Mercury CCD diffractometer at room temperature. The data was integrated by the Crystal Clear program. The intensities were corrected by Lorentz polarization, air absorption, and absorption attributable to the variation in the path length through the detector faceplate. The crystal structure of NaPO₃NH₃ was solved through the directed methods and then confirmed by full-matrix least-squares fitting on F² using SHELXL on the Olex2 package.^36,37 The PLATON program was used to check for the structure, and no higher symmetry was found. Crystallographic data and structure refinements of NaPO₃NH₃ were given in Table S1.† Atomic coordinates, equivalent isotropic displacement parameters, selected bond lengths and angles, and anisotropic displacement parameters for NaPO₃NH₃ were shown in Tables S2–S4.†

Power X-ray diffraction

The power X-ray diffraction (XRD) patterns were collected on the Miniflex-600 powder X-ray diffractometer (Cu Kα radiation with λ = 1.540598 Å) from 10° to 80° at room temperature with a scanning speed of 5° min⁻¹. The experimental powder XRD patterns and the simulated patterns of the single crystal are coincident.

Energy-dispersive X-ray spectroscopy analysis

Energy dispersive X-ray spectroscopy (EDS) was tested on the FESEM, SU-8010 scanning electron microscope (SEM) with an X-ray spectroscope. A clean NaPO₃NH₃ crystal was mounted on an aluminum sample stage with carbon conductive tape.

Thermal analysis

The data thermogravimetric analysis (TG) and differential thermal analysis (DTA) were carried out on the Netzsch STA449C simultaneous analyzer. NaPO₃NH₃ power was packed in an Al₂O₃ crucible and heated from room temperature to 900 °C at a rate of 10 °C min⁻¹.

UV-Vis diffuse reflectance spectroscopy

The UV-Vis diffuse reflection spectrum was measured on PerkinElmer Lamda-950 UV-Vis spectrophotometer scanning with the scope of 190–800 nm to determine the cutoff edge of the NaPO₃NH₃ crystal. BaSO₄ is used as the standard of 100% reflectance.

Birefringence measurement

The birefringence measurement was performed on the polarizing microscope (ZEISS Axio Scope. A1) equipped with a Berek compensator. The light source is 546.1 nm. The formula for calculating is expressed as follows: Δn = ΔR/T, while ΔR means the optical path difference, Δn signifies the birefringence, and T represents the thickness of the crystal.

Second harmonic generation measurements

The test of polycrystalline powder Second Harmonic Generation (SHG) was executed on a Q-switched Nd: YAG solid-state laser with laser radiation wavelengths of 1064 nm. The sample was divided into 24–45, 45–62, 62–75, 75–109, 109–150, and 150–212 μm, respectively. KH₂PO₄ (KDP) is used as the standard reference. The samples were pressed between two round glass plates, each sealed with 8 mm diameter and 1 mm thick rubber rings. Then the samples were placed in the antiglare box and irradiated with the pulsed laser from an OPO laser. The signals are detected with a photomultiplier tube connected to a RIGOL DS1052E 50 MHz oscilloscope. The ratio of the SHG intensity outputs for the samples and KDP was calculated.

Theoretical calculations

The molecular orbitals and polarizability calculations were carried out using density functional theory (DFT)³⁸ and couple cluster with singles and doubles substitutions (CCSD) implemented in the Gaussian09 package 41. HOMO–LUMO Gap (E_g) of anionic groups were calculated through CCSD/Aug-cc-PVDZ, and the polarizability anisotropy (δ) and hyperpolarizability tensor were calculated through wB97/Aug-cc-PVDZ.

