Large Mid-IR Second-Order Nonlinear-Optical Effects Designed by the Supramolecular Assembly of Different Bond Types without IR Absorption

Centrosymmetric CaBaMF8 and Noncentrosymmetric Li2CaMF8 (M = Zr, Hf): Dimension Variation and Nonlinear Optical Activity Resulting from an Isovalent Cation Substitution-Oriented Design

Zirconium/hafnium fluorides have recently been recognized as potential nonlinear optical (NLO) materials with short ultraviolet (UV) cutoff edges, which is significant in laser science and industry. The synthesis of noncentrosymmetric (NCS) materials based on centrosymmetric (CS) compounds by an isovalent cation substitution-oriented design is an emerging strategy in the NLO territory. Here, two isostructural and novel fluorides, CaBaMF8 (M = Zr (1), Hf (2)), have been synthesized through the combination of alkaline earth metals, zirconium/hafnium, and fluorine elements. They feature zero-dimensional and CS structures composed by an isolated MF8 (M = Zr, Hf) dodecahedron and dissociative Ca2+ and Ba2+ cations, and they display short UV cutoff edges (<200 nm) as well. Two three-dimensional fluorides Li2CaMF8 (M = Zr (3), Hf (4)) are obtained by replacing Ba with alkali metal Li atom, which not only represent phase-matchable second-harmonic-generation activities (0.36, 0.30× KH2PO4 (KDP)) at 1064 nm but also maintain short UV cutoff edges with high reflectance. This work has largely enriched the family of NCS zirconium/hafnium fluorides reaching the short UV region.

The first polyanion-substitution-driven centrosymmetric-to-noncentrosymmetric structural transformation realizing an excellent nonlinear optical supramolecule [Cd4P2][CdBr4]

Crystallographically, noncentrosymmetricity (NCS) is an essential precondition and foundation of achieving nonlinear optical (NLO), pyroelectric, ferroelectric, and piezoelectric materials. Herein, structurally, octahedral [SmCl6]3- is substituted by the acentric tetrahedral polyanion [CdBr4]2-, which is employed as a templating agent to induce centrosymmetric (CS)-to-NCS transformation based on the new CS supramolecule [Cd5P2][SmCl6]Cl (1), thereby providing the NCS supramolecule [Cd4P2][CdBr4] (2). Meanwhile, this replacement further results in the host 2D ∞2[Cd5P2]4+ layers converting to yield the twisted 3D ∞3[Cd4P2]2+ framework, which promotes the growth of bulk crystals. Additionally, phase 2 possesses well-balanced NLO properties, enabling considerable second-harmonic generation (SHG) responses (0.8-2.7 × AgGaS2) in broadband spectra, the thermal expansion anisotropy (2.30) together with suitable band gap (2.37 eV) primarily leading to the favorable laser-induced damage threshold (3.33 × AgGaS2), broad transparent window, and sufficient calculated birefringence (0.0433) for phase-matching ability. Furthermore, the first polyanion substitution of the supramolecule plays the role of templating agent to realize the CS-to-NCS transformation, which offers an effective method to rationally design promising NCS-based functional materials.

Symmetry breaking of A3M2X9-type perovskite derivatives induced by polar quaternary ammonium cations: achieving efficient nonlinear optical properties

Low-dimensional organic-inorganic metal halides, especially lead-free perovskites, are attracting increasing attention because of their environmentally friendly processing, flexible structures, chemical stability, and promising nonlinear optical properties. Herein, we report a new stable polar 0D lead-free hybrid bismuth chloride to enable the second-harmonic generation (SHG) active material (BTA)₃Bi₂Cl₉ (BTA = benzyltriethylammonium, C₆H₅CH₂N(C₂H₅)₃⁺) that was obtained by the antisolvent vapor diffusion method and crystallized in the polar Cc space group. Its structure features organic cations surrounded by face-sharing [Bi₂Cl₉]^3- dimers. (BTA)₃Bi₂Cl₉ exhibits a wide direct bandgap (3.21 eV) and a strong phase-matchable SHG conversion efficiency (1.39 × KDP). Theoretical calculation reveals that the SHG response is owing to the synergistic effect of distorted inorganic [Bi₂Cl₉]^3- anions and polar organic BTA⁺ cations. This work not only enriches the family of organic-inorganic A₃M₂X₉ (A = monovalent cations; M = trivalent metal ions; and X = halide ions) NLO crystals but also provides the possibilities for further designing novel lead-free semiconducting piezoelectric, pyroelectric and ferroelectric materials.

Material research from the viewpoint of functional motifs

As early as 2001, the need for the ‘functional motif theory’ was pointed out to assist the rational design of functional materials. The properties of materials are determined by their functional motifs and by how they are arranged in the materials. Uncovering the functional motifs and their arrangements is crucial in understanding the properties of materials and rationally designing new materials of desired properties. The functional motifs of materials are the critical microstructural units (e.g. constituent components and building blocks) that play a decisive role in generating certain material functions, and could not be replaced with other structural units without losing or significantly suppressing the relevant functions. The role of functional motifs and their arrangements in materials with representative examples was presented. These examples could be classified into six types of material microscopic structures on a length scale smaller than ∼10 nm with maximum subatomic resolution, i.e. the crystal, magnetic, aperiodic, defect, local, and electronic structures. The method of functional motif analysis could be employed in the function-oriented design of materials, as elucidated by taking infrared nonlinear optical materials as an example. Machine learning is more efficient in predicting material properties and screening materials with high efficiency than high-throughput experimentation and high-throughput calculations. In extracting the functional motifs and finding their quantitative relationships, developing sufficiently reliable databases for material structures and properties is imperative.

