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Birefringent materials play a key role in modulating the polarization of light and thus in optical communication as well as in laser techniques and science. Designing new, excellent birefringent materials remains a challenge. In this work, we designed and synthesized the first antimony(III) fluoride oxalate birefringent material, KSb2C2O4F5, by a combination of delocalized π‐conjugated [C2O4]2− groups, stereochemical active Sb3+ cations, and the most electronegative element, fluorine. The [C2O4]2− groups are not in an optimal arrangement in the crystal structure of KSb2C2O4F5; nonetheless, KSb2C2O4F5 exhibits a large birefringence (Δn=0.170 at 546 nm) that is even better than that of the well‐known commercial birefringent material α‐BaB2O4, even though the latter features an optimal arrangement of π‐conjugated [B3O6]3− groups. Based on first‐principles calculations, this prominent birefringence should be attributed to the alliance of planar π‐conjugated [C2O4]2− anions, highly distorted SbO2F2 and SbOF3 polyhedra with a stereochemically active lone pair. The combination of lone‐pair electrons and π‐conjugated systems boosts the birefringence to a large extent and will help the development of high‐performance birefringent materials. [ABSTRACT FROM AUTHOR]

The species diversity of silicon (including traditional tetrahedral coordinated silicon and hypervalent penta‐ and hexa‐coordinate silicon) gives rise to the structural richness and diverse properties of silicates. Among these silicon species, hypervalent silicon is very rare, not to mention almost unexplored mixed‐anion hypervalent fluoroxosilicate species. In this work, we successfully obtained a mixed‐anion fluorooxosilicophosphate Na4Si2PO4F9 consisting of two uncommon hypervalent fluoroxosilicate species, namely, trans‐SiO2F4 species and SiOF5 species. To the best of our knowledge, such hypervalent silicon species are reported for the first time in inorganic compounds. Remarkably, the coexistence of two distinct hypervalent fluoroxosilicate species in one compound is somewhat conflicted with Pauling's parsimony rule, but it indeed achieves an unlikely connection by PO4 and our phonon dispersion calculation confirms the structure stability of Na4Si2PO4F9. Temperature‐dependent conductivity measurements show that Na4Si2PO4F9 is a promising solid ionic conductor with a high conductivity of 4.0×10−5 S⋅cm−1 at 700 K and a low active energy of about 53.1 KJ⋅mol−1. This work will enrich the structure chemistry of silicates and may provide a new platform for solid ionic batteries. [ABSTRACT FROM AUTHOR]

Sulfates have long been ignored as nonlinear optical materials over the past decades. Here we report a new sulfate deep-ultraviolet nonlinear optical material Li8NaRb3(SO4)6·2H2O synthesized by the facile water solution method. It crystallizes in the asymmetric monoclinic space group C2 (No. 5). Its single-crystal structure features a three-dimensional framework made up of SO4 and LiO4 tetrahedra. Powder second-harmonic generation tests demonstrate that Li8NaRb3(SO4)6·2H2O is phase-matchable with a second-harmonic generation response of about 0.5 × KH2PO4. Ultraviolet-visible-near-infrared diffuse reflectance spectra illustrate that the ultraviolet cutoff edge of Li8NaRb3(SO4)6·2H2O may be as low as λ ＜ 190 nm. Theoretical calculations reveal that the optical properties of Li8NaRb3(SO4)6·2H2O are mainly attributed to S–O groups. All these results reveal that Li8NaRb3(SO4)6·2H2O may possess potential use in the deep-ultraviolet nonlinear optics field. We believe that the discoveries in our work will attract scientists’ attention to sulfate systems for their potential as deep-ultraviolet nonlinear optical materials. [ABSTRACT FROM AUTHOR]

Few nonlinear optical (NLO) materials are available for deep-UV second-harmonic generation (SHG), limited by the fundamental requirements of the structure-directing optical properties. KBe2BO3F2 is the sole exception, but it suffers from the high toxicity of the compositional beryllium element and a serious layering growth tendency. Here, we report a beryllium-free deep-UV NLO phosphate with honeycomb-like topological structural layers, namely, CsNaMgP2O7. The phosphate exhibits an enhanced SHG response of 1.1 times that of KH2PO4, which is comparable to that of KBe2BO3F2. Based on theoretical analysis, the enhanced SHG response can be well rationalized. Meanwhile, CsNaMgP2O7 possesses a reinforced interlayer bonding of about 2.95 times that of KBe2BO3F2, which is very favorable to mitigate the layering growth tendency. This work will be helpful to better understand the structure–property relationships in deep-UV NLO materials. [ABSTRACT FROM AUTHOR]

Accurately designing and synthesizing new deep-ultraviolet (deep-UV) nonlinear optical (NLO) crystals that are limited by the so-called "200 nm wall" on their transparency windows remain challenging. On the basis of a bandgap-directed computer-aided material design approach, two new NLO sulfates, KMgSO 4 F and KZnSO 4 F, are designed and successfully synthesized. They feature three-dimensional frameworks closely related to the commercial NLO crystal, KTiOPO 4 (KTP). Remarkably, the transmittance spectrum based on a single crystal indicates that the transparency window of KZnSO 4 F is significantly blue-shifted to <190 nm from 350 nm for KTP. The microscopic origin of this significant transparent window blue shift is illustrated well by first-principles calculations. This work pushes the transparency windows of KTP-like NLO sulfates into the deep-UV spectral region for the first time and will pave a prospective way to the accurate design and synthesis of new deep-UV NLO materials.

