Deep-Ultraviolet Bialkali-Rare Earth Metal Anhydrous Sulfate Birefringent Crystal

Birefringence is an important linear optical property of anisotropic crystals that plays a significant role in regulating light polarization. A new bialkali-rare earth metal sulfate, NaRbY2(SO4)4 compound, consisting of non-π-conjugated alkali metals and rare earth metal-centered dodecahedral YO8 has been synthesized. The structure analysis suggests that the three-dimensional (3D) structure of the compound is found to be attributable to the combination of dodecahedral YO8 and tetrahedral SO4 groups with Na+ and Rb+ located in the cavities. The ultraviolet, visible, and near-infrared (UV-vis-NIR) spectra reveal that the compound exhibits transparency at a wavelength of less than 200 nm. The observed birefringence of the compound is 0.045@550 nm, which is comparatively larger than that of most deep-ultraviolet (DUV) birefringent crystals. The birefringence mainly originated from the YO8 dodecahedron, which is suggested by first-principles calculations. This research work can provide a useful perspective to explore new DUV sulfates with excellent birefringence.

A UV non-hydrogen pure selenite nonlinear optical material for achieving balanced properties through framework-optimized structural transformation

For non-centrosymmetric (NCS) oxides intended for ultraviolet (UV) nonlinear optical (NLO) applications, achieving a wide band gap, large second harmonic generation (SHG) intensity, and sufficient birefringence to satisfy phase matching is a significant challenge due to their inherent incompatibility. To address this issue, this study proposes a strategy called framework-optimized structural transformation. Building upon centrosymmetric (CS) NaGa(SeO3)2 as a foundation, an original UV selenite NLO material, NaLu(SeO3)2, was successfully synthesized. The derived NaLu(SeO3)2 exhibits a balanced comprehensive performance, including a band gap (5.3 eV), an SHG response (2.7 × KDP), a UV cut-off edge (210 nm), a laser-induced damage threshold (LIDT) (151.69 MW cm-2), birefringence (Cal: 0.138@546 nm, Exp: 0.153@546 nm), thermal stability (∼575 °C) and environmental stability. Notably, its SHG effect, band gap, LIDT, and birefringence are all the largest among UV non-hydrogen pure selenite materials. Such progress can be attributed to the successful arrangement of the SeO3 groups by optimizing the cations on the framework of the parent compound.

UV-Transparent SHG Material Explored in an Alkali Metal Sulfate Selenite System

The first examples of alkali metal selenite sulfates, namely, Na8(SeO3)(SO4)3 (1), Na2(H2SeO3)(SO4) (2), and K4(H2SeO3)(HSO4)2(SO4) (3), were successfully synthesized by hydrothermal reactions. Their structures display three different zero-dimensional configurations composed of isolated sulfate tetrahedra and selenite groups separated by alkali metals. Na8(SeO3)(SO4)3 (1) features a noncentrosymmetric structure, while Na2(H2SeO3)(SO4) (2) and K4(H2SeO3)(HSO4)2(SO4) (3) are centrosymmetric. Powder second-harmonic-generation measurements revealed that Na8(SeO3)(SO4)3 (1) shows a phase-matchable SHG intensity about 1.2 times that of KDP. UV-vis-NIR diffuse reflectance spectroscopic analysis indicated that Na8(SeO3)(SO4)3 (1) has a short UV cutoff edge and a large optical band gap, which makes it a possible UV nonlinear optical material. Theoretical calculations revealed that the birefringence of Na8(SeO3)(SO4)3 (1) is 0.041 at 532 nm, which is suitable for phase-matching condition. This work provides a good experimental foundation for the exploration of new UV nonlinear crystals in an alkali metal selenite sulfate system.

Hg4(Te2O5)(SO4): A Giant Birefringent Sulfate Crystal Triggered by a Highly Selective Cation

Sulfate crystals are often criticized for their low birefringence. The small anisotropic SO4 group is becoming the biggest bottleneck hindering the application of sulfates in optical functional materials. In this study, we report a new method to significantly enhance the birefringence of sulfates. The title compound increases the birefringence recording of sulfates to 0.542@546 nm, which is significantly larger than that of the commercial birefringent crystal of TiO2 (0.306@546.1 nm). At the infrared wavelength, the birefringence of Hg4(Te2O5)(SO4) can be up to 0.400@1064 nm, which is also much larger than the infrared birefringent crystal of YVO4 (0.209@1064 nm). In addition, it also has a wide transparency range, high thermal stability, and excellent environmental stability, making it a potential birefringent material. Hg4(Te2O5)(SO4) features a novel two-dimensional layered structure composed of [Hg4(Te2O5)]2+ layers separated by isolated (SO4)2- tetrahedra. This compound was designed by introducing a highly selective cation in a tellurite sulfate system. The low valence low coordination cations connect with tellurite groups only, making the sulfate isolated in the structure. The steric repulsive action of the isolated SO4 tetrahedra may regulate the linear and lone pair groups arranged in a way that favors large birefringence. This method can be proven by theoretical calculations. PAWED studies showed that the large birefringence originated from the synergistic effect of (Hg2O2)2-, (Te2O5)2-, and (SO4)2- units, with a contribution ratio of 42.17, 37.92, and 19.88%, respectively. Our work breaks the limitation of low birefringence in sulfates and opens up new possibilities for their application as birefringent crystals.

