A tri-alkali/alkaline-earth metal fluorophosphate deep-ultraviolet birefringent crystal with coexisting PO₃F and PO₂F₂

Abstract

Deep-ultraviolet (deep-UV) birefringent crystals capable of generating or modulating deep-UV polarized lights are essential for modern laser technologies. However, there are few commercial deep-UV birefringent crystals, except MgF₂, which exhibit a tiny birefringence of 0.012@532 nm. Herein, by incorporating multiple alkali/alkaline-earth metal elements, the first tri-alkali/alkaline-earth metal fluorophosphate KBaSr(PO₂F₂)(PO₃F)₂ (KBSPF) with the coexistence of (PO₃F)²⁻ and (PO₂F₂)⁻ has been successfully synthesized using the hydrothermal method. Experimental results indicate that KBSPF not only shows a wide transparency window down to the deep-UV spectral region, but also exhibits a relatively large birefringence of 0.042@550 nm, exceeding those of commercial MgF₂ and most alkali/alkaline-earth metal phosphates or fluorophosphates. Theoretical calculations reveal that (PO₃F)²⁻ and (PO₂F₂)⁻ anions are responsible for the optical properties of KBSPF. Our work not only provides a promising deep-UV birefringent crystal but also offers new insights to enrich the solid-state chemistry of fluorophosphates.

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Introduction

Deep-ultraviolet (deep-UV) birefringent crystals play a crucial role in advancing optical technologies, such as photoemission spectroscopy, laser micromachining, and high-density storage data, by enabling the manipulation of light at wavelengths below 200 nm.^1–3 From an application perspective, a large birefringence is highly desirable, as it allows for more compact and efficient optical devices. To date, commercially available deep-UV birefringent crystals remain extremely scarce, as most crystals with sufficient birefringence are opaque in the deep-UV spectral region. Although MgF₂, the most widely used deep-UV birefringent crystal, exhibits a wide transparency window, it has a tiny birefringence of 0.012@532 nm.⁴ Therefore, it is of urgent demand and has importance in developing novel excellent deep-UV birefringent crystals with enhanced birefringence.

In order to achieve large birefringence, researchers have proposed several effective strategies, including introducing planar π-conjugated units,^5,6 metal cations with stereochemically active lone pairs,^7,8 d⁰ metal cations exhibiting second-order Jahn–Teller distortions,⁹ and d¹⁰ transition metals.^10,11 However, metal cations with lone pairs or transition metals always narrow the band gap, reducing their suitability for deep-UV applications. In addition, π–π interactions in planar π-conjugated units, such as (BO₃)³⁻, (CO₃)²⁻, and (NO₃)⁻, lead to significantly lower band gaps compared to non-π-conjugated units like (BO₄)⁵⁻, (PO₄)³⁻, and (SO₄)²⁻.¹² As a result, non-π-conjugated units are often preferred as the building blocks for deep-UV optical crystals.^12,13 However, non-π-conjugated units inherently exhibit drawbacks of low anisotropy, which limits their ability to achieve large birefringence, as seen in KLa(PO₃)₄ (Δn = 0.0084@1064 nm),¹⁴ Rb₂Ba₃(P₂O₇)₂ (Δn = 0.002@1064 nm),^15,16 and RbNaMgP₂O₇ (Δn = 0.0034@1064 nm).¹⁶ To overcome low-anisotropy constraints, it has been experimentally demonstrated that fluorine-induced structural distortion in regular tetrahedral frameworks can enhance anisotropic polarizability via electron density redistribution. Compared to (BO₄)⁵⁻, (PO₄)³⁻, and (SO₄)²⁻ units, the resultant tetrahedral groups like (BO_xF_4−x) (x = 1, 2, 3), (PO_xF_4−x) (x = 1, 2, 3), and (SO₃F)⁻ exhibit greater anisotropic polarizability.^17–19 The corresponding crystals also exhibit enhanced birefringence, such as KLa(PO₂F₂)₄ (Δn = 0.019@1064 nm),²⁰ Cs₂PO₃F (Δn = 0.017@1064 nm),²¹ and Ba(PO₂F₂) (Δn = 0.011@1064 nm).²²

It is well-known that alkali/alkaline-earth metals are beneficial for deep-UV transparency due to their electronic configuration lacking unfavourable d–d or f–f transition. In this work, the first tri-alkali/alkaline-earth metal fluorophosphate, KBaSr(PO₂F₂)(PO₃F)₂ (KBSPF), which coexists with (PO₃F)²⁻ and (PO₂F₂)⁻, has been successfully synthesized via the hydrothermal method. Fluorine substitution is beneficial for bandgap widening. In addition, the coexistence of functional anionic groups enhances microscopic anisotropic polarizability, consequently leading to improved birefringence. KBSPF not only demonstrates a wide transparency window down to the deep-UV spectral region, but also exhibits a significant birefringence of 0.042@550 nm.

