A Joint Strategy To Evaluate the Microscopic Origin of the Second-Harmonic-Generation Response in Nonpolar ABCO3F Compounds

RbCl·(H2SeO3)2: A Salt-Inclusion Selenite Featuring Short UV Cut-Off Edge and Large Birefringence

Birefringent materials are key components to control the light polarization in laser science and technology as well as optical communication. However, the performance of current commercial birefringent materials has been limited by the magnitude of birefringence, optical transparency range, or the attainability of large-scale single crystals. To explore new birefringent materials, we strategically incorporated a lone pair cation (Se⁴⁺) with large optical anisotropy, an alkali metal, and halogen ions (Rb⁺ and Cl^-) with superior UV transparent capacity; thus a new compound, RbCl·(H₂SeO₃)₂, was successfully discovered with the aid of the facile hydrothermal method. Interestingly, Rb-Cl chains locate in the [H₂SeO₃]_∞ skeleton, which makes RbCl·(H₂SeO₃)₂ a salt-inclusion selenite. Millimeter-sized single crystals (up to 4 × 2 × 1 mm³) were obtained, and the transmittance spectrum revealed that its UV cut-off edge can be as low as 230 nm. In addition, the calculated birefringence of RbCl·(H₂SeO₃)₂ is 0.14 at 589 nm that is similar to the birefringent value of famous α-BaB₂O₄. Wide UV transparency, large birefringence, and feasible crystal growth make RbCl·(H₂SeO₃)₂ a new member of birefringent materials for UV light applications.

BaZn3(BO3)2F2: a new beryllium-free zincoborate with a KBBF-type structure

A new beryllium-free zincoborate, BaZn₃(BO₃)₂F₂, with a KBBF-type structure has been synthesized for the first time. The electrostatic force of interaction in BaZn₃(BO₃)₂F₂ provides better linkage in neighboring [ZnBO₃F]_∞ single layers. BaZn₃(BO₃)₂F₂ is the first case of borates with both [ZnO₃F] tetrahedra and [ZnO₆] octahedra, enriching the structural chemistry of borate system. All the coplanar [BO₃] triangles align in the same direction with a high density, which endows BaZn₃(BO₃)₂F₂ with a large birefringence of cal. 0.076 at 1064 nm. This work is of great significance to design beryllium-free borates with a KBBF-type structure.

Hydroxyfluorooxoborate Na[B3 O3 F2 (OH)2 ]⋅[B(OH)3 ]: Optimizing the Optical Anisotropy with Heteroanionic Units for Deep Ultraviolet Birefringent Crystals

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.

RbMT3 (BO3 )2 O3 (M=Ba, Sr; T=Al, Ga): New Double-Layered Oxyborates Constructed from [BO3 ] Triangles and [TO4 ] Tetrahedra

Four new alumino-/galloborates RbMT₃ (BO₃ )₂ O₃ (M=Ba, Sr; T=Al, Ga) have been synthesized for the first time by using a high-temperature solution method. All the title compounds have Sr₂ Be₂ B₂ O₇ -like structures, in which the [BO₃ ] triangles and [TO₄ ] tetrahedra form the final double-layered configurations with the M- and Rb-site atoms located between and in the double layer, respectively. The structure evolution from Sr₂ Be₂ B₂ O₇ to RbMT₃ (BO₃ )₂ O₃ series is discussed. The broader energy bandgaps in Al-based borates when compared with Ga-based ones can be entirely attributed to the location of Al/Ga s orbitals near the Fermi surface. Both experimental and computational approaches were used to study their structure-property relationships.

Discovery of First Magnesium Fluorooxoborate with Stable Fluorine Terminated Framework for Deep-UV Nonlinear Optical Application

The generated light can be tuned to cover almost the entire spectral range from deep-ultraviolet to terahertz wavelengths by utilizing the nonlinear optical crystals with a simple frequency doubling process. Among them, the discovery of novel candidates for the production of deep-ultraviolet light is by extension a great challenge toward realizing the vast potential. Actually, the availability for this process mainly depends on whether the critical performance can be well coexisted in one practical crystal. Herein, the first magnesium fluorooxoborate MgB₅ O₇ F₃ was synthesized as a new competitive candidate for deep-ultraviolet nonlinear optical application. It has a sufficiently large nonlinearity and a deep-ultraviolet phase matching wavelength, indicating that it holds great potential for the production of coherent light below 200 nm. The critical performance enhancement of MgB₅ O₇ F₃ when compared with its isomorphic phases was demonstrated and discussed. More importantly, we proposed that fluorooxoborate system with the general formula of MB₅ O₇ F₃ (M=divalent metal) possesses stable fluorine terminated framework, which makes them tend to retain their crystallized space groups unchanged.

Large optical polarizability causing positive effects on the birefringence of planar-triangular BO3 groups in ternary borates

The structure-property relationship of photoelectric functional materials has been recognized as a hot topic. The study of the inner link between the band gaps and birefringence of optical materials is extremely crucial for the design and creation of novel optical devices, but still remains rather unexplored. In this work, taking a series of borates with only planar-triangular BO₃ groups, α-/β-TM₃(BO₃)₂ (TM = Zn, Cd), Cd₂B₂O₅, and M₃(BO₃)₂ (M = Hg, Mg, Ca, Sr) as the research subject, the comprehensive relationship between their electronic structures and linear optical properties has been systematically investigated. Through combining experimental measurements and theoretical calculations, the effect of optical polarizability on the birefringence of these borates was clarified. Based on the present discussion, the relationship between the O (2p) bandwidth of the highest valence band and the HSE06 band gaps is opposite. Meanwhile, a method involving the determination of so-called optical permittivity Δε to evaluate the magnitude of birefringence Δn is found to be feasible. A large Δε makes a positive contribution to Δn. In addition, the experimentally measured band gaps and IR vibrations are in good agreement with theoretical results for the compounds α-Cd₃(BO₃)₂ and Cd₂B₂O₅.