PbTeB4O9: a lead tellurium borate with unprecedented fundamental building block [B4O10] and large birefringence

Herein, the first lead tellurium borate, PbTeB4O9, with an unprecedented fundamental building block [B4O10] was successfully synthesized. The near-parallel alignment of [B4O10] groups and [TeO3] polyhedra resulted in a high birefringence (0.099@1064 nm). The structure-property relationship was discussed by using the first-principles calculations.

YSO4F·H2O: A Deep-Ultraviolet Birefringent Rare-Earth Sulfate Fluoride with Enhanced Birefringence Induced by Fluorinated Y-Centered Polyhedra

Birefringent crystals can modulate and detect the polarization of light and are important optical functional materials. The birefringence is positively correlated to the anisotropy of the structure. By partially substituting sulfate anion with large electronegative fluorine in the parent compound Y2(SO4)3·8H2O, a new fluorinated rare-earth sulfate YSO4F·H2O with enhanced anisotropy was achieved. YSO4F·H2O features a dense 3D structure constructed by the polarizable [YOF] polyhedra and [SO4] tetrahedra. The diffuse reflectance spectrum reveals that it has a short UV absorption edge of below 200 nm. The substitution of the F- ion enhances the optical anisotropy, making the material exhibit an enhanced birefringence (0.0357 at 546 nm), which is 5.1 times that of the parent compound and is also larger than most deep-UV birefringent sulfates. It is expected that this work may shed useful insights in the exploration of deep-UV birefringent materials with enhanced optical performances..

Sb4O3(TeO3)2(HSO4)(OH): An Antimony Tellurite Sulfate Exhibiting Large Optical Anisotropy Activated by Lone Pair Stereoactivity

A new birefringent crystal of Sb₄O₃(TeO₃)₂(HSO₄)(OH) was achieved by incorporating two stereochemically active lone pair (SCALP) cations of Sb(III) and Te(IV) into sulfates simultaneously. The Sb³⁺ and Te⁴⁺ ions display highly distorted coordination environments due to the SCALP effect. Sb₄O₃(TeO₃)₂(HSO₄)(OH) displays a 3D structure composed of [Sb₄O₃(TeO₃)₂(OH)]_∞⁺ layers bridged by [SO₃(OH)]^- tetrahedra. It possesses a large birefringence and a wide optical transparent range, making it a new UV birefringent crystal. First-principles calculation analysis suggests that the synergistic effect of the cooperation of SCALP effect of Sb³⁺ and Te⁴⁺ cations make a dominant contribution to the birefringence. The work highlights that units with SCALP cations have advantages in generating large optical anisotropy and are preferable structural units for designing novel birefringent materials.

Host-Guest Symmetry Matching in Two Crystalline Magnesium Sulfate Oxalates Obtained Via a Solvent-Free Route

The combination of π-conjugated oxalate anion with sulfate group has been explored in the solvent-free synthesis of two new magnesium sulfate oxalates. One of them has a layered structure crystallized in the noncentrosymmetric space group Ia, while the other has a chainlike structure crystallized in the centrosymmetric space group P2₁/c. The noncentrosymmetric solid has a wide optical bandgap and exhibits a moderate second-harmonic-generation response. Density functional theory calculations were carried out to disclose the origin of its second-order nonlinear-optical response.

The First UV Nonlinear Optical Selenite Material: Fluorination Control in CaYF(SeO3 )2 and Y3 F(SeO3 )4

It is a great challenge to develop UV nonlinear optical (NLO) material due to the demanding conditions of strong second harmonic generation (SHG) intensity and wide band gap. The first ultraviolet NLO selenite material, Y₃ F(SeO₃ )₄ , has been obtained by control of the fluorine content in a centrosymmetric CaYF(SeO₃ )₂ . The two new compounds represent similar 3D structures composed of 3D yttrium open frameworks strengthened by selenite groups. CaYF(SeO₃ )₂ has a large birefringence (0.138@532 nm and 0.127@1064 nm) and a wide optical band gap (5.06 eV). The non-centrosymmetric Y₃ F(SeO₃ )₄ can exhibit strong SHG intensity (5.5×KDP@1064 nm), wide band gap (5.03 eV), short UV cut-off edge (204 nm) and high thermal stability (690 °C). So, Y₃ F(SeO₃ )₄ is a new UV NLO material with excellent comprehensive properties. Our work shows that it is an effective method to develop new UV NLO selenite material by fluorination control of the centrosymmetric compounds.

Hg3(Te3O8)(SO4): a new sulfate tellurite with a novel structure and large birefringence explored from d10 metal compounds

Our exploration of novel inorganic solids with large birefringence in the d10 Metal-Te4+-SO42- system afforded four new sulfate tellurites, Ga2(TeO3)(SO4)(OH)2, In2(TeO3)2(SO4)(H2O), Zn4(Te3O7)2(SO4)2(H2O) and Hg3(Te3O8)(SO4). Notably, Hg3(Te3O8)(SO4) exhibits the largest birefringence at 532 nm (0.184) and 1064 nm (0.166) among the reported metal sulfate tellurites.

An antimony(iii) borate with large birefringence exhibiting unwonted [B5O11] fundamental building blocks and dimeric [Sb2O6] clusters

By combining π-conjugated [B₃O₆] groups and Sb³⁺ cations with SCALP, we successfully obtained the second antimony(iii) borate SbB₅O₉ with a large birefringence (0.151 @ 1064 nm), which exhibits unwonted [B₅O₁₁] FBBs and dimeric [Sb₂O₆] clusters.