Vacuum-ultraviolet (VUV, 100–200 nm) light sources are essential for semiconductor lithography, high-resolution spectroscopy, and emerging applications such as the 229Th nuclear clock. Their realization critically depends on nonlinear optical (NLO) materials capable of efficient frequency conversion, where phase-matching is a key requirement for VUV generation. Recent advances, including record-short birefringent-phase-matching at 158.9 nm in fluorooxoborates, full-wavelength phase-matching near 193 nm, twist-phase-matching in low-dimensional materials, and additional periodic phase engineering in conventional crystals, have significantly expanded the accessible design space for short-wavelength NLO frequency conversion. However, the relationships between crystal dimensionality, structural motifs, and phase matching mechanisms remain fragmented. In this Review, we establish a unified structure–dimension–mechanism–performance framework that connects crystal dimensionality with phase matching mechanisms and VUV performances. We systematically integrate birefringent-phase matching, quasi-phase-matching, additional periodic phase-phase-matching, and twist-phase-matching across bulk, superlattice, and low-dimensional systems. We further highlight how structural evolution and chemical modulation, particularly fluorination, enable simultaneous control of bandgap, anisotropy, and nonlinear response, thereby providing insight into current VUV phase-matching limits and pathways toward overcoming the 150 nm barrier.

Nonlinear optical (NLO) crystals operating in the short-wavelength ultraviolet (UV) region are essential for all-solid-state lasers. For such materials, a moderate birefringence represents a key requirement. Borates with planar pi-conjugated [B3O6] rings are promising candidates for short-wavelength UV NLO crystals, yet their tendency to adopt parallel alignment often leads to excessive birefringence and detrimental beam walk-off. Herein, we propose a charge-compensation strategy mediated by heterovalent cation occupation to rationally tailor the arrangement of [B3O6] units. Finally, a new noncentrosymmetric crystal that adopts a unique zigzag packing of [B3O6] groups distinct from previously reported structural types, RbBaB3O6, was designed and synthesized. This configuration yields a balanced set of optical properties: a birefringence of 0.075@532 nm, a moderate phase-matching SHG response, a short UV cutoff edge below 196 nm, and the shortest type-I phase-matching wavelength of 236 nm, confirming its potential as a viable UV NLO crystal. This work demonstrates a practical route for fine-tuning birefringence in borate-based UV NLO materials, offering a viable candidate for all-solid-state laser systems operating in the UV region.

Birefringent materials are crucial optical materials that enable the control of polarized light in optical technologies, yet achieving high birefringence in systems dominated by non-pi-conjugated units in the deep-ultraviolet (UV) region remains a significant challenge. Herein, a birefringent material, [C(NH2)(3)](2)PO3(OH), was successfully synthesized by integrating pi-conjugated [C(NH2)(3)] units and heteroleptic tetrahedra [PO3(OH)]. The short UV cutoff edge (<200 nm) and large birefringence (0.086 @ 546 nm) make [C(NH2)(3)](2)PO3(OH) a potential deep-UV birefringent crystal. This study provides the experience for the design of deep-UV optical crystals.

The development of modern science urgently needs the vital tool of deep-ultraviolet (DUV, lambda < 200nm) all-solid-state lasers, while the exploration of DUV nonlinear optical (NLO) crystals is limited by their strict application conditions. Herein, we propose a structural design strategy for developing novel DUV NLO optical crystals through synergistic assembly of fluorinated polyhedral and planar B & horbar;O groups with balanced optical properties, namely, large second-harmonic generation (SHG) response, appropriate birefringence, and wide band gap. The key to achieve consistent alignment of pi-conjugated functional groups lies in leveraging the "shearing" effect and directional polymerization capability of fluorinated polyhedra. Furthermore, based on this strategy, a series of alkali-metal fluorooxoborates Mx(NH4)(2-)xB(8)O(12)F(2) (M = K, Rb; 0 < x < 2; KABF, RABF) and CsNH4B8O12F2 (CABF) were successfully designed and synthesized. Driven by uniformly arranged [BO3F](4-) and chain-like aggregated [BO3](3-) functional groups, these compounds achieve a well-balanced optimization of strong SHG response (1.6-1.7 x KDP, 1064-532 nm; 0.4-0.5 x BBO, 532-266 nm, respectively) and the shortest type-I phase-matching wavelength (161.5-168.6 nm). The inspiring results highlight their potential as candidates for DUV NLO crystals and provide support for the structure construction strategy using a synergistic combination of fluorinated polyhedra and polymerized BO3 units.

