HgBr2: an easily growing wide-spectrum birefringent crystal.

Birefringent materials are of great significance to the development of modern optical technology; however, research on halide birefringent crystals with a wide transparent range remains limited. In this work, mercuric bromide (HgBr2) has been investigated for the first time as a promising birefringent material with a wide transparent window spanning from ultraviolet (UV) to far-infrared (far-IR) spectral regions (0.34-22.9 µm). HgBr2 has an exceptionally large birefringence (Δn, 0.235 @ 546 nm), which is 19.6 times that of commercial MgF2. The ordered linear motif [Br-Hg-Br] with high polarizability anisotropy within the molecule is the inherent source of excellent birefringence, making it an efficient building block for birefringent materials. In addition, HgBr2 can be easily grown under mild conditions and remain stable in air for prolonged periods. Studying the birefringent properties of HgBr2 crystals would provide new ideas for future exploration of wide-spectrum birefringent materials.

Salt-inclusion chalcogenides with d-orbital components: unveiling remarkable nonlinear optical properties and dual-band photoluminescence.

Metals containing d-orbitals are typically characterized by strong deformation and polarization, yet they tend to induce narrow bandgaps that render them little-appreciated by high-power nonlinear optical (NLO) crystals. Incorporating highly electropositive polycations into d-orbital-containing chalcogenides to modify them into salt-inclusion chalcogenides (SICs) that are competitive in NLO materials, is a viable solution to this predicament. In the present work, two isostructural SICs [K4Cl][MGa9S16] (M = Mn, 1; Hg, 2) are successfully synthesized by the high-temperature molten-salt growth method. Both compounds demonstrate commendable second-harmonic-generation (SHG) responses (0.6-1.0 × AgGaS2 @1910 nm), which can be attributed to their well-designed [MGa9S16]3- anionic frameworks; and compound 2 exhibits the widest optical bandgap (3.41 eV) among the Hg-based NLO chalcogenides. Also, an interesting dual-band photoluminescence emission centered at ∼650 and ∼718 nm is detected in 1 at 77 K, with long lifetimes of 0.94 and 1.35 ms, respectively.

[K2 PbX][Ga7 S12 ] (X = Cl, Br, I): The First Lead-Containing Cationic Moieties with Ultrahigh Second-Harmonic Generation and Band Gaps Exceeding the Criterion of 2.33 eV.

In contrast to anionic group theory of nonlinear optical (NLO) materials that second-harmonic generation (SHG) responses mainly originate from anionic groups, structural regulation on the cationic groups of salt-inclusion chalcogenides (SICs) is performed to make them also contribute to the NLO effects. Herein, the stereochemically active lone-electron-pair Pb2+ cation is first introduced to the cationic groups of NLO SICs, and the resultant [K2 PbX][Ga7 S12 ] (X = Cl, Br, I) are isolated via solid-state method. The features of their three-dimensional structures comprise highly oriented [Ga7 S12 ]3- and [K2 PbX]3+ frameworks derived from AgGaS2 , which display the largest phase-matching SHG intensities (2.5-2.7 × AgGaS2 @1800 nm) among all SICs. Concurrently, three compounds manifest band gap values of 2.54, 2.49, and 2.41 eV (exceeding the criterion of 2.33 eV), which can avoid two-photon absorption under the fundamental laser of 1064 nm, along with the relatively low anisotropy of thermal expansion coefficients, leading to improved laser-induced damage thresholds (LIDTs) values of 2.3, 3.8, and 4.0 times that of AgGaS2 . In addition, the density of states and SHG coefficient calculations demonstrate that the Pb2+ cations narrow the band gaps and benefit SHG responses.

Breaking the bottleneck of simultaneously wide band gap and large nonlinear optical coefficient by a "pore reconstruction" strategy in a salt-inclusion chalcogenide.

The template-based design of the crystal structure is a direct and highly efficient method to achieve optimal nonlinear optical (NLO, meaning second-order NLO) performances. The structural flexibility of porous salt-inclusion chalcogenides (SICs) provides an alternative platform for modulating the enlargement of the band gap (that is generally positive with laser-induced damage threshold) and second harmonic generation (SHG) response simultaneously. By applying the "pore reconstruction" strategy to SIC [K3Cl][Mn2Ga6S12] (1), a new derivative K3Rb3[K3Cl][Li2Mn4Ga12S27] (2) is successfully isolated, which unusually features a heterologous nanopore framework with inner diameters of 8.90 and 9.16 Å. Guided by such a strategy, compound 2 possesses the widest band gap (3.31 eV) among the magnetic NLO chalcogenides; this finding is dominantly attributed to the porous structure and the "dimensional deduction" effect. Moreover, phase 2 displays a remarkable phase-matchable SHG intensity (1.1 × AgGaS2 at the incident laser of 1910 nm) that originated from the oriented alignment of NLO-functional motifs, as well as the rich terminal S atoms in the nanopore structure. Furthermore, the "pore reconstruction" strategy offers an efficient pathway to explore potential NLO candidates with excellent comprehensive performances; in particular, it settles the conflicting issue of enhancing the band gap (>3.0 eV) and SHG intensity (>1.0 × AgGaS2) concurrently.

Broad transparency and wide band gap achieved in a magnetic infrared nonlinear optical chalcogenide by suppressing d-d transitions.

