An Antimony(III) Fluoride Oxalate with Large Birefringence.

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, KSb2 C2 O4 F5 , by a combination of delocalized π-conjugated [C2 O4 ]2- groups, stereochemical active Sb3+ cations, and the most electronegative element, fluorine. The [C2 O4 ]2- groups are not in an optimal arrangement in the crystal structure of KSb2 C2 O4 F5 ; nonetheless, KSb2 C2 O4 F5 exhibits a large birefringence (Δn=0.170 at 546 nm) that is even better than that of the well-known commercial birefringent material α-BaB2 O4 , even though the latter features an optimal arrangement of π-conjugated [B3 O6 ]3- groups. Based on first-principles calculations, this prominent birefringence should be attributed to the alliance of planar π-conjugated [C2 O4 ]2- anions, highly distorted SbO2 F2 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.

Pushing KTiOPO4-like Nonlinear Optical Sulfates into the Deep-Ultraviolet Spectral Region.

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, KMgSO4F and KZnSO4F, are designed and successfully synthesized. They feature three-dimensional frameworks closely related to the commercial NLO crystal, KTiOPO4 (KTP). Remarkably, the transmittance spectrum based on a single crystal indicates that the transparency window of KZnSO4F 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.

A Deep-UV Nonlinear Optical Borosulfate with Incommensurate Modulations.

Despite borate-sulfates composed of isolated borate and sulfate groups have been discovered for more than one century, borosulfates of B-O-S bonding have not been synthesized until 2012 and their properties remain rarely explored. Here, we report a new borosulfate, namely LiRb4 [B(SO4 )4 ], which shows potential as a bran-new class of deep-UV nonlinear optical (NLO) material. Remarkably, according to the in situ single-crystal X-ray diffraction, LiRb4 [B(SO4 )4 ] undergoes successive first-order phase transitions with incommensurate structural modulations under room temperature and low temperature with (3 + 1)D superspace groups of X2(α0γ)0 and P21 (α0γ)0, respectively. The first-principle calculations and structural analyses reveal that NLO properties, phase transitions, and incommensurate modulations are closely related to the [B(SO4 )4 ]5- super-tetrahedral units with B-O-S bonding. We anticipate that the unprecedented coexistence of deep-UV transparency, NLO responses, and incommensurate structural modulations in one borosulfate will bring new possibilities to understand the underlying concepts of physics and chemistry in aperiodic materials.

Designing a Deep-UV Nonlinear Optical Fluorooxosilicophosphate.

Structures composed of SiOxF6-x (x = 1, 2, 3, 4, 5) or SiOxF4-x (x = 1, 2, 3) species have thus far been observed in only a few compounds, and their functional properties are completely unknown in silicate chemistry. By introducing the least electronegative element, cesium, and the most electronegative element, fluorine, into the silicophosphate system, we successfully designed the first noncentrosymmetric fluorooxosilicophosphate with Si-F bonds, CsSiP2O7F, whose structure consists of an unprecedented SiP2O10F moiety containing hexacoordinate SiO5F species. The experimental results highlight CsSiP2O7F as the first fluorooxosilicophosphate deep-UV nonlinear optical (NLO) material. The first-principles calculations reveal that the SiP2O10F moiety is a new type of NLO-active unit and that both cesium and fluorine increase the deep-UV transparency of CsSiP2O7F. This work provides a new source of deep-UV NLO materials and insights into obtaining noncentrosymmetric structures that are indispensable to functional materials in nonlinear optics, piezoelectricity, ferroelectric, pyroelectricity, etc.

An Unprecedented Antimony(III) Borate with Strong Linear and Nonlinear Optical Responses.

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 SbB3 O6 . Remarkably, SbB3 O6 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 KH2 PO4 , exceeding those of most borates. Theoretical calculations reveal that the coexistence of strong linear and nonlinear optical responses in SbB3 O6 should be attributable to the synergistic effect of π-conjugated B-O anionic groups and Sb3+ with stereochemically active lone pair. This work provides a new class of optical bi-functional materials with potential prospects in integrated optical devices.

An Exceptional Peroxide Birefringent Material Resulting from d-π Interactions.

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 Rb2 VO(O2 )2 F is reported, the single crystals of which keep transparency after being exposed in the air for two weeks. Interestingly, Rb2 VO(O2 )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 O2 2- anions and V5+ 3d orbitals, which may be also favorable to the stability in the air for Rb2 VO(O2 )2 F. These findings distinguish peroxides as a brand-new class of birefringent materials that may possess birefringence superior to the traditional oxides.

Two Non-π-Conjugated Deep-UV Nonlinear Optical Sulfates.