The development of anisotropic materials, particularly within the borate family, is a key focus for achieving large birefringence in short‐wave ultraviolet optical applications. Layered structures comprising interconnected planar fluorooxoborate units have emerged as an effective structural model. Recently, the incorporation of planar cations into fluorooxoborates has proven to be an effective strategy for enhancing optical anisotropy. In previously reported structures, these cations and fluorooxoborate anions typically form independent and alternating layers to maintain charge balance. Here, we report a novel compound in which the planar [C 3 N 2 H 5 ] cation is integrated into the fluorooxoborate layer via hydrogen bonding, resulting in a unique single‐layer packing arrangement. To our knowledge, this compound represents the first instance of a structure containing a new [B 7 O 12 F 2 ] fundamental building block. Owing to the above structural innovations, the compound exhibits a remarkably high birefringence of 0.223 at 546 nm. This work establishes a new structural model wherein planar cations and fluorooxoborate anions are unified within a single layer, offering a promising strategy for enhancing the anisotropy of fluorooxoborates.

The direct correlation between chirality and second-harmonic generation (SHG) remains underexplored in nonlinear optical (NLO) materials. Herein, we report a pair of dextroisomer and enantiomer crystals, 4-nitro-d-phenylalanine nitrate, (d-NPA, P212121) and its racemate rac-4-nitro-phenylalanine nitrate (rac-NPA, P21/c). d-NPA exhibits excellent SHG performance (2.8 & times; KDP) and a large birefringence (Delta nobv. = 0.131), while rac-NPA is SHG inactive. This pair directly demonstrates that the chirality plays a decisive role in the formation of the NCS structure and SHG properties. Theoretical calculations indicate that the SHG primarily originates from the benzene rings and nitro groups in the organic component, their contribution rates are 56% and 47% respectively, and the large birefringence is closely related to the parallel alignment of the NO3- groups.

The development of robust strategies for designing high-performance ultraviolet (UV) birefringent materials has become a critical challenge. The combination of alkali metal and Ca2+ cations, which do not exhibit d-d or f-f transitions, with [NO3]- anions that possess high polarizability anisotropy results in the synthesis of three hydrated alkali metal calcium nitrates, ACa(NO3)3·2H2O (A = NH4, Rb, Cs). The title compounds were synthesized at room temperature using the water solution-evaporation technique, featuring a unique chain-like structure composed of [CaO9] polyhedra and [NO3]- units. The compounds exhibit notable optical properties, including short UV absorption edges (220-235 nm) and moderate birefringence (0.077-0.086 at 546 nm). Detailed structural investigation and theoretical analyses reveal that the birefringence is primarily due to the planar triangular arrangement of the [NO3]- groups. This research not only increases the structural diversity within inorganic materials but also provides a key approach for further studies of all-inorganic birefringent optical materials in the UV region.

Non-centrosymmetric (NCS) materials underpin numerous emerging technologies, yet their targeted design remains a formidable challenge. Here, we introduce a novel "molecular modification strategy" for rationally designing ultraviolet (UV) nonlinear optical (NLO) crystals. Based on this strategy, two glycine sulfonates [C2NO2H6][SO3CF3] (I) and [C4N2O4H11][SO3CF3] (II) were synthesized, and a transition from CS compound I to NCS compound II was successfully achieved through delicate molecular design. II achieves an optimal balance between moderate second-harmonic generation (1 & times; KDP), a short-UV cutoff edge (216 nm) and ideal birefringence (0.078@546 nm). In addition, compound II exhibits favorable crystal growth habits, resulting in a large single crystal measuring 18 & times; 4 & times; 2 mm3. By comparing the CS and NCS compounds, this work not only reveals the origin of symmetry breaking but also quantitatively demonstrates how fine-tuning the molecular structure can simultaneously optimize three core parameters-SHG, birefringence and bandgap-in one step. These findings provide valuable insights for the rational design of high-performance UV NLO materials.

By introducing the SCALP cation Sb 3+ into sulfate, (NH 4 ) 2 Sb 2 SO 4 F 6 and Na 2 Sb 2 SO 4 F 6 were synthesized. They own cutoff edge of 236 nm and 218 nm, large birefringence of 0.055 and 0.100@1064 nm, indicating potential as UV birefringent crystals.

Deep-ultraviolet (DUV) nonlinear optical (NLO) crystals are key materials for creating tunable DUV lasers for frequency conversion technologies. However, limited by the transparency range, very few crystals can satisfy this technical requirement, and the innovation of novel candidates has become a challenge for researchers. Hence, studies to date have chiefly focused on finding potential competitive candidates by changing the coordination environment through the use of highly electronegative anionic functional groups. Herein, we introduce a DUV-NLO concept by exploring a new chemical space with the [BO2] unit, and present the design of a novel NLO crystal, CsBe2B3O7, by combining molecular engineering and first-principles calculations. CsBe2B3O7, with a new DUV NLO functional [BO2] module, exhibits excellent DUV optical properties such as a suitable SHG response of similar to 0.8 x KH2PO4 (KDP), a short DUV cutoff edge of 192 nm and a large birefringence of 0.108 at 1064 nm. The optimized optical properties endow CsBe2B3O7 with a DUV frequency doubling ability close to its cutoff edge (similar to 192 nm). Additionally, the first-principles calculations demonstrate that our constructed [Be2B3O7] units maintain a densely stacked layered structure without F atoms. This work not only extends structural and functional diversity but also provides a new direction for the invention of novel high-performance NLO crystals with the [BO2] unit.

