Birefringent crystals are indispensable for polarization manipulation and phase-matching in advanced photonic and optical technologies. However, the development of short-wavelength ultraviolet (SWUV) birefringent materials remains challenging, attributed to the intrinsic conflict between wide bandgaps and strong optical anisotropy. Herein, we propose a density-compensated polarizability anisotropy strategy to overcome this limitation. The linear cyanate anion, featuring a large HOMO-LUMO gap and a small van der Waals volume, is identified as an ideal functional unit and combined with d10 transition-metal cationic anchors to construct four new inorganic birefringent crystals with SWUV transparency: K[Hg(NCO)2]Cl (1), K[Cd(NCO)3] (2), K2[Hg(NCO)4] (3), and K2[Cd(NCO)4] (4). Compounds 1-4 exhibit SWUV cutoff edges at 254 (1), 228 (2), 253 (3), and 229 nm (4), together with large birefringence values of 0.273 (1), 0.329 (2), 0.524 (3), and 0.511 (4) at 546 nm. The enhanced birefringence originates from the increased density and optimized alignment of linear (NCO)- groups within the crystal lattices. Notably, the small van der Waals volume of (NCO)- enables exceptionally high group densities and well-aligned arrangements in compounds 3 and 4, endowing them with record-high birefringence among all reported SWUV inorganic birefringent crystals. This work demonstrates the strong potential of linear cyanate groups for SWUV birefringent applications and establishes a general density-compensated polarizability anisotropy strategy based on linear functional units, providing a new paradigm for the rational design of high-performance inorganic birefringent crystals that simultaneously achieve large birefringence and wide bandgaps.

Developing bulk nonlinear optical (NLO) crystals that simultaneously combine outstanding optical performance with excellent crystal growth capability remains a major challenge. Here, we introduce an intrinsic coordination asymmetry strategy that exploits highly polarizable tetrahedral Hg-S coordination and conformationally flexible ligands to amplify local structural distortions within discrete homoanionic modules. Guided by this concept, Cc-(NH4)2Hg(SCN)4 (Cc-AHT) and its C2/c polymorph were synthesized. Structural and theoretical investigations reveal that the flexible (SCN)ligands propagate distortions originating in the [HgS4] core throughout the [Hg(SCN)4]2-module, establishing an atomic-shell distortion amplification model that quantitatively traces the evolution of local coordination asymmetry. Cc-AHT exhibits a giant second-harmonic generation response (14 × KH2PO4 and 1.2 × KTiOPO4 at 1064 nm), a band gap of 3.25 eV, and large birefringence (Δn = 0.341 at 1064 nm), while centimeter-sized single crystals can be readily grown under mild conditions. This work establishes intrinsic coordination asymmetry and its atomic-shell amplification as a chemically guided strategy for designing high-performance homoanionic NLO materials and provides a general framework for understanding how local structural distortions propagate through coordination modules containing conformationally flexible ligands.

Structural alteration based on existing molecules is an effective way to study new substances. However, a limited control range results, especially when the band gaps are close together, from the difficulty of achieving high optical anisotropic enhancement in compounds with the same chemical formula as comparable behaviors of cations and anions that have the same organic and inorganic characteristics. This article presents a methodical investigation of the highly versatile hydroxyfluorooxoborate family formulated A(2)[B3O3F4(OH)] (A = monovalent cations), which coordinates various inorganic and organic cations with the identical heteroanionic anion of [B3O3F4(OH)]. We found more than 10 times the optical anisotropy enhancement in this family and validated the anion-anion and tethered-heteroanion interaction for this highly tunable optical anisotropy. Furthermore, it is noted that the recently discovered new member [CN4H7](2)[B3O3F4(OH)] achieved a well-balanced optical property with a short deep-ultraviolet cutoff edge and strong birefringence. Our results confirm that it is feasible to achieve highly tunable optical anisotropy in compounds with identical molecular formulas.

Metal borates are excellent source materials for exploring short-wavelength nonlinear optical (NLO) crystals. Galloborates show rich structural chemistry with various coordination configurations of Ga cation and B-O anionic units and are suitable candidates as ultraviolet NLO crystals. Up to now, the shortest cut-off edge of galloborates was reported to be down to 190 nm in KCs2Ga(B5O10)(OH), while the largest second harmonic generation (SHG) effect of galloborates was reported to be up to 4.6 times that of KH2PO4 (KDP) in Na5Ga[B7O12(OH)]22B(OH)3. Herein, we give a detailed summary of the recent progress in NLO inorganic galloborates, where these galloborates are grouped into two types in terms of their compositions: (1) alkali/alkaline earth metal galloborates and (2) alkali/alkaline earth metal galloborate halides. We discuss their structural features, band gaps, and SHG intensities. Finally, we give future perspectives in this field. Noncentrosymmetric galloborates show rich structural chemistry with various B-O anionic units and the [GaO4] tetrahedron and are suitable candidates as ultraviolet nonlinear optical crystals.