Rb3BaTeB7O15: a novel [B7O16] fundamental building block in a new telluroborate with [TeO3] polyhedra

A new telluroborate Rb₃BaTeB₇O₁₅ with a new [B₇O₁₆] fundamental building block has been synthesized, and it is the first telluroborate that is constructed only by using [TeO₃] polyhedra.

Research and Development of Zincoborates: Crystal Growth, Structural Chemistry and Physicochemical Properties

Borates have been regarded as a rich source of functional materials due to their diverse structures and wide applications. Therein, zincobrates have aroused intensive interest owing to the effective structural and functional regulation effects of the strong-bonded zinc cations. In recent decades, numerous zincoborates with special crystal structures were obtained, such as Cs₃Zn₆B₉O₂₁ and AZn₂BO₃X₂ (A = Na, K, Rb, NH₄; X = Cl, Br) series with KBe₂BO₃F₂-type layered structures were designed via substituting Be with Zn atoms, providing a feasible strategy to design promising non-linear optical materials; KZnB₃O₆ and Ba₄Na₂Zn₄(B₃O₆)₂(B₁₂O₂₄) with novel edge-sharing [BO₄]^5- tetrahedra were obtained under atmospheric pressure conditions, indicating that extreme conditions such as high pressure are not essential to obtain edge-sharing [BO₄]^5--containing borates; Ba₄K₂Zn₅(B₃O₆)₃(B₉O₁₉) and Ba₂KZn₃(B₃O₆)(B₆O₁₃) comprise two kinds of isolated polyborate anionic groups in one borate structure, which is rarely found in borates. Besides, many zincoborates emerged with particular physicochemical properties; for instance, Bi₂ZnOB₂O₆ and BaZnBO₃F are promising non-linear optical (NLO) materials; Zn₄B₆O₁₃ and KZnB₃O₆ possess anomalous thermal expansion properties, etc. In this review, the synthesis, crystal structure features and properties of representative zincoborates are summarized, which could provide significant guidance for the exploration and design of new zincoborates with special structures and excellent performance.

Targeting the Next Generation of Deep-Ultraviolet Nonlinear Optical Materials: Expanding from Borates to Borate Fluorides to Fluorooxoborates

Coherent light radiation down to the deep-ultraviolet spectral range (λ < 200 nm) produced by common laser sources is extensively used in diverse fields ranging from ultrahigh-resolution photolithography to photochemical synthesis to high-precision microprocessing. Actually, it is hard to immediately obtain certain wavelengths, deep-ultraviolet coherent light in particular, from commercial laser sources. However, the direct second harmonic generation process governed in part by nonlinear optical crystals is a feasible and effective approach to generate deep-ultraviolet coherent light, which motivates chemists and materials scientists to find potential deep-ultraviolet nonlinear optical materials that can practically meet the scientific requirements. The research progress required to go from a new single-crystal structure to final device applications involves many pivotal steps and is highly time-consuming and challenging, and therefore, it is necessary to commence systematic studies aimed at shortening the research cycle and accelerating the rational design of deep-ultraviolet nonlinear optical materials. In this Account, we choose borates as raw materials because they have ever-greater possibilities to form desired noncentrosymmetric structures, wide optical transparency windows, rich structural chemistry, and also large polarizabilities to guarantee the coexistence of large second-order nonlinear optical coefficients and suitable birefringence. Besides, the effects of fluorine atoms on the structural chemistry and optical properties of borates have been summarized and analyzed. On the basis of these favorable influences, three specific rational design strategies, including experimental and theoretical methods, have been proposed in order to shorten the investigational cycle of discovering the new expected compounds with high physicochemical performances required for practical applications. In this way, the progress of searching for candidates for the next generation of deep-ultraviolet nonlinear optical materials was accelerated from borates to borate fluorides to fluorooxoborates with three effective strategies: (1) expansion of the frontier from borates to borate fluorides with the introduction of fluorine to achieve enhanced optical performance; (2) computer-assisted design of new deep-ultraviolet nonlinear optical materials with a newly introduced systematic global structure optimization method; and (3) expansion of the frontier from borate fluorides to fluorooxoborates by proposed functionalized oxyfluoride [BO _xF_{4- x}]^{( x+1)-} ( x = 1, 2, 3) chromophores to balance multiple criteria. The preliminary development of fluorooxoborates exhibiting high performance as a new fertile field to search for deep-ultraviolet nonlinear optical materials is highly encouraging and inspiring and can guide chemists and materials scientists with new directions and thoughts aimed at finding the next generation of practical deep-ultraviolet nonlinear optical materials.

Li4Na2CsB7O14: a new edge-sharing [BO4]5− tetrahedra containing borate with high anisotropic thermal expansion

Li₄Na₂CsB₇O₁₄, a new edge-sharing [BO₄]⁵⁻ tetrahedra containing borate with high anisotropic thermal expansion, was obtained under the atmospheric pressure conditions.

Experimental characterization and first principles calculations of linear and nonlinear optical properties of two orthophosphates A3Al2(PO4)3 (A = Rb, K)

We report the experimental characterization and first principles calculation of linear and nonlinear optical properties of two orthophosphates A₃Al₂(PO₄)₃ (A = Rb, K).

Ion-induced structural and optical performance evolution in LBO-like crystals: experimental and theoretical investigation

A new alkali metal borate fluoride Li₃KB₉O₁₅F has been obtained for the first time with a UV cut-off edge below 175 nm.