Three isostructural ACa(NO 3 ) 3 ·2H 2 O (A = NH 4 , Rb, Cs) compounds were synthesized by the water evaporation method, composed of [CaO 9 ] polyhedra and [NO 3 ] − units with enhanced optical properties. RbCa(NO 3 ) 3 ·2H 2 O is a representative example.

Two new potassium hepta-borates deep-UV birefringent crystals, KB 7 O 9 (OH) 4 and KB 7 O 10 (OH) 2 , exhibit deep-UV transparency and enhanced birefringence due to structural reconstruction ([B 7 O 10 (OH) 4 ] to [B 7 O 12 (OH) 2 ]) of functional basic units.

The exploration of deep-ultraviolet (DUV) nonlinear optical materials remains challenging. Herein, we report three novel compounds, NaK3Rb2[(CO3)(HCO3)]2·2H2O (I), NaK3.6Rb1.4[(CO3)(HCO3)]2·2H2O (II), and NaK5[(CO3)(HCO3)]2 (III), all exhibiting cutoff edges below 200 nm. Notably, I and II represent the first non-centrosymmetric examples in this system, showing moderate SHG responses (0.2-0.3 × KDP) and encouraging further exploration of carbonate-bicarbonate DUV materials.

The vast chemical diversity of crystalline phosphates and the high cost of first‐principles calculations hinder rapid discovery of wide‐bandgap materials. Here, we develop an interpretable machine‐learning framework for phosphate bandgap ( E g ) prediction and descriptor‐guided design using 474 structures from the NOEMD database. A Bayesian‐optimized Categorical Boosting (CatBoost) regressor achieves high accuracy on an independent test set ( R 2 = 0.94, MAE = 0.18 eV). SHapley Additive exPlanations reveal two governing, physically meaningful descriptors—the standard deviation of d ‐valence electron counts (), and the standard deviation of atomic density ()—that dominate E g regulation across diverse compositions and structures. and are negatively associated with E g , linking electronic‐configuration heterogeneity and packing‐density inhomogeneity to bandgap narrowing. We translate these descriptor‐property relationships into actionable screening and design guidelines. This work provides an interpretable strategy to accelerate the discovery of phosphate‐based wide‐bandgap crystals for ultraviolet/deep‐ultraviolet and related optoelectronic applications.

Birefringent crystals play indispensable roles in laser and telecommunication technologies. For UV birefringent crystals with suitable cutoff edges (<300 nm), the key to developing novel crystals with large birefringence (Δn >0.15) lies in strategically introducing π-conjugated groups with high polarizability anisotropy and reducing the dihedral angles between π-conjugated groups to minimize the cancellation of optical anisotropy. However, when multiple strategies are simultaneously operative, the relative importance of distinct strategies needs to be systematically evaluated. In this work, five new oxalate UV birefringent crystals and the known (CN4H7)2C2O4 crystal were synthesized through a facile evaporation method. By employing three distinct strategies, halogen-centered secondary building unit (SBU) modulation, diversifying intermolecular hydrogen bond types, and group modification, remarkable birefringence enhancement was achieved. Through correlation analyses between polarizability anisotropy of groups, group density, dihedral angle, and birefringence across the six crystals and over 20 diverse π-conjugated systems, we reveal the hierarchical contributions of distinct birefringence-enhancement strategies. This work establishes a systematic framework for evaluating the birefringence potential of π-conjugated structures and provides a rational design principle for the development of next-generation UV birefringent crystals.

Deep-ultraviolet (deep-UV) nonlinear optical (NLO) crystals are crucial for generating deep-UV lasers, and their performance is determined by the type, ratio, and arrangement of microscopic NLO functional units. Currently, there are no suitable materials capable of achieving deep-UV phase-matching (PM) laser output via direct second harmonic generation (SHG) at around 148.3 nm - a key requirement for the 2 2 9Th nuclear clock. Here, we proposed a functional-units-ratio design principle to address this bottleneck. Applying this strategy to the Li-B-O-F system, we designed two novel compositions, LiB3O4F2 and Li2B4O5F4. Subsequent crystal structure prediction identified C2-LiB3O4F2 as an exceptional candidate, exhibiting a record-short PM wavelength of 145.2 nm and a strong SHG response of 3.4 & times; KH2PO4. The prediction also revealed several other metastable phases with outstanding performance, including Cc-LiB3O4F2 (149.7 nm), P21-Li(2)B(4)O5F4 (151.6 nm), P21-LiB3O4F2-5 (156.1 nm), P21-LiB3O4F2-9 (156.8 nm), and Cm-LiB(3)O(4)F2-7 (158.2 nm), all of which surpass the previous record and have a high synthesis probability. Crucially, the combination of [BO3] and [BO2F2] functional units enables deep-UV PM with a moderate birefringence (similar to 0.05 @1064 nm), effectively circumventing the traditional performance trade-off. This work provides a generalizable design strategy for next-generation deep-UV NLO materials and paves the way for the practical development of the 229Th nuclear clock.

