(NH4)3B11PO19F3: a deep-UV nonlinear optical crystal with unique [B5PO10F]∞ layers

Abstract Deep-ultraviolet (DUV) nonlinear optical (NLO) crystals that can extend the output range of coherent light below 200 nm are pivotal materials for solid-state lasers. To date, KBe2BO3F2 (KBBF) is the only usable crystal that can generate DUV coherent light by direct second harmonic generation (SHG), but the layered growth habit and toxic ingredients limit its application. Herein, we report a new fluoroborophosphate, (NH4)3B11PO19F3 (ABPF), containing four different functional units: [BO3], [BO4], [BO3F] and [PO4]. ABPF exhibits a KBBF-like structure while eliminating the limitations of KBBF crystal. The unique [B5PO10F]∞ layers enhance ABPF’s performance; for example, it has a large SHG response (1.2 × KDP) and a sufficient birefringence (0.088 at 1064 nm) that enables the shortest phase-matching wavelength to reach the DUV region. Meanwhile, the introduction of strong B-O-P covalent bonds decreases the layered growth habit. These findings will enrich the structural chemistry of fluoroborophosphate and contribute to the discovery of more excellent DUV NLO crystals.


INTRODUCTION
Deep-ultraviolet (DUV) nonlinear optical (NLO) materials can expand the frequency range of allsolid-state lasers through cascaded second harmonic generation (SHG), which has important applications in lithography, semiconductor manufacturing and many other fields [1,2]. There are at least three basic requirements for DUV NLO materials: a large NLO coefficient (d ij > 0.39 pm V −1 ) to improve laser conversion efficiency; a short cutoff edge in the DUV region (λ cutoff ≤ 200 nm); and suitable birefringence ( n: 0.05-0.10) to meet the phasematching (PM) condition in the DUV region [3,4]. These mutually constraining indicators (d ij , λ cutoff and n) are mainly determined by the electronic structures and microscopic properties (hyperpolarizability, highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gap and polarizability anisotropy) of the anionic groups and their arrangements [5][6][7]. Unfortunately, a single anionic group can barely balance all three conditions due to the intrinsic limitations.
Traditionally, the exploration of UV NLO crystals is mainly focused on borate and phosphate C The Author(s) 2022. Published by Oxford University Press on behalf of China Science Publishing & Media Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited. systems, and typical crystals include LiB 3 O 5 (LBO), β-BaB 2 O 4 (BBO) and KH 2 PO 4 (KDP) [20][21][22][23]. Borophosphate, as a mixed-anionic system, is also a source of NLO crystals, and BPO 4 (BPO) and MBPO 5 (M = Sr, Ba) have been reported as NLO crystals with excellent properties [8,24]. Recently, we proposed a 'fluorination strategy' by substituting fluorine for oxygen atoms in borates to regulate the structure of NLO crystals, so as to achieve the balance of the three parameters mentioned above (i.e. d ij , λ cutoff and n) [5,6,25]. Also, this strategy was further extended to the phosphate system. Consequently [26][27][28][29][30][31][32][33][34]. However, fluoroborophosphate, as a system with even more functional anionic groups, has been left behind. To date, only 14 cases of fluoroborophosphates (organic-inorganic hybrids and mineral compounds are not included in the statistical data here) have been reported and deposited in the international inorganic crystal structure database (ICSD) [35][36][37][38][39][40][41][42][43]. As shown in Supplementary  4 ]-was successfully designed and synthesized. Fascinatingly, ABPF exhibits a new type of KBBF-like structure with unique [B 5 PO 10 F] ∞ layers connected by shared oxygen atoms forming the final 3D framework. It inherits the excellent properties of KBBF, such as a wide transparency range, a large SHG response and a suitable birefringence to satisfy the DUV PM condition. Beyond these, ABPF has a non-layered growth habit and is chemically benign. These properties make ABPF a promising DUV NLO crystal. In addition, the contributions of multiple anionic groups to the linear and NLO properties of ABPF were confirmed by the first-principles calculations. Our results highlight the synergistic effect of multiple anionic groups on the design of DUV NLO materials and open up new possibilities for exploring DUV NLO materials in fluoroborophosphates.

RESULTS AND DISCUSSION
Polycrystalline samples of ABPF were synthesized via the high-temperature solution method in a closed system, and the photograph of ABPF crystals is shown in Supplementary Fig. 1. Crystallographic data are contained in CCDC 2153289 in crystallographic information file format. The purity of the phase was checked by powder X-ray diffraction (XRD; see Supplementary Fig. 2). The results of thermogravimetric analysis-differential scanning calorimetry (TG-DSC) curves and powder XRD patterns show that ABPF begins to decompose after 180 • C, and BPO 4 was found in decomposition products (see Supplementary Figs 2 and 3). The constituent elements and the anion units are further confirmed by elemental analysis and infrared (IR) spectroscopy (see Supplementary Figs 4

