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Investigating ultraviolet (UV) birefringent crystals is a focal point of research in recent years. The development of superior birefringent materials faces substantial challenges, primarily due to the need to pinpoint optimal fundamental building blocks and to perfect their geometric arrangement within the crystal lattice. By selecting a planar π-conjugated [C(NH2)3] group and a staggered [S2O6] group, we have successfully synthesized an organic–inorganic hybrid crystal, [C(NH2)3]2S2O6. Similar to the famous inorganic crystal Sr2Be2B2O7(SBBO), [C(NH2)3]2S2O6exhibits a two-dimensional double-layered crystal structure connected by N–H···O hydrogen bonding. Due to the ideal spatial arrangements of the planar π-conjugated [C(NH2)3] group, this crystal displays a large birefringence (0.150 at 546 nm) among sulfate derivatives in the short-wave UV region. Meanwhile, [C(NH2)3]2S2O6has a large band gap of 5.23 eV. This work indicates that the [S2O6] unit, acting as a direct structural motif, is beneficial for the discovery of UV birefringent crystals.

Investigating novel nonlinear optical (NLO) active units serves as a valuable method for broadening the research landscape of NLO materials. This study showcases the potential of the cytosinium cation (C4H6N3O)+as a novel NLO-active motif through theoretical calculations. The title compound exhibited a wide band gap of 3.85 eV, along with a moderate second harmonic generation (SHG) response of 1.65 times that of KH2PO4(KDP) and significant birefringence of 0.47. Its exceptional optical properties are primarily attributed to the synergy interaction between cations and anionic groups in the asymmetric unit.

Three thiophosphates including noncentrosymmetric Na6Pb3P4S16and centrosymmetric K2MIIP2S6(MII= Mg and Zn) were successfully synthesized in vacuum-sealed silica tubes. Note that interesting multiple six membered-rings (6-MRs) including 6-NaS6-MRs and 6-KSn-MRs (n= 6 and 7) formed by A+-centered polyhedra were discovered in the structures of title thiophosphates and these MR-composed three-dimensional (3D) tunnels show great possibility to facilitate the filling of various structural blocks (such as zero-dimensional (0D) Pb3S10trimers or one-dimensional (1D) (MIISn)nchains). Na6Pb3P4S16exhibits the strongest nonlinear optical (NLO) response (5.4 × AgGaS2) with phase-matching (PM) behavior among the known Pb-based PM NLO sulfides, which is much larger than that of Pb3P2S8(3.5 × AgGaS2); it was verified that such large second harmonic generation (SHG) response in Na6Pb3P4S16can be attributed to the huge contribution of stereochemically active PbS4units based on the SHG-density and dipole-moment calculations. Moreover, title thiophosphates show large birefringences (Δn= 0.102–0.21), which indicates that incorporation of [P2S6] dimers or polarized PbS4units into structures provides positive benefits for the onset of strong optical anisotropy.

Two acentric chalcogenide compounds, K2CdGe3S8and K2CdGe3Se8, were synthesized viaconventional high-temperature solid-state reactions. The crystal structures of K2CdGe3S8and K2CdGe3Se8were accurately determined by single-crystal X-ray diffraction and crystallize in the K2FeGe3S8structure type. K2CdGe3S8is isostructural to K2FeGe3S8with superior nonlinear optical properties. For the second harmonic generation (SHG) response, K2CdGe3S8is 18× K2FeGe3S8for samples of particle size of 38–55 μm. The superior nonlinear optical properties of K2CdGe3S8over K2FeGe3S8are mainly contributed by the chemical characteristics of Cd compared with Fe, which are elucidated by nonlinear optical property measurements, electronic structure calculations, and density functional theory calculations. The [CdS4] tetrahedra within K2CdGe3S8exhibit a higher degree of distortion and larger volume compared to the [FeS4] tetrahedra in K2FeGe3S8. This study possesses a good platform to investigate how d-block elements contribute to the SHG response. The fully occupied d10-elements are better for SHG susceptibility than d6-elements in this study. K2CdGe3S8is a good candidate as an infrared nonlinear optical material of high SHG response (2.1× AgGaS2, samples of particle size of 200–250 μm), type-I phase-matching capability, high laser damage threshold (6.2× AgGaS2), and good stability.

