[Ge 2 S 5 (S 2 )] 4− , A NLO‐Active Unit Leading to an Asymmetric Structure Discovered in Li 2 Cs 4 Ge 2 S 5 (S 2 )Cl 2 : An Experimental and Theoretical study

Excellent Nonlinear Optical M[M4 Cl][Ga11 S20 ] (M = A/Ba, A = K, Rb) Achieved by Unusual Cationic Substitution Strategy

The typical chalcopyrite AgGaQ2 (Q = S, Se) are commercial infrared (IR) second-order nonlinear optical (NLO) materials; however, they suffer from unexpected laser-induced damage thresholds (LIDTs) primairy due to their narrow band gaps. Herein, what sets this apart from previously reported chemical substitutions is the utilization of an unusual cationic substitution strategy, represented by [[SZn4 ]S12 + [S4 Zn13 ]S24 + 11ZnS4 ⇒ MS12 + [M4 Cl]S24 + 11GaS4 ], in which the covalent Sx Zny units in the diamond-like sphalerite ZnS are synergistically replaced by cationic Mx Cly units, resulting in two novel salt-inclusion sulfides, M[M4 Cl][Ga11 S20 ] (M = A/Ba, A = K, 1; Rb, 2). As expected, the introduction of mixed cations in the GaS4 anionic frameworks of 1 and 2 leads to wide band gaps (3.04 and 3.01 eV), which exceeds the value of AgGaS2 , facilitating the improvement of high LIDTs (9.4 and 10.3 × AgGaS2 @1.06 µm, respectively). Furthermore, compounds 1 and 2 exhibit moderate second-harmonic generation intensities (0.84 and 0.78 × AgGaS2 @2.9 µm, respectively), mainly originating from the orderly packing tetrahedral GaS4 units. Importantly, this study demonstrates the successful application of the cationic substitution strategy based on diamond-like structures to provide a feasible chemical design insight for constructing high-performance NLO materials.

Improved Birefringence Activated by Tetrahedra Decorated with a Single Linear Unit

Performance enhancement induced by structural modification has long been the target in materials science fields. Direct evidence to witness the effectivity of one strategy is challenging and necessary. In this work, a tetrahedra-decoration strategy was proposed to improve the birefringent performance sharply, namely decorating the tetrahedra with a single linear [S2 ] unit. The strategy was verified by comprehensive characterization of two thiogermanates K2 BaGeS4 and K2 BaGeS5 , which crystallize in the same space group, have similar unit cells and the same units arrangements. Theoretical characterization verified that the [GeS5 ] group has much larger polarization anisotropy than [GeS4 ], further demonstrated that the linear [S2 ] led to the sharp birefringence enlargement of K2 BaGeS5 (0.19 vs 0.03 of K2 BaGeS4 ). This work provides a new guiding thought to improve the birefringence performance.

KHg4Ga3S9: A Hg-Based Sulfide with Nonlinear-Optical Activity in the A-MII-MIII-Q (A = Alkali Metal; MII = d10 Metal; MIII = Ga, In; Q = S, Se) System

The search for new high-performance infrared (IR) nonlinear-optical (NLO) materials is a hot topic in the fields of laser chemistry and inorganic solid-state chemistry. Here, a new Hg-based sulfide KHg4Ga3S9 in the family of A-MII-MIII-Q (A = alkali metal; MII = d10 metal; MIII = Ga, In; Q = S, Se) was synthesized. It crystallizes in the orthogonal system of the C2221 structure, which is rare for IR NLO chalcogenides. Its anionic framework {[Hg4Ga3S9]-}∞ is constructed by two types of interconnected helical chains, viz., the inner layer ({[Hg6Ga2S29/3]4/3-}∞) and the outer layer ({[Hg2Ga4S25/3]2/3-}∞). It exhibits a moderate NLO response and a high laser-induced damage threshold. Theoretical calculations indicate that the HgS4 unit accounts for its much larger NLO response compared to RbCd4Ga3S9. The influence of alkali metals and d10 metals on the initial phase-matching wavelength is also discussed. This work provides inspiration for improving the properties of NLO materials' properties.