Results and discussion

Polycrystalline samples of KBSPF were synthesized using the modified hydrothermal method. Single-crystal X-ray diffraction (XRD) analysis reveals that KBSPF crystallizes in the centrosymmetric space group P2₁/c (No. 14) with a = 7.0638(1) Å, b = 7.4306(1) Å, c = 21.3633(3) Å, β = 93.386(1)°, V = 1119.37(3) ų, and Z = 4, and detailed crystallographic information is provided in Tables S1–S5.† Within one asymmetric unit, there are one crystallographically unique K atom, one Ba atom, one Sr atom, three P atoms, four F atoms, and eight O atoms. Among them, the P(1) and P(2) atoms are tetrahedrally bonded to three O atoms and one F atom, forming distorted (PO₃F)²⁻ tetrahedra. The other P(3) atom is surrounded by two O atoms and two F atoms, generating the (PO₂F₂)⁻ tetrahedra. P–O bonds and P–F bonds range from 1.521(2) to 1.586(2) Å and from 1.504(2) to 1.556(2) Å, respectively. As shown in Fig. 1a, K⁺, Sr²⁺, and Ba²⁺ cations are eight-, eight-, and ten-coordinated, forming KO₅F₃, SrO₅F₃ and BaO₅F₅ cationic polyhedra with bond lengths in the ranges of 2.701(2) to 3.026(2) Å, 2.731(2) to 2.932(2) Å and 2.708(2) to 3.013(2) Å. According to previous reports, all these bond lengths are consistent with previously reported values.^18,22,23 The KO₅F₃, BaO₅F₅ and SrO₅F₃ cationic polyhedra are further interconnected by isolated (PO₃F)²⁻ and (PO₂F₂)⁻ anionic tetrahedra together via corners, edges, and faces, constructing the overall three-dimensional framework with channels (Fig. 1b).

Fig. 1 The crystal structure and elemental characterization of KBSPF. (a) The coordination of K, Ba and Sr. (b) The structure of KBSPF viewed along the a axis. (c) Elemental mapping by scanning electron microscopy. (d) ¹⁹F NMR spectrum. (e) ¹⁹F–³¹P solid-state cross-polarization NMR spectrum.

The phase purity of polycrystalline KBSPF was confirmed by the high consistency between the measured and simulated powder XRD patterns, as shown in Fig. S1.† Energy dispersive X-ray spectroscopy (EDS) mapping verified the presence of K, Ba, Sr, P, O, and F elements in a single crystal, with these elements being evenly distributed throughout the crystal (Fig. 1c and S2†). To examine the stability in air, KBSPF was exposed to air for one week. As displayed in Fig. S3† the surface of the exposed crystal is nearly unchanged. Thermogravimetric and differential thermal analyses were performed to evaluate the thermal stability of KBSPF and exclude the possibility of hydroxyl groups (Fig. S4†). KBSPF remains stable up to approximately 573 K, beyond which it undergoes continuous weight loss. To confirm the transmission wavelength range of KBSPF, UV/Vis/NIR diffuse reflectance spectra were recorded from 200 to 800 nm. As demonstrated in Fig. S5,† the absorption edge of KBSPF is below 200 nm.

The infrared spectrum of KBSPF is shown in Fig. S6,† and the assignment of related absorption peaks helps identify the corresponding functional units and chemical bonds. The peak at 869 cm⁻¹ corresponds to the stretching vibration of the P–F bond. The peak at 496 cm⁻¹ corresponds to the vibrational bending of the (PO₂F₂)⁻ group.^20,22,24 The peak at 430 cm⁻¹ corresponds to the vibrational bending of the (PO₃F)²⁻ group.^25,26 To further verify the coexistence of (PO₃F)²⁻ and (PO₂F₂)⁻ anions, we measured the ¹⁹F solid-state nuclear magnetic resonance (NMR) spectrum and ¹⁹F–³¹P solid-state cross-polarization magic angle spinning (MAS) NMR spectrum. The ¹⁹F MAS NMR spectrum shows exactly four resonances at δ = −10.91, −61.95, −88.53, and −96.58 ppm (Fig. 1d), which are attributed to the F signals from four different coordination environments.¹⁸ Moreover, the ¹⁹F–³¹P solid-state cross-polarization MAS spectrum reveals two resonances at δ = −0.41 and −2.14 ppm (Fig. 1e), confirming the presence of two distinct P–F bonds in (PO₃F)²⁻ and (PO₂F₂)⁻ anions.²⁶

Given that KBSPF crystallizes in a non-cubic space group, it is expected to be birefringence-active. The experimental birefringence of KBSPF was investigated using a high-quality, colorless crystal plate under an orthogonally polarized microscope. The setup was equipped with a charge-coupled device (CCD) camera to capture transmitted images, as demonstrated in Fig. 2a. Fig. 2b presents transmitted images of the KBSPF crystal under orthogonally polarized light. As the tested single crystal was rotated, the brightness of the transmitted image changed periodically every 45°. This behaviour arises because when the incident polarized light is parallel to the fast or slow axis of the crystal, its polarization state remains perpendicular to the polarizer, resulting in the darkest image. In contrast, when the incident light is oriented at an angle of 45° to the fast or slow axis, the polarized optical images display the greatest brightness of the crystal. These observations confirm that KBSPF has birefringence to modulate the polarization of light.