Seven acentric sulfides Ba6(CuxZy)Sn4S16(Z = Mg, Mn, Zn, Cd, In, Bi, Sn) were grown by a high-temperature salt flux method. The crystal structures of the Ba6(CuxZy)Sn4S16(Z = Mg, Mn, Zn, Cd, In, Bi, Sn) compounds were determined by single-crystal X-ray diffraction with the aid of solid-state NMR spectroscopy. The Ba6(CuxZy)Sn4S16(Z = Mg, Mn, Zn, Cd, In, Bi) compounds are isostructural and crystallize in the Ba6Ag4Sn4S16structure type. The Sn-containing compound exhibits high structural similarity to Ba6(CuxZy)Sn4S16(Z = Mg, Mn, Zn, Cd, In, Bi) with the presence of an interstitial atomic position partially occupied by Sn atoms. The chemical bonding characteristics of Ba6(Cu2.9Sn0.4)Sn4S16were understood with electron localization function calculations coupled with crystal orbital Hamilton population calculations. The Ba–S and Cu–S interactions are dominantly ionic, but the Sn–S interactions consist of strong covalent bonding characteristics in Ba6(Cu2.9Sn0.4)Sn4S16. The monovalent Cu atoms, mixed with certain metals with various oxidation states, significantly shift the optical properties of the Ba6(CuxZy)Sn4S16(Z = Mg, Mn, Zn, Cd, In, Bi) compounds. This results in a good balance between the second-harmonic-generation (SHG) response and laser damage threshold (LDT). Ba6(Cu1.9Zn1.1)Sn4S16possesses a high SHG response and a high LDT of 2.8 × AGS and 3 × AGS, respectively. A density functional theory calculation revealed that CuS4and SnS4tetrahedra significantly contribute to the SHG response in Ba6(Cu2Mg)Sn4S16, which also confirmed that CuS4tetrahedra are crucial for the stability and optical properties of the Ba6(CuxZy)Sn4S16(Z = Mg, Mn, Zn, Cd, In, Bi, Sn) compounds revealed by electronic structure analysis.

Two triclinic A2ZnSi3S8(A = Rb and Cs) with layered structures were successfully synthesized, and their physicochemical performances including optical bandgap, thermal behavior, and optical anisotropy were investigated. A2ZnSi3S8could be viewed as the first discovered Si-based examples in the known A2MIIMIV3Q8family (2–1–3–8 system; A = monovalent alkali metal; MII= divalent transition metal; MIV= group 14 metal; Q = chalcogen). The A2MIIMIV3Q8family members crystallize in five different space groups (P1̅, P21, P21/n, P212121, and Pa3̅), and their structural transformation and optical performances (bandgap, NLO coefficient, and birefringence) were systematically studied based on the first-principles calculation among 13 A2MIIBMIV3Q8(MIIB= Zn, Cd, and Hg) compounds without cubic β-K2ZnSn3S8. Research result shows that the above 13 compounds exhibit the layered structures, but diverse wavelike layers and their optical anisotropy (Δn) undergo an increasing trend range from the triclinic to orthorhombic systems. Moreover, P212121compounds have very weak NLO effects compared with those of the P21compounds since the polarization directions of anionic groups (MIIBQ4and MIVQ4) in P212121compounds are directing oppositely and almost completely canceled out by the dipole moment calculation, which further indicates that P21compounds exhibiting the relatively strong NLO effect and large optical anisotropy could be expected as potential IR NLO candidates.

The birefringent crystals capable of modulating the polarization of lights are of the current research interests. Although many oxide crystals have been discovered and widely used in UV and visible regions, the birefringent crystals in the infrared (IR) region are still rare. Herein, two new chalcogenides, K2Na2Sn3S8and Rb3NaSn3Se8, have been synthesized by the solid-state method. We have used the single crystal X-ray diffraction to determine their structures. K2Na2Sn3S8crystallizes in the monoclinic space group C2/cand exhibits a three-dimensional framework constructed by the corner-sharing SnS4and SnS5units, whereas Rb3NaSn3Se8crystallizes in the tetragonal space group P4/nbmand features a zero-dimensional [Sn3Se8]4–trimer built by the three edge-sharing SnSe4tetrahedra. The physical property measurements indicate that Rb3NaSn3Se8has a wide IR transparent window up to 20 μm and large birefringence, ∼0.196, suggesting its potential application as a birefringent crystal in the IR region. However, compared with Rb3NaSn3Se8, the birefringence of K2Na2Sn3S8is relatively small, ∼0.070. The study of their structure-property relationship indicates that the different connection modes of SnQn(Q = S, Se; n= 4, 5) polyhedra are the main reason for the large difference of birefringence between the two compounds. These studies will provide a new insight for the origin of birefringence and will facilitate the exploration of new IR birefringent crystals.

Multifunctional monolayer materials with attractive properties and novel applications are of present research interest. In this contribution, we design a new monolayer Be2CO3F2(BCF) by taking a KBe2BO3F2unique crystal that is able to produce a high energetic laser by a direct second harmonic generation method, as a parent. The cohesive energy, positive phonon modes, and elastic constants reveal that the BCF monolayer is dynamically and mechanically stable, and the appropriate cleavage energy predicts the experimental realization possibility. The property investigations demonstrated that the monolayer BCF has the significantly superiority in flexibility over the representative flexible optoelectronic material MoS2based on the calculation of Young’s modulus. Additionally, the monolayer BCF possesses both a large band gap (5.2 eV) and a second harmonic generation response. These results demonstrate that the monolayer BCF may provide better applications as a promising multifunctional material in the flexible nonlinear optical fields. We hope that this research will pave a new way for designing new generation multifunctional devices.