Two antimony sulfate fluoride birefringent crystals, (NH4)2Sb2SO4F6 and Na2Sb2SO4F6, were successfully synthesized via a mild aqueous solution method. Compared with the alkali metal sulfates, they exhibit enhanced optical anisotropy with calculated birefringence of 0.055 and 0.100 at 1064 nm, respectively, which mainly depend on the stereochemically active [SbO2F3] and [SbOF3] polyhedra.

Abstract Discovering deep-ultraviolet (deep-UV) nonlinear optical (NLO) crystals is hampered by the vast structural search space and the cost of first-principles screening. Here, we establish an integrated framework coupling machine learning interatomic potential (MLIP) construction, MLIP-assisted crystal structure prediction (CSP), and first-principles property calculations and apply it to the LiB2O3F system. A total of 40 low-energy candidate structures with formation energies above the thermodynamic convex hull (Ehull) ≤ 50 meV/atom were identified. Notably, seven of these structures are thermodynamically competitive with zero/near-zero Ehull values, namely, LiB2O3F-1 (P3, Z = 6), LiB2O3F-2 (R3c, Z = 6), LiB2O3F-3 (R3, Z = 3), LiB2O3F-4 (P3, Z = 3), LiB2O3F-5 (P31c, Z = 2), LiB2O3F-6 (P3, Z = 6), and LiB2O3F-7 (P63, Z = 2). First-principles calculations further reveal that these thermodynamically competitive phases exhibit wide band gaps ranging from 7.737 to 8.133 eV at the HSE06 level, suitable second harmonic generation (SHG) coefficient magnitudes ranging from 0.367 to 0.711 pm/V, and shortest phase-matching wavelengths ranging from 162.4 to 164.7 nm, highlighting the potential of LiB2O3F as a deep-UV NLO material. These results demonstrate that MLIP-assisted CSP is an effective strategy for discovering new deep-UV NLO materials.

The nonlinear optical performance of materials is closely related to the type and arrangement of their anionic frameworks. One-dimensional (1D) anionic frameworks exhibit strong anisotropy and facilitate the alignment of functional units. Here, two low-energy metastable borate fluorides compounds, SrBO2F and BaBO2F containing 1D [BO2]∞ chains were predicted using the evolutionary algorithm USPEX combined with density functional theory. Both crystallize in the monoclinic non-centrosymmetric space group C2. Phonon and electronic structure calculations revealed a reduced dimensionality of the energy bands, with 2D and 1D dispersion relations present in the band structures. Independent-particle approximation and density functional perturbation theory calculations have shown large birefringence and second-harmonic generation coefficients comparable to KH2PO4. The real-space atom-cutting method revealed that the second harmonic generation of SrBO2F and BaBO2F is primarily contributed by the [BO2]∞ infinite chains. The combination of deep-ultraviolet transparency (cut-off edge 184 nm for SrBO2F and 195 nm for BaBO2F), non-centrosymmetric crystal structure, and optical anisotropy identifies SrBO2F and BaBO2F as promising candidates for DUV applications.

Solar-blind ultraviolet nonlinear optical crystals are difficult to design because efficient frequency conversion requires the simultaneous realization of noncentrosymmetry, phase-matchable birefringence, and a sufficiently wide band gap. Here, we introduce a local conformational-confinement strategy that transforms flexible malonate into chelated [C3H2O4BFR]- (R = F, Me, CF3) chromophores, in which a six-membered ring fixes the relative orientation of two carboxylate π-conjugated units. This geometric locking enhances polarizability anisotropy and microscopic hyperpolarizability without sacrificing the band gap. In parallel, substitution at the boron-bound R site tunes the ground-state dipole moment of the anionic functional unit, suppressing antiparallel dipole packing and promoting noncentrosymmetric crystallization. Guided by this dual design principle, we obtained NaMaBFMe, which combines a UV cutoff edge near 220 nm, birefringence of 0.131 at 1064 nm, a phase-matchable SHG response of 2.4 × KDP, and a laser-induced damage threshold of 1.258 GW·cm-2. These results establish conformational locking coupled with dipole engineering as a general route to high-performance solar-blind UV NLO crystals.