Second-harmonic generation (SHG) is a fundamental optical property of nonlinear optical (NLO) crystals. Thus far, it has proved difficult to engineer large SHG responses, particularly in the mid-infrared region, owing to the difficulty in simultaneously controlling the arrangement and density of functional NLO-active units. Herein, a new assembly strategy employing functional modules only, and aimed at maximizing the density and optimizing the spatial arrangement of highly efficient functional modules, has been applied to the preparation of NLO crystals, affording the van der Waals crystal MoO2 Cl2 . This exhibits the strongest powder SHG response (2.1×KTiOPO4 (KTP) @ 2100 nm) for a transition-metal oxyhalide, a wide optical transparency window, and a sufficient birefringence. MoO2 Cl2 is the first SHG-active transition-metal oxyhalide effective in the infrared region. Theoretical studies and crystal structure analysis suggest that the densely packed, optimally-aligned [MoO4 Cl2 ] modules within the two-dimensional van der Waals layers are responsible for the giant SHG response.

Borate crystals can be chemically and functionally modified by the fluorination strategy, which encourages the identification of emerging fluorooxoborates with a structure and set of characteristics not seen in any other oxide parents. However, the bulk of fluorooxoborates have been found accidentally, rational methods of synthesis are required, particularly for the infrequently occurring poly-fluorinated components. Herein, we reported the use of bifluoride salts as a potent source of fluorine to prepare fluorooxoborates that contain rarely tri-fluorinated [BF3 X] (X=O and CH3 ) tetrahedra and eleven compounds were found. We identified the optical properties of the organofluorinated group [CH3 BF3 ] and their potential for nonlinear optics for the first time. Among these, two non-centrosymmetric components hold potential for the production of 266 nm harmonic coherent light for nonlinear optics, and more crucially, have the benefit of growing large size single crystals. Our study establishes experimental conditions for the coexistence of the diverse functional groups, enabling the production of poly-fluorinated optical crystals.

Borates provide an excellent platform for investigating the optical nonlinearity and linearity of crystals as photoelectric functional materials. In our work, borate derivatives with isolated [B3O3] six-membered rings as structural features are the preferred system due to their simple functional units and excellent properties. Herein, by utilizing the target-oriented synthesis, a series of borate derivatives, A2[B3O3F4(OH)] (A= NH4, Rb, Cs) (ABOFH), K2.3Cs0.7B3O3F6 (KCsBOF), and Cs3[B3O3(OH)3]Cl3 (CsBOHCl), with novel heteroanionic groups containing [BOxF4-x] (x = 0-3) and/or [BO2(OH)] units were obtained. ABOFH, KCsBOF, and CsBOHCl construct different two-dimensional pesudolayers featuring [B3O3F4(OH)], [B3O3F6], and [B3O3(OH)3] units, respectively. Also, the optical properties and the arrangement information of these anionic groups were studied. Among the total five compounds, (NH4)2[B3O3F4(OH)] and Cs3[B3O3(OH)3]Cl3 with enlarged birefringence and sufficient band gaps were screened out as promising birefringent crystals due to the optimally aligned configuration of birefringence-active heteroanionic units. The successful results of target-oriented synthesis indicate a more profound conclusion that the borate system now has more diversified structural chemistry, and an effective strategy was proposed to modify the arrangement and species of anionic units to optimize the performance of optical crystals.

Discovery of crystals functionalized by optically active units is of great significance to laser science and industry since these species have the potential for frequency conversion and light modulation. This spurred researchers on to find new optical crystals with such functionality, especially in new unexplored systems. Herein, inspired by the idea of introducing different optically active units into one structure, difluoro(oxalato)borate was defined as a new source of short-wavelength ultraviolet (UV) optical crystals. Based on this, three difluoro(oxalato)borates with [BF2C2O4] bifunctional units were synthesized and characterized. Among them, NaBF2C2O4 shows a large nonlinear optical effect (1.1 x KDP), a wide band gap (E-g = 4.48 eV), and a large birefringence (delta n(exp.) = 0.162@546.1 nm), making it a potential short-wavelength UV nonlinear optical crystal. Among them, LiBF2C2O4 and Sr(BF2C2O4)(BF4), with giant birefringence (delta n(cal.) = 0.317 and 0.130@546 nm) as well as wide band gaps (5.54 and 4.76 eV), were characterized as short-wavelength UV birefringent crystals. Theoretical analysis clarifies that [BF2C2O4] constructed by both pi- and non-pi-conjugated units shows balanced microscopic properties and that it can be regarded as bifunctional units to cause a considerable nonlinear optical effect and birefringence simultaneously. Our study confirms the feasibility of exploring optical crystals with balanced performance in the difluoro(oxalato)borate system.