and 5).
ABPF crystallizes in the trigonal space group R3 (see Supplementary Table 2), and the basic structure is shown in Fig. 1 (Fig. 1a). Three FBBs are closed to form a large 18membered ring (MR), and further polymerized to unprecedented 2D [B 5 PO 10 F] ∞ layers extending in the ab-plane (Fig. 1b). Amazingly, similar layers with 18-MRs were also found in NH 4 B 4 O 6 F (ABF), and play important roles in its excellent NLO properties [26] (see Supplementary Fig. 6). Different from the 2D [B 4 O 6 F] ∞ layers in the structure of ABF, the layers in ABPF are further connected by shared oxygen atoms of [BO 4 ] and [PO 4 ] tetrahedra, stacking along the c-direction to form a 3D framework. Also, the interlayer spacing of ABPF is 3.97Å, less than that of KBBF (6.25Å), and NH 4 + cations are filled in the interlayer (Fig. 1c and d) The interference pattern of polarized light indicates that ABPF is a uniaxial crystal (Fig. 2a). The transmittance spectrum demonstrates that its UV cutoff edge is 183 nm (the corresponding band gap is 6.78 eV), indicating that ABPF has a wide DUV transparency window (Fig. 2b). Based on the charge-transfer model and Mulliken analysis [44,45], the bond valence of O atoms is in the range of 1.7-2.0 e (see Supplementary Fig. 7), which confirms that the introduction of non-π -conjugated [BO 4 ] and [BO 3 F] units are beneficial to the partial elimination of the dangling bond, thus obtaining a DUV transparency. So, the short UV cutoff edge of ABPF is mainly attributed to the large HOMO-LUMO gaps of its microscopic anionic groups [5,6] Moreover, the SHG capabilities of ABPF were measured by the Kurtz-Perry method [46] under incident laser 1064 and 532 nm, respectively. Two standard NLO crystals, KDP and BBO, were used as the references. The output SHG response of ABPF is 1.2 × KDP at 1064 nm and 0.2 × BBO at 532 nm in the 200-250 μm particle size range, respectively ( Fig. 2c and d).
To further explore the structure-property relationship of ABPF, electronic structures and optical proprieties were calculated by the first-principles calculations based on density functional theory (DFT). The direct band gap of ABPF under a generalized gradient approximation (GGA) framework is 5.96 eV (see Supplementary Fig. 8), which is slightly smaller than the experimental value of 6.78 eV due to the discontinuity of exchange-correlation energy functional. To keep the band gap consistent with the realistic condition, a scissors operation (0.  [47,48]. The largest tensor, d 11 , was analyzed by the SHG-density method to understand the contribution of NLO-active electron states and units. It shows that the virtual electron (VE) process is dominant in the SHG process, and the contributions of occupied states are mainly determined by the nonbonding O-2p and F-2p, while unoccupied states are mainly determined by the orbitals of B-2p, N-2p, O-2p and F-2p ( Fig. 3c and d). In fact, the orbitals of non-centrosymmetric sublattices near the top of valence bands from [BO 3 ] are mainly responsible for the SHG effect [49]. Meanwhile, the contribution origins of the SHG response were analyzed by the real-space atom-cutting method [48], and the result indicates that [BO 3 ] units contribute most to the SHG response, while other units contribute relatively little (see Supplementary Table 9).
Suitable birefringence ( n) and mild dispersion are essential for realizing the PM conditions that foster a practical DUV laser output. The birefringence and PM wavelength were calculated by first-principles calculations based on DFT. To the best of our knowledge, ABPF has the largest birefringence of 0.088 at 1064 nm among all fluoroborophosphates reported so far (Table S1)  differences ( ρ), based on the response electron distribution anisotropy (REDA) [50] (Fig. 3b), which suggests that ABPF has potential applications in the DUV field.

CONCLUSION
In conclusion, a new type of KBBF-like compound, ABPF, with four different units, has been successfully obtained, and the synergistic effect of πconjugated units and non-π -conjugated units means it exhibits excellent optical properties, namely, the highest NLO coefficients, the largest birefringence and the shortest PM SHG limit among all fluoroborophosphates. Owing to a beryllium-free, nolayered growth habit, and excellent optical properties, ABPF has a promising future as DUV NLO crystal. Moreover, we propose that the introduction of strong covalent bonds between layers can enhance the interlayer interaction force while simultaneously maintaining the large optical anisotropy of layered structures. More importantly, the emergence of ABPF once again proves the advancement of the 'fluorination strategy' in the DUV NLO field. These findings will facilitate the discovery of more DUV NLO materials with optimal and practical performance.

Synthesis
Crystals were obtained via the high-temperature solution method in a closed system. NH 4 PF 6 (95%, Aladdin), NH 4

Characterizations
Powder XRD data were collected using a Bruker D2 PHASER diffractometer at room temperature. The single-crystal XRD data were collected using a Bruker D8 Venture diffractometer and the crystal structure was solved using Olex2. The interference pattern of polarized light was measured using a polarizing microscope (ZEISS Axioscope 5). TG-DSC were measured on a simultaneous NETZSCH STA 449 F3 thermal analyzer instrument under a flowing N 2 atmosphere. The sample was placed in a Pt crucible and heated from 40 to 800 • C at a rate of 5 • C min −1 . Elemental analysis was analyzed on the single crystal surface by a field emission scanning electron microscope (SEM, SUPRA 55VP) equipped with an energy dispersive X-ray spectroscope (EDX, BRUKER x-flash-sdd-5010). IR spectroscopy was measured by Shimadzu IR Affinity-1 Fourier transform infrared spectrometer. The transmittance measurement of a transparent crystal was measured by Shimadzu SolidSpec-3700DUV spectrophotometer under a flowing N 2 atmosphere. Powder SHG intensity was measured via the Kurtz-Perry method using a Q-switched Nd: YVO 4 solidstate laser (Cnilaser, DPS-1064-Q) at 1064 nm and 532 nm, for visible and UV SHG, respectively. Polycrystalline samples were ground and sieved into the following particle size ranges: 38-55, 55-88, 88-105, 105-150, 150-200 and 200-250 μm. The samples were loaded into a 1-mm-thick aluminum holder with an 8-mm-diameter hole. The sieved KDP and β-BBO samples were used as references.

SUPPLEMENTARY DATA
Supplementary data are available at NSR online.