Band structure and density of states (DOS) were determined using DFT calculations provided by the CASTEP code in the Material Studio Package. The exchange interaction³⁸ was treated by the generalized gradient approximation (GGA) of Perdew–Burke–Ernzerhof (PBE).³⁹ The energy cutoff for the plane wave was chosen as 750 eV, and the Monkhorst–Pack k-point sampling of 2 × 2 × 2 in the Brillouin zone of the unit cell was selected for calculation. The valence electrons are Na 2s² 2p⁶ 3s¹, P 3s² 3p³, O 2s² 2p⁴, N 2s² 2p³, and H 1s. The real part of the dielectric function was obtained by the Kramers–Kronig transform, and then the refractive index was determined. The SHG coefficients were obtained by the formula originally proposed by Sipe et al.⁴⁰ and developed by Cheng et al.⁴¹ The SHG density of d₃₃ is calculated out by the band-resolved method.⁴²

Results and discussion

In order to investigate the potential structure-directing optical properties of amino-phosphate, we performed calculations and constructed molecular orbital diagrams for phosphate family groups. As shown in Fig. 1 HOMO, the nonbonding O 2p orbitals occupied the HOMO in three groups, while the F atom and NH₃ group were not involved. This indicates that the electrons within the group are not evenly distributed, suggesting that the polarizability anisotropy values of (PO₃F)²⁻ and (PO₃NH₃)⁻ are not lower than those of (PO₄)³⁻. From HOMO to LUMO, the electron distribution in (PO₃NH₃)⁻ shifts from O to NH₃. This extensive electron redistribution correlates with a large electric field, which may enhance the SHG effect. Additionally, we calculated three parameters for (PO₄)³⁻, (PO₃F)²⁻ and (PO₃NH₃)⁻ as references (Table 1). The computed HOMO–LUMO gap of (PO₃NH₃)⁻ is comparable to that of (PO₄)³⁻, suggesting that amino-phosphate could maintain the advantage of broad UV transmittance characteristic of phosphate. Concurrently, (PO₃NH₃)⁻ exhibits superior hyperpolarizability, which can be rationalized by the values of the “flexible” chemical bonds within the NLO-active group.⁴³ The flexibility index of P–N in (PO₃NH₃)⁻ (F = 0.14) is greater than that of P–F in (PO₃F)²⁻ (F = 0.09), indicating that the more flexible P–N bonds are beneficial for further enhancing the SHG response. In conclusion, based on anionic group theory, the slightly larger polarizability anisotropy and superior hyperpolarizability could result in exceptional NLO properties.

Table 1 Calculated properties of the optimized (PO₄)³⁻, (PO₃F)²⁻ and (PO₃NH₃)⁻ groups

Units	δ	|β|	E g (eV)
Polarizability anisotropy (δ), hyperpolarizability tensor (β), and HOMO–LUMO gap (Eg).
(PO 4 ) ^3−	0	0	7.24
(PO 3 F) ^2−	6.03	60.8	9.52
(PO 3 NH 3 ) ^−	2.16	194	7.82

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Fig. 1 Molecular orbitals of the (PO₄)³⁻, (PO₃F)²⁻, and (PO₃NH₃)⁻ anionic groups. The HOMO and LUMO are shown in the middle and lower panels, respectively.

NaPO₃NH₃ (NPNH) powder samples were synthesized using the two-step method reported by E. Hobbs et al.³⁵ To obtain large crystals for subsequent measurements, we employed a U-tube gelatin gel crystallization technique. The experimental powder X-ray diffraction (PXRD) pattern in Fig. S1† is consistent with the calculated values, confirming phase purity. Energy-dispersive X-ray spectroscopy (EDS) analysis reveals that the crystal contains Na, P, O, and N elements (Fig. S2†). Thermogravimetric (TG) and differential thermal analysis (DTA) curves for NPNH are presented in Fig. S3,† indicating stability up to 260 °C, which is higher than the thermal stability of NaNH₄PO₃F·H₂O (under 100 °C),²⁹ KNO₃SO₃NH₃ (∼150 °C),³⁴ and KLi(HC₃N₃O₃)·2H₂O (125 °C).⁴⁴ NPNH is insoluble in water and alcohol and remains stable in air at room temperature for at least three months.