Wide band gap thiophosphates ASrPS4 (A = Li, Na, K, Rb, Cs): cation size effect induced successive structural transformation

Metal thiophosphates have aroused much research interest due to their structural chemistry and possible applications as infrared functional materials. In this study, five quaternary Sr-based alkali metal thiophosphates ASrPS₄ (A = Li, Na, K, Rb, Cs) were obtained. Their structural comparison shows that their symmetry undergoes transformation from tetragonal (I4₁/acd) to monoclinic (P2₁/c) to orthorhombic (Pnma) system, which is induced by the cation size effects and coordination features of different alkali metal cations. The experimental and theoretical results demonstrate that the band gaps of all title compounds are large, namely 3.6-3.9 eV (experimental results) and 3.78-4.12 eV (HSE06). Theoretical analyses indicate that the [PS₄] group could be regarded as a good unit for designing wide band gap compounds, and the birefringence of NaSrPS₄ is 0.08 at the fundemental 1064 nm wavelength, which shows that it may be a potential infrared birefringent material.

AIn4S6Cl (A = Rb and Cs) and Pb5Sn3Q10Cl2 (Q = S and Se): quaternary chalcohalides with mixed anionic coordination exhibit photocurrent responses

Mixed-anionic compounds have caught considerable attention due to their flexible coordination manners and abundant physical properties. Four new chalcohalides RbIn₄S₆Cl (1), CsIn₄S₆Cl (2), Pb₅Sn₃S₁₀Cl₂ (3) and Pb₅Sn₃Se₁₀Cl₂ (4) were successfully obtained by the high-temperature halide salt flux method. Compounds 1 and 2 have layered structures that consist of octahedral InS₆ and aliovalent-anionic InS₃Cl units. Compounds 3 and 4 feature 3-D structural frameworks built by [Pb₄SnQ₈Cl₄]^6- and [PbSn₂Q₆]^2- (Q = S and Se) polyhedral chains, in which partial Pb²⁺ cations are coordinated by Q^2- and Cl^- anions. Compounds 1-4 have optical band gaps close to the wavelength range of visible light and exhibit significant photocurrent responses of 28.75 nA cm^-2, 55.12 nA cm^-2, 19.58 mA cm^-2, and 36.12 μA cm^-2 with on/off ratios 30.0, 2.5, 15.7 and 2.6, respectively, implying their potential for photovoltaic applications. To the best of our knowledge, compound 3 has the largest photocurrent response among all non-oxides. In addition, the activation energies of 1-4 are well below 0.3 eV, which makes these compounds interesting for potential applications in electrochemical devices. This work sheds light on the exploration of promising photocurrent response materials in the mixed-anionic compound system.

Na6 MQ4 (M=Zn, Cd; Q=S, Se): Promising New Ternary Infrared Nonlinear Optical Materials

Four sodium-based ternary IR nonlinear optical (NLO) materials, Na₆ MQ₄ (M=Zn, Cd; Q=S, Se), were prepared through a high-temperature flux method. The crystal structure of the compounds is built up of isolated [MQ₄ ] tetrahedra and a 3D framework formed by the NaQ_n (n=4, 5) units. The two selenides, Na₆ MSe₄ (M=Zn, Cd), as promising IR NLO materials, show moderate second-harmonic generation (SHG) responses (0.9 and 0.5×AgGaS₂ ) with good phase-matching behavior, as well as high laser damage thresholds (2 and 1.9×AgGaS₂ ). The two sulfides, Na₆ MS₄ (M=Zn, Cd), exhibit higher laser damage thresholds (13 and 4×AgGaS₂ ), but smaller SHG responses (0.3 and 0.2×AgGaS₂ ). Theoretical calculations and statistical analyses indicate that the SHG effect and band gap in the compounds originate mainly from the distorted NaQ₄ NLO-active units with a short Na-S bond length, which provides a new insight into the design of novel IR NLO materials.