A new hybrid fluorooxoborate was discovered following the strategy of combining two birefringence-active units. The large birefringence and short absorption edge make it a potential short-wavelength ultraviolet birefringent crystal.

Polarization modulation of deep-UV light is of significance to current technologies, and to this end, the birefringent crystal has emerged as an invaluable material as it allows for effective light modulation. Herein, a double-modification strategy driven by F and OH anions that makes double effects towards the critical property enhancement of deep-UV birefringent crystals is proposed. This leads to a new hydroxyborate (NH4 )4 [B12 O16 F4 (OH)4 ] with giant cluster as a deep-UV birefringent crystal with large birefringence (Δnexp. =0.12@546.1 nm). This birefringence is a record among inorganic hydroxyborates with experimentally measured birefringence. Structural analysis shows that the near-plane arrangement of [B12 O16 F4 (OH)4 ] cluster is responsible for the large optical anisotropy. Theoretical calculations indicate that its π-conjugated [BO3 ] and [BO2 OH] units are the main source of this large optical anisotropy.

Discovery of new efficient nonlinear optical (NLO) materials with large second-order nonlinearity for the short-wave ultraviolet spectral region (λPM ≤266 nm, PM=phase-matching) is still very challenging. Herein, a new beryllium-free borate CaZn2 (BO3 )2 with Sr2 Be2 B2 O7 (SBBO) double-layered like configuration was rationally designed, which not only preserves the structural merits but also eliminates the limitations of the SBBO crystal. CaZn2 (BO3 )2 shows a large PM second harmonic generation (SHG) reponse of 3.8×KDP, which is 38 times higher than that of its barium analogue. This enhancement mainly originates from the 1 [Zn2 O6 ]∞ polar chains with a large net dipole moment and [BO3 ] units with a high NLO active density. Our findings show the great significance of the [ZnO4 ] tetrahedra introduced strategy to design beryllium-free SBBO-type NLO crystals and also verify the feasibility of using simple non-isomorphic substitution to induce giant second-order nonlinearity enhancement.

Metal-free borates have emerged as a new class of solid-state chemistry and related crystalline materials with high performance. However, the discovery of metal-free borate crystals that can meet the phase-matching behavior in the short-wavelength ultraviolet (UV) spectral region is extremely limited. Against this background, two new guanidinium fluorooxoborates with the chemical formula of [C(NH2)(3)][B3O3F2(OH)(2)] and [C(NH2)(3)](2)[B3O3F4(OH)] have been discovered and characterized as high-performance candidates for short-wavelength UV nonlinear optical (NLO) applications. The optimally aligned configuration of coplanar [C(NH2)(3)] cations and [B3O3F2(OH)(2)]/[B3O3F4(OH)] heteroanionic units makes both crystals have a sufficiently large NLO coefficient, birefringence, and band gap, which indicate that both metal-free hydroxyfluorooxoborates are promising NLO crystals. The well-ordered configurations of OH/F anions in two borates were identified using various approaches, and the origin of high optical performance was validated by additional measurements and calculations. Our findings verified the feasibility of searching NLO crystals in the short-wavelength region in a metal-free borate system via an eco-friendly and low-cost way.

A case of hydrous borate with the formula of Ba2B13O19(OH)(5)center dot 5H(2)O was synthesized by the traditional hydrothermal method. It crystallizes in the chiral P1 space group and has a unique two-dimensional (2)[B13O19(OH)(5)](infinity) layer that is constructed by the hydrogen involved [B13O22(OH)(5)] fundamental building blocks (FBBs). The Ba atoms and H2O molecules are filled in the three-dimensional pore constructed by the (2)[B13O19(OH)(5)](infinity) layer. Interestingly, Ba2B13O19(OH)(5)center dot 5H(2)O is the first case of hydrous borates that has the above rare [B13O22(OH)(5)] FBB and (2)[B13O19(OH)(5)](infinity) layer, which enriching the structural chemistry of borate system. The diffuse reflection spectrum result shows that the ultraviolet (UV) cutoff edge of Ba2B13O19(OH)(5)center dot 5H(2)O is below 200 nm. Its second harmonic generation (SHG) response is approximately 1.0 times that of potassium dihydrogen phosphate at the 1064 nm fundamental wavelength and it is also phase-matchable. Both results indicate that the title compound is a nonlinear optical crystal for UV spectral region. Theoretical simulation calculation shows that the occupied VE states are mainly contributed by the O-2p orbitals, and therefore [B13O22(OH)(5)] groups are the main sources of SHG effects. (C) 2021 Elsevier B.V. All rights reserved.