NPNH crystallizes in the hexagonal asymmetric space group P6₃ (No. 173) with lattice parameters a = 5.7805(1) Å, c = 6.0265(1) Å, which aligns with the findings of E. Hobbs et al. The structure of NPNH is characterized by a single anionic structural motif, (PO₃NH₃)⁻. As illustrated in Fig. 2d, each phosphorus atom is tetrahedrally coordinated by three oxygen atoms and an NH₃ group, forming a (PO₃NH₃)⁻ tetrahedron. The P–O distances are approximately 1.509 Å, and the P–N bond length is 1.779 Å. Intriguingly, the (PO₃NH₃)⁻ groups are interconnected via hydrogen bonds, facilitating the formation of a three-dimensional (3D) structure in NPNH. Sodium cations are embedded within this network, ensuring overall charge neutrality (Fig. 2b). This bonding pattern is reminiscent of the interactions observed between (PO₃F)²⁻ groups in NaNH₄PO₃F·H₂O. When contrasted with NaNH₄PO₃F·H₂O, NPNH exhibits a more neatly arrangement of anionic groups (Fig. 2e and f). In NaNH₄PO₃F·H₂O, the (PO₃F)²⁻ tetrahedra within each layer are anchored by water molecules and NH₄⁺ groups, forming a robust hydrogen-bonded network that aligns nearly parallel to the c-axis. In contrast, in NPNH, the (PO₃F)²⁻ and NH₄⁺ groups are combined into a single (PO₃NH₃)⁻ group (Fig. 2c and d), and (PO₃NH₃)⁻ units are able to achieve complete alignment along the c-axis due to their inherent hydrogen bonds (Fig. 2a and b). Consequently, this uniform alignment of polar P–N bonds results in the superposition of dipoles along the c-axis, maximizing macroscopic polarization. This leads to a significant enhancement of the (010) in-plane anisotropy, enabling NPNH to exhibit birefringence along the c-axis. Furthermore, the hydrogen bonds between anions and Na–O ionic bonds give rise to a 3D structure without any layering tendency (Fig. S4†). The lack of crystal water in NPNH potentially enhances its thermal stability.

Fig. 2 (a) the structure of NaNH₄PO₃F·H₂O; (b) the structure of NaPO₃NH₃; (c and d) transformation from [PO₃F]²⁻ to [PO₃NH₃]⁻; (e and f) the arrangement of anionic groups in NaNH₄PO₃F·H₂O and NaPO₃NH₃, respectively.

The Ultraviolet–Visible (UV-Vis) diffuse reflectance spectra of NPNH are presented in Fig. 3a, illustrating an absorption edge below 190 nm, which corresponds to a band gap exceeding 6.5 eV. The electronic and optical properties of NPNH were computed using the first-principles method, indicating a band gap of 5.320 eV (Fig. S5†). The discrepancy between the experimental and computed band gaps can be attributed to the discontinuity of the exchange–correlation energy function.

Fig. 3 (a) The UV/Vis diffuse-reflectance spectroscopy of NaPO₃NH₃; (b) power SHG measurement at 532 nm; (c and d) the SHG density of d₃₃ in occupied state and unoccupied state.

The SHG response of NPNH was evaluated under fundamental wave laser radiation (λ = 1064 nm), utilizing polycrystalline KDP as a reference. As depicted in Fig. 3b, the SHG response of NPNH is approximately 1.2 times that of KDP within the particle size range of 150–212 μm, exhibiting phase-matchable behavior in accordance with the rule proposed by Kurtz and Perry.⁴⁵ Given that the space group is P6₃, two independent SHG tensor components (d₃₁, d₃₃) must be considered. Through the application of the “velocity-gauge” formula developed by Sipe et al.,^40,46 the largest tensor component d₃₃ of NPNH is 0.583 pm V⁻¹ at a wavelength of 1064 nm (1.165 eV), while the largest tensor component d₁₄ of KDP is 0.381 pm V⁻¹ at the same wavelength (Fig. S6†). Accordingly, the calculated SHG intensity is approximately 1.5 times that of KDP, aligning closely with the experimental value. Furthermore, the SHG-weighted electron density of d₃₃ was investigated to discern the origin of the SHG responses (refer to Fig. 3c and d). The occupied and unoccupied states suggest that the primary SHG effect stems from the (PO₃NH₃)⁻ units, which is consistent with the anionic group theory.⁴⁷