SnPQ3 (Q = S, Se, S/Se): A Series of Lone-Pair Cationic Chalcogenophosphates Exhibiting Balanced NLO Activity Originating from SnQ8 Units

Two chalcogenophosphates, SnPS_2.86Se_0.14 (1) and SnPSe₃ (2), are isostructural and crystallize in the monoclinic noncentrosymmetric space group Pn. Their three-dimensional (3D) structures are constructed by [Sn(1)Q₈] hendecahedra and [Sn(2)Q₈] dodecahedra by sharing Q vertices and edges, leaving cavities for isolated [P₂Q₆] (Q = S/Se, Se) dimers. A second-harmonic-generation (SHG) measurement indicates that 1 is phase-matchable with a response of approximately 1.2 × AgGaS₂ (AGS), which is verified by the theoretical calculation result. The powder sample of 1 exhibits a high laser-induced damage threshold of 3.9 × AGS. For comparison, the known SnPS₃ (3) was also synthesized and evaluated using the same method. The chemical composition-NLO performance relationship of 1-3 is also discussed. Dipole moment calculation results suggest that [SnQ₈] polyhedra make the main contribution to their excellent nonlinear optical (NLO) performance.

A Series of Pentanary Salt-Inclusion Chalcogenoborates Containing a B12Q12 (Q = S, Se) Cluster Exhibiting a Kleinman-Forbidden Frequency-Doubling Effect

A series of pentanary chalcogenoborates (A₃X)[InB₁₂(InQ₄)₃] (A = K, Rb, Cs; X = Cl, Br, I; Q = S, Se) were obtained by high-temperature solid-state reactions. These salt-inclusion chalcogenides crystallize in the hexagonal space group P6₃22, a type of Kleinman-forbidden noncentrosymmetric structure. Their structures feature a [InB₁₂(InQ₄)₃] open framework built by InQ₆ octahedra and InQ₄ tetrahedra consolidated B₁₂ cluster, which accomodates octahedral cavities for XA₆ units. They are second-harmonic generation active, and their optical properties were studied experimentally and theoretically. This work can evoke more interest in chalcogenoborate-based second-order nonlinear optical materials.

Structural modulation induced by MIIIA metals in Ba3MQ4X (M = Al, Ga, In; Q = S, Se; X = Cl, Br): an experimental and computational analysis

The optical properties of Ba₃MQ₄X (M = Al, Ga, In; Q = S, Se; X = Cl, Br) have been studied and their structures have been compared.

Advantageous Units in Antimony Sulfides: Exploration and Design of Infrared Nonlinear Optical Materials

The exploration of infrared (IR) nonlinear optical (NLO) materials remains attractive because of the urgent requirements in the laser field. Meanwhile, the deepened cognition of structure-property relationships is necessary to help guide the exploration of IR NLO materials. So far, the family of antimony sulfides is an important system with a lot of attention, and a series of antimony sulfides are reported. However, it is urgent to reveal how different Sb-S units, like SbS₃, SbS₄, and more complex combinations, affect apparent properties. Here, taking ternary metal antimony sulfides as examples, the sources of some essential optical properties, such as second harmonic generation (SHG) and birefringence, are systematically analyzed through first-principles calculations, and the mechanisms of the performances with various magnitudes are also presented to clarify the structure-property relationships. The results indicate that the SbS₄ unit among antimony sulfides is an advantageous NLO-active unit, which can balance the contradiction between the band gap and SHG response. Introduction of transition metals in the Sb-S anionic frameworks can tune the magnitude of birefringence. Besides, the substitution of a cation from a transition metal to an alkali metal can notably enlarge the band gap and maintain a large SHG response. These design strategies are beneficial to explore potential IR NLO materials with Sb-S units.

A new pnictidehalide with van der Waals host–guest interactions exhibiting both geometric spin frustration and resistive humidity sensitivity

The structure and electric and magnetic properties of a new metal pnictidehalide (Hg₆P₄)(CrCl₆)Cl are reported in detail. The weak van der Waals interactions in its structure are responsible for its geometric spin frustration and good resistive humidity sensitivity.

Partial Geometric Frustration in Inorganic Supramolecular Spin Systems with One-Dimensional Trigonally Aligned Magnetic Chains ∞1(MCl4)2- (M = Fe2+, Co2+)

Exploration of new spin systems with low-dimensional subunits have been of great interest in the past decades due to their interesting physical properties and potential applications in molecular spintronics. Two inorganic supramolecular complexes, (Hg₃S₂)(FeCl₄) and (Hg₃S₂)(CoCl₄), with trigonally aligned 1-D infinite magnetic _∞¹(FeCl₄)²⁻ or _∞¹(CoCl₄)²⁻ chains have been prepared by solid-state reactions. They exhibit 3-D long-range spin order with strong field dependence and field induced metamagnetic behavior. The intrachain and interchain magnetic coupling constants were estimated by DFT+U and DFT+U+SOC calculations and the both complexes can be regarded as partially frustrated spin systems. The spin Hamiltonian was constructed, the ground state is proposed to be incommensurate spiral spin order, which differs from the commensurate 120° spin structure ground state of fully frustrated trigonal case by a little canted angle. This study shows that cooperative magnetic ordering induced by geometric frustration can be realized in inorganic supramolecular systems assembled by weak van der Waals’ interactions.