Magnetic infrared (IR) nonlinear optical (NLO) materials, particularly those containing d-block metals, have attracted considerable attention due to the contributions of d-orbitals to large NLO efficiency. However, the d-d transitions from the d-block metals lead to strong optical absorption and narrow band gap, seriously hindering their practical applications. The structural flexibility of salt-inclusion systems provides a good opportunity for modulating the crystal field of magnetic ions to suppress the d-d transitions but allowing the NLO-active d-s and d-p transitions. These ideas afford a new salt-inclusion sulfide [K3Cl][Mn2Ga6S12], which features a rare nanoporous [MnGa3S6]- framework with tunnels of inner diameter of 9.0 Å and possesses a broad transparency (0.39-25.0 µm) and the widest band gap (3.17 eV) among all magnetic IR NLO chalcogenides. Remarkably, it exhibits a strong phase-matchable second-harmonic generation intensity (0.8 × AgGaS2 at 1910 nm and 3.1 × AgGaS2 at 1064 nm) and a high laser-induced damage threshold (12.5 × AgGaS2 at 1064 nm), achieving the important criteria of an advanced IR NLO material.

AAg3Ga8Se14 (A = Rb, Cs): second-harmonic generation responses realized through the parallel arrangement of AgSe4 and GaSe4 tetrahedrons.

Two new quaternary selenides AAg3Ga8Se14, (A = Rb, 1; Cs, 2) were synthesised via solid-state reaction in sealed silica tubes. Compounds 1 and 2 crystallised in the monoclinic space group Cm (no. 8) and their three-dimensional [Ag3Ga8Se14]- anionic frameworks were comprised of AgSe4 and GaSe4 tetrahedrons. Their UV-Vis-near infrared diffuse reflectance spectra showed that 1 and 2 possessed wide band gaps of 2.17 and 2.10 eV, respectively. Notably, under incident laser irradiation at 1910 nm, compounds 1 and 2 presented moderate second-harmonic generation responses of 0.6 and 0.7 × AgGaS2, respectively, with phase-matching behaviours due to the parallel arrangement of nonlinear optical (NLO) functional tetrahedral AgSe4 and GaSe4 units. The laser-induced damage thresholds of 1 and 2 were estimated to be 25.4 and 18.0 MW cm-2, respectively, which were 2.1 and 1.5 times the threshold of AgGaS2. This study revealed that the title selenides, which were constructed from tetrahedral units arranged in a parallel array, are promising infrared NLO materials.

Photocurrent, humidity sensitivity and proton conductivity properties of a new sulfide semiconductor CsCuS4.

Copper chalcogenides have drawn considerable attention due to their prominent semiconducting properties. A new Cu-containing semiconductor, namely, CsCuS4 (1), was obtained by a halide salt flux method. Its structure featured 1D infinite ∞1[CuS4] - chains, where the polysulfide anion S42- was relatively rare in Cu chalcogenides. The compound was multifunctional and exhibited significant photocurrent, humidity sensitivity, and proton conductivity properties. Specifically, it exhibits an "on" state photocurrent response of 0.95 µA cm-2 and an "off" state photocurrent response of 0.55 µA cm-2 with good reversibility. The humidity-sensitive resistance in dry air (10% RH) could reach up to six orders of magnitude higher than that in wet air (100% RH). Compound 1 showed an activation energy of 0.19 eV and may have potential electrochemical applications.

Optimizing the Nonlinear Optical Performance of an A-N-M-Q (A: Alkali Metal; N: d10 Metal; M: Main Group Metal; Q: Chalcogen) System.

Exploring new infrared nonlinear optical (IR NLO) materials with superior overall properties is scientifically and technically important. However, large second-order harmonic generation (SHG) efficiencies and high laser-induced damage thresholds (LIDT) are incompatible, which makes realizing this goal a challenge. The IR NLO performance of an A-NIIB-MIIIA-Q (Q: chalcogen) system was optimized by simultaneously modulating A/(M + N) and M/N ratios (A: alkali metal; N, M: tetra-coordinated metals), and SHG-LIDT balance was achieved. Three new sulfides, KCd3Ga5S11 (1), RbCd4Ga3S9 (2), and Cs2Cd2Ga8S15 (3), containing the same CdS4 and GaS4 but with different A/(Ga + Cd) and Ga/Cd ratios were obtained. Among these compounds, compound 3 exhibits both the largest SHG efficiency (0.5 × AgGaS2) and LIDT (35 × AgGaS2), which can be ascribed to the Ga/Cd modulation for enhancing the NLO functional motif distortions and SHG efficiency as well as the A/(Ga + Cd) modulation for enlarging the band gap and LIDT. Remarkably, compound 3 is the first phase-matchable IR NLO material in the A-NIIB-MIIIA-Q family. This article proposes a novel avenue to explore infrared nonlinear materials with superior comprehensive properties by modulating the A/(M + N) and M/N ratios.

Balanced infrared nonlinear optical performance achieved by modulating the covalency and ionicity distributions in the electron localization function map.

The nonlinear optical (NLO) efficiency (dij) and laser-induced damage threshold (LIDT) of a material are mainly determined by their covalency and ionicity, respectively, the incompatibility between which makes balancing the dij and LIDT challenging in an IR NLO material. The topological feature (fractal dimension) of the electron localization function (ELF) map (distribution of covalency and ionicity) was evaluated for a series of NLO materials, and, phenomenologically, the fine mixing of covalency and ionicity will benefit a balanced dij and LIDT. Chemical bonds with different interaction strengths were introduced simultaneously to mix the covalency and iconicity finely, and three new IR NLO sulfides, A2Ba3Li6Ga28S49 (A = K, 1; Rb, 2; Cs, 3), were obtained. They exhibit a strong NLO efficiency (1.9-2.1 × AgGaS2 at 1064 nm and 0.5-0.6 × AgGaS2 at 1910 nm) and high LIDTs (16.7-18.0 × AgGaS2), which fulfill the criteria of being promising IR NLO candidates. This study provides a new method for designing high-performance IR NLO materials based on the topological features of the ELF.