Although phosphates are a rich source of deep-ultraviolet optical materials, the realization of large optical anisotropy in them still remains a challenge because of the small polarizability anisotropy of [PO4] units. Inspired by the fluoridation strategy and hydrogen bond interaction, a new metal-free monofluorophosphate, (N2H6)[HPO3F]2, was synthesized, which exhibits a large birefringence (cal. 0.077) and wide band gap (∼6.51 eV). Such a large birefringence in (N2H6)[HPO3F]2 sets a new record among available fluorophosphates, and the [HPO3F] unit is theoretically confirmed to be a new birefringence-active unit.

A new borate exhibiting the unprecedented combination of the largest two highly polymerized B–O clusters with novel B9O18 FBB was obtained. And the corresponding design strategies for borates with two polymerized B–O clusters are proposed.

Three new tellurates, namely, K2BaPb0.62Ba0.38Te2O9 (1), Rb2BaPb0.7Ba0.3Te2O9 (2), and Ba2KLiTe2O9 (3), with hexagonal triple-perovskite structures have been designed via a cation substitution strategy. All three centrosymmetric compounds crystallize in the same hexagonal space group P6(3)/mmc. In their structures, the [BTe2O9] (B = Pb/Ba for 1 and 2 and Li for 3) groups construct the final three-dimensional framework using face-sharing [Te2O9] dimmers and vertex-linked [BO6] linkages. Meanwhile, the theoretical calculations demonstrate the numerical differences of their birefringence. Thermal stability analysis and differential scanning calorimetry were performed, and UV-vis-NIR diffuse reflectance spectra and infrared spectra of the three title compounds were also characterized and analyzed.

Maximizing the optical anisotropy in birefringent materials has emerged as an efficient route for modulating the polarization-dependent light propagation. Currently, the generation of deep-ultraviolet (deep-UV) polarized light below 200 nm is essential but challenging due to the interdisciplinary significance and insufficiency of high-performing birefringent crystals. Herein, by introducing multiple heteroanionic units, the first sodium difluorodihydroxytriborate-boric acid Na[B3 O3 F2 (OH)2 ]⋅[B(OH)3 ] has been characterized as a novel deep-UV birefringent crystal. Two rare heteroanionic units, [B3 O3 F2 (OH)2 ] and [B(OH)3 ], optimally align to induce large optical anisotropy and also the dangling bonds are eliminated with hydrogens, which results in an extremely large birefringence and band gap. The well-ordered OH/F anions in [B3 O3 F2 (OH)2 ] and [B(OH)3 ] were identified and confirmed by various approaches, and also the origin of large birefringence was theoretically discussed. These results confirm the feasibility of utilizing hydrogen involved heteroanionic units to design crystals with large birefringence, and also expand the alternative system of deep-UV birefringent crystals with new hydroxyfluorooxoborates.

A new alkali-metal iodate, K6(IO6H4)(HI2O6)(HIO3)2(IO3)4·2H2O (KIOH), was successfully grown at room temperature by a slow evaporation method. To our knowledge, the title compound is the first alkali-metal iodate containing isolated [I5+O3] and [I7+O6] units in one structure. Both the bond valence sum and X-ray photoelectron spectroscopy confirmed this phenomenon, which is consistent with the single-crystal data. Also, the theoretical calculation results showed that the title compound is a potential birefringent material. What is more, the low-cost growth of centimeter-sized crystals for the title compound greatly enriches the structural chemistry of the iodate system.

Under hydrothermal conditions, a new lanthanide coordination polymer, [Er-3(oba)(4)(na)](n) (1, oba = 4,4'-oxybis(benzoate), na = nicotinic acid) has been synthesized. Compound 1 shows a 3D framework by incorporating tri-nuclear [Er-3(COO)(6)] unit and the mixed ligands of oba(2-) and na. It is interesting that decarboxylation occurred in the ortho position and 2,3-pyridinedicarboxylic acid was partially transformed into na under hydrothermal conditions. Compound 1 can be reduced into a (3,11)-connected net and displays weak second-harmonic generation response. Furthermore, the PXRD, TGA and IR spectra were also studied.