The non-π-conjugated sulfate system has long been overlooked as potential deep-UV nonlinear optical (NLO) materials. Here we report two asymmetric anhydrous sulfates, namely, NH4NaLi2(SO4)2 (Ι) and (NH4)2Na3Li9(SO4)7 (Π), which consist of non-π-conjugated [SO4]2- anions. Their single crystals can be readily grown by a facile evaporation method from water solution. Both sulfates are transparent down to the deep-UV region. Interestingly, there is a large NLO gap between I and Π, with phase-matching NLO responses of 1.1 and 0.5 times that of the benchmark KH2PO4, respectively. The first-principles studies reveal that the non-π-conjugated [SO4]2- anions are the dominate NLO-active groups, and the large NLO gap between I and Π can be ascribed to the nonbonding O 2 p orbitals of different orientations in the crystallographically independent S1O4 groups. This work provides an innovative non-π-conjugated source that is distinct from the traditional π-conjugated ones for deep-UV NLO materials.

Abrupt Structural Transformation in Asymmetric ABPO4F (A = K, Rb, Cs).

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, RbBPO4F and CsBPO4F. Both compounds crystallize in cubic space group P213 (No. 198) with three-dimensional (3D) gismondine-like structures. Remarkably, in spite of the same basic structural units BO3F and PO4, both structures are distinct from the previously reported derivative KBPO4F, 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.

A Langbeinite-Type Yttrium Phosphate LiCs2Y2(PO4)3.

Langbeinite-type inorganic compounds have a wide range of applications, but nonlinear-optical properties are rarely mentioned in this series. Here, we report a new orthophosphate, LiCs2Y2(PO4)3, with a langbeinite-type structure, which is the first example in the system of mixed alkali-metal yttrium phosphates. Notably, LiCs2Y2(PO4)3 exhibits a moderate second-harmonic-generation efficiency of 0.9KH2PO4 and is transparent down to 200 nm. In addition, the thermal stability and theory calculations, including the electronic band structure, second-order nonlinear-optical coefficients, and dipole moment analysis, are also reported. This work not only expands the langbeinite-type system but also inspires a study on their nonlinear-optical properties.

An organic-inorganic hybrid birefringent material with diverse functional groups.

Birefringent materials are vital materials to modulate the polarization of light, and play a key role in plorization devices such as linear optical devices, optical communication devices, and fiber optic sensors. It is still a challenge to design excellent birefringent materials. Herein, we report an organic-inorganic hybrid oxalate birefringent material, (CN4H7)SbC2O4F2(H2O)0.5, by introducing organic delocalized π-conjugated [CN4H7]+ and [C2O4]2- groups, and stereochemical active inorganic SbO4F2 polyhedra. (CN4H7)SbC2O4F2(H2O)0.5 exhibits a large birefringence (Δn = 0.126@546 nm) that is almost equal to that of the well-known birefringent material α-BaB2O4. Theoretical calculations reveal that the distinguished birefringence should stem from the synergistic arrangement of π-conjugated [CN4H7]+ and [C2O4]2- planar groups, and highly distorted SbO4F2 polyhedra with a stereochemically active lone pair. The synergistic effect of π-conjugated systems and the lone pair electrons greatly boosts the birefringence, which is helpful for the development of high-performance birefringent materials.

Two Covalent Ultraviolet Nonlinear Optical Crystals.

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, B2 O3 I and B2 O3 II. According to the first-principles calculations, B2 O3 I and II have extremely short absorption edges of about 134 nm and 141 nm, large NLO coefficients of d22 =1.38 pm/V and d24 =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-SiO2 and are comparable to those of the commercial UV NLO crystal LiBO3 with Li+ cation. Furthermore, the birefringences are significantly larger than that of α-SiO2 , which are favorable to the phase matching for both crystals. These results reveal that B2 O3 I and B2 O3 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.

An Uncommon Hypervalent Fluorooxosilicophosphate.

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 Na4 Si2 PO4 F9 . Temperature-dependent conductivity measurements show that Na4 Si2 PO4 F9 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.

A unique dysprosium selenoarsenate(iii) exhibiting a photocurrent response and slow magnetic relaxation behavior.

A redox, substitution and self-assembly reaction offers a novel dysprosium selenoarsenate(iii) with As3+ [Dy2(tepa)2(µ2-OH)2Cl2]3[As3Se6]2 (1, tepa = tetraethylenepentamine), which provides the first example of chair conformation ring [As3Se6]3- combined with lanthanide complexes as counterions. 1 exhibits a photocurrent response and slow magnetic relaxation behavior.