Birefringent materials play a key role in modulating the polarization of light and thus in optical communication as well as in laser techniques and science. Designing new, excellent birefringent materials remains a challenge. In this work, we designed and synthesized the first antimony(III) fluoride oxalate birefringent material, KSb 2 C 2 O 4 F 5 , by a combination of delocalized π-conjugated [C 2 O 4 ] 2- groups, stereochemical active Sb 3+ cations, and the most electronegative element, fluorine. The [C 2 O 4 ] 2- groups are not in an optimal arrangement in the crystal structure of KSb 2 C 2 O 4 F 5 ; nonetheless, KSb 2 C 2 O 4 F 5 exhibits a large birefringence (Δn=0.170 at 546 nm) that is even better than that of the well-known commercial birefringent material α-BaB 2 O 4 , even though the latter features an optimal arrangement of π-conjugated [B 3 O 6 ] 3- groups. Based on first-principles calculations, this prominent birefringence should be attributed to the alliance of planar π-conjugated [C 2 O 4 ] 2- anions, highly distorted SbO 2 F 2 and SbOF 3 polyhedra with a stereochemically active lone pair. The combination of lone-pair electrons and π-conjugated systems boosts the birefringence to a large extent and will help the development of high-performance birefringent materials., (© 2020 Wiley-VCH GmbH.)

Birefringent materials, which can modulate the polarization of light, are almost exclusively limited to oxides. Peroxides have long been overlooked as birefringent materials, because they are usually not stable in air. Now, the first peroxide birefringent material Rb 2 VO(O 2 ) 2 F is reported, the single crystals of which keep transparency after being exposed in the air for two weeks. Interestingly, Rb 2 VO(O 2 ) 2 F does not feature an optimal anisotropic structure, but its birefringence (Δn=0.189 at 546 nm) exceeds those of the majority of oxides. According to the first-principles calculations, this exceptional birefringence should be attributed to the strong electronic interactions between localized π orbital of O 2 2- anions and V 5+ 3d orbitals, which may be also favorable to the stability in the air for Rb 2 VO(O 2 ) 2 F. These findings distinguish peroxides as a brand-new class of birefringent materials that may possess birefringence superior to the traditional oxides., (© 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Antimony(III) borates with a stereochemical active lone pair remained unknown, although the first antimony borate was reported more than twenty years ago. Now, the first antimony(III) borate in a closed system is successfully synthesized, namely SbB 3 O 6 . Remarkably, SbB 3 O 6 not only exhibits an exceptional linear optical response, that is, birefringence of Δn=0.290 at the wavelength of 546 nm, which is the largest among borates, but also has a strong nonlinear optical response of 3.5 times larger than the benchmark KH 2 PO 4 , exceeding those of most borates. Theoretical calculations reveal that the coexistence of strong linear and nonlinear optical responses in SbB 3 O 6 should be attributable to the synergistic effect of π-conjugated B-O anionic groups and Sb 3+ with stereochemically active lone pair. This work provides a new class of optical bi-functional materials with potential prospects in integrated optical devices., (© 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Nonlinear optical (NLO) crystals are the vital components of laser science and technology, as they can convert lasers in common wavelengths into new wavelength bands for ultraviolet (UV), IR, and even terahertz laser output. Known UV NLO crystals mainly focus on crystals containing cations, but covalent crystals have rarely been reported. Here we report two covalent NLO crystals, B 2 O 3 I and B 2 O 3 II. According to the first-principles calculations, B 2 O 3 I and II have extremely short absorption edges of about 134 nm and 141 nm, large NLO coefficients of d 22 =1.38 pm/V and d 24 =0.702 pm/V, as well as sufficient birefringences of 0.037 and 0.031, respectively. Notably, the absorption edges are almost the shortest among NLO crystals. Meanwhile, the NLO coefficients are evidently larger than that of another well-known covalent NLO crystal α-SiO 2 and are comparable to those of the commercial UV NLO crystal LiBO 3 with Li + cation. Furthermore, the birefringences are significantly larger than that of α-SiO 2 , which are favorable to the phase matching for both crystals. These results reveal that B 2 O 3 I and B 2 O 3 II are excellent candidates for UV NLO applications. In-depth calculations are carried out to reveal the origin of excellent NLO properties. These covalent crystals provide a new direction for the research of UV NLO crystals., (© 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

An asymmetric structure is the necessary requirement for second-order nonlinear-optical (NLO) materials, which have important applications in modern science and technology. Here we report two isostructural asymmetric compounds, RbBPO 4 F and CsBPO 4 F. Both compounds crystallize in cubic space group P2 1 3 (No. 198) with three-dimensional (3D) gismondine-like structures. Remarkably, in spite of the same basic structural units BO 3 F and PO 4 , both structures are distinct from the previously reported derivative KBPO 4 F, which crystallizes in a monoclinic space group Cc (No. 9) with a two-dimensional (2D)-layered structure. Careful structural analysis reveals that this structural transformation (from a monoclinic 2D structure to cubic 3D structures) should be aroused by the different alkaline ionic radii. To the best of our knowledge, such an abrupt structural transformation by alkaline elements is reported in all-inorganic asymmetric compounds for the first time. The structural transformation from 2D to 3D structures is favorable to eliminate the layered growth habit. This study will shed deep insight in the structural modulation of asymmetric compounds.