Fig. 2 Birefringence properties of KBSPF. (a) The schematic diagram illustrates the birefringence measurement using a polarizing microscope. (b) Polarized optical observations of a KBSPF crystal. The angle φ at which the KBSPF crystal achieves extinction is defined as 0°. (c) Original interference of the KBSPF crystal under orthogonally polarized light. (d) The corresponding crystal realizing complete extinction. (e) The orientation of the KBSPF plate used for measurement. (f) The thickness of the KBSPF crystal. (g) Comparison of the birefringence of KBSPF with many well-known phosphates and fluorophosphate optical crystals.

As shown in Fig. 2c and d, further analysis revealed the original interference colour of second-order purple for the tested single crystal, corresponding to an optical path difference of R = 1095.7 nm, measured using a Berek compensator. The crystal plane of the tested crystal was verified by single-crystal XRD. As depicted in Fig. 2e, the selected crystal plane is along the (001) direction. The thickness of the measured crystal was determined to be d = 27 μm, as shown in Fig. 2f. Therefore, the experimental birefringence was calculated as Δn(001)exp = 0.042@550 nm based on the birefringence formula Δn = R/d. The calculated wavelength-dependent refractive index at λ = 550 nm is shown in Fig. S7.† The calculated birefringence Δn(001)cal ≈ n[100] − n[010] for the (001) crystal plane is 0.042@550 nm (Fig. S7†), which agrees well with the experimental value (Δn(001)exp = 0.042@550 nm). It should be noted that this relatively large birefringence of KBSPF is about four times that of the commercial deep-UV birefringent crystal MgF₂ (Δn = 0.012@532 nm).²⁷ Moreover, this birefringence far exceeds those of many famous phosphates and fluorophosphate optical crystals (Fig. 2g and Table S6†).^{14–16,20–22,23,25,26,28–41}

First-principles calculations were performed to investigate the micro-origin of the optical properties of KBSPF.^42–45 Fig. 3a shows that the electronic band structure of KBSPF is direct, with a bandgap of 5.80 eV, in good agreement with the experimental result. The density of states (DOS) and partial DOS for KBSPF (Fig. 3b) indicate that O 2p orbitals predominantly occupy the top part of the valence band. Additionally, P 3p, O 2p, and F 2p orbitals are mainly positioned at the bottom of the conduction band. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) for KBSPF were also calculated. Fig. 3c and d show that the HOMO is dominated by O 2p orbitals, while the LUMO is mainly composed of P 3p, O 2p and F 2p orbitals. All these calculations indicate that (PO₃F)²⁻/(PO₂F₂)⁻ anions contribute dominantly to the optical properties of KBSPF.

Fig. 3 Theoretical calculations of KBSPF. (a) Electronic band structure. (b) DOS and partial DOS. (c) The HOMO map. (d) The LUMO map. The atoms K, Ba, Sr, P, O and F are denoted using yellow, blue, red, grey, pink, and green balls, respectively.

Conclusion

In summary, by incorporating multiple alkali and alkaline-earth metal elements, we have successfully synthesized and reported the first tri-alkali/alkaline-earth fluorophosphate, namely KBaSr(PO₂F₂)(PO₃F)₂. This compound also represents a rare example of a material, in which both (PO₃F)²⁻ and (PO₂F₂)⁻ anions coexist. KBaSr(PO₂F₂)(PO₃F)₂ demonstrates a wide transparency window down to the deep-UV spectral region and exhibits significant birefringence (0.042@550 nm), surpassing those of most alkali/alkaline-earth metal phosphates and fluorophosphates. The absence of d–d/f–f electron transitions in Ba²⁺ and Sr²⁺ cations enables an enlarged band gap. Furthermore, fluorine substitution not only further widens the band gap but also enhances the anisotropy of microscopic polarizability in the structural units, thereby amplifying the birefringence. Detailed theoretical calculations show that the (PO₃F)²⁻ and (PO₂F₂)⁻ anions contribute to the linear optical properties and ultra-wide bandgap of KBaSr(PO₂F₂)(PO₃F)₂.

Data availability

All data supporting this article have been included within the manuscript and ESI.† Crystallographic data for KBaSr(PO₂F₂)(PO₃F)₂ have been deposited at the CCDC under number 2428547. The article provides sufficient experimental evidence and data to ensure reproducibility of the work.

Conflicts of interest

The authors declare no competing interests.

Acknowledgements

This work acknowledges the financial support from the National Natural Science Foundation of China (22193042 and 22405273), the Natural Science Foundation of Fujian Province (2022J02012), the Key Research Program of Frontier Sciences of the Chinese Academy of Sciences (ZDBS-LY-SLH024), the Postdoctoral Fellowship Program of CPSF under Grant Number GZB20240747, and the China Postdoctoral Science Foundation (2023M743498).

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2428547. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d5qi00752f

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