The birefringence measurement was conducted using a ZEISS Axio Scope. A1 polarizing microscope, equipped with a Berek compensator. The resultant values for ΔR and T were 1219.7 nm and 19.64 μm, respectively, as shown in Fig. S7† and Fig. 4a. The calculated birefringence at 546.1 nm, derived using the formula Δn = ΔR/T,⁴⁸ was 0.062, closely aligning with the theoretical value of 0.063, following the trend of n_a = n_b > n_c (Fig. 4b). It is concluded that NPNH exhibits the highest birefringence among DUV phosphates, surpassing that of NaNH₄PO₃F·H₂O (0.053@589.3 nm), as illustrated in Fig. 4e and Table S7.† This significant birefringence can be primarily attributed to two reasons. First, the (PO₃NH₃)⁻ unit displays a greater polarizability anisotropy, which contributes to enhanced birefringence. Second, these units possess an optimal arrangement, superior to that of NaNH₄PO₃F·H₂O, as the angle between the P–N bond and the crystallophysical axis c is eliminated (Fig. 4c). The computed dipole moment, as shown in Table S6,† vividly reflects this optimized arrangement. The net dipole moment of (PO₃NH₃)⁻ units is calculated to be 10.02 D in the polar c direction, with 0 D on the a and b directions, indicating that the polarization direction of all (PO₃NH₃)⁻ units is along the c-axis. This optimal arrangement potentially compensates for the diminished birefringence effect caused by the reduced anisotropy of the (PO₃NH₃)⁻ unit compared to (PO₃F)²⁻. As shown in Fig. 4d, the uniform arrangement of polar chemical bonds considerably reduces the electron density in the c-axis direction, resulting in a significant decrease in electron density in the c direction compared to the a and b directions. Consequently, the refractive index in the c direction is substantially lower than that in the a and b directions, leading to high birefringence.⁴⁹ This is further corroborated by the calculated refractive index dispersion curve.

Fig. 4 (a) Photograph of NaPO₃NH₃ in the size of 19.64 μm; (b) calculated the refractive index of NaPO₃NH₃; (c) the angular correlation between the P–N bond and the crystallophysical axis c is 0°; (d) the calculated electron density map of NPNH; (e) the birefringence of phosphates whose λ_cut-off below 200 nm.

Conclusions

We report the successful synthesis and growth of a novel NLO crystal, NaPO₃NH₃, utilizing the newly developed polar NLO-active (PO₃NH₃)⁻ unit. Structural analysis reveals that these polar (PO₃NH₃)⁻ units achieve complete alignment along the c-axis, resulting in a uniform arrangement of polar P–N bonds. This unique alignment facilitates the superposition of dipoles along the c-axis, thereby maximizing macroscopic polarization. The resultant effect is an enhanced SHG response and a significantly amplified (010) in-plane anisotropy. Furthermore, NaPO₃NH₃ demonstrates a DUV absorption edge, indicating its potential as a promising DUV NLO crystal. This study not only introduces a novel NLO phosphate anion group but also proposes a new strategy for obtaining NLO crystals with large birefringence.

Author contributions

Prof. Min Luo provided research ideas, supervised the entire research, and revised the manuscript. Lingli Wu wrote the initial draft and completed the entire experiment and measurement work. Haotian Tian assisted in the experiments and measurements. Dr Chengshen Lin and Xin Zhao were responsible for theoretical calculations. Dr Huixin Fan, Pengxiang Dong, Dr Shunda Yang, and Prof. Ning Ye offered help with data and experiment analysis. All authors participated in the discussion.

Conflicts of interest

The authors declare no competing financial interests.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No. 22222510, 21975255, 21921001, and 52302008), the Natural Science Foundation of Fujian Province (2023J02026), the Foundation of Fujian Science & Technology Innovation Laboratory (2021ZR202), and the Self-deployment Project Research Program of Haixi Institutes, Chinese Academy of Sciences (CXZX-2022-JQ01).

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Footnote

† Electronic supplementary information (ESI) available: Crystallographic data and structure date (Tables S1–S5), Dipole moment (Table S6), Comparative table of phosphates (Table S7), and Figures about the measurement results and detail of NaPO₃NH₃ (Fig. S1–S7). CCDC 2307797 for NaPO₃NH₃. See DOI: https://doi.org/10.1039/d3qi02465b

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