PbBeB2O5: A High-Performance Ultraviolet Nonlinear-Optical Crystal with Functional [BeB2O8]8- Group.

High-performance nonlinear-optical (NLO) crystals need to simultaneously meet multiple basic and conflicting performance requirements. Here, by using a partial chemical substitution strategy, the first noncentrosymmetric (NCS) PbBeB2O5 crystal with a BeB2O8 group was synthesized, exhibiting a two-dimensional [BeB2O5]∞ layer constructed by interconnecting BeB2O8 groups and bridged PbO4 with an active lone pair. The crystal shows a promising UV NLO functional feature, including a strong SHG effect of 3.5 × KDP (KH2PO4), large birefringence realizing phase matchability in the whole transparency region from 246 to 2500 nm, a short UV absorption edge of 246 nm, and single-crystal easy growth. Remarkably, theoretical studies reveal that the BeB2O8 group has high nonlinear activity, which could stimulate the discovery of a series of excellent NLO beryllium borates.

Phase Transitions and Nonlinear Optical Property Modifications in BaGa4Se7.

Phase transitions can change the crystal structure and modify the physical properties of crystals. In this work, we investigate the phase transition behavior in BaGa4Se7, an important middle infrared (mid-IR) nonlinear optical (NLO) crystal, in the temperature range from room temperature to 1173 K. Interestingly, the BaGa4Se7 crystal undergoes a reversible ferroelastic phase transition at T = 528 K, resulting in the presence of a newly discovered phase (γ-phase) at the higher temperature. The experimental temperature dependence of optical birefringence, as well as the first-principles birefringence and NLO coefficients, reveals that the γ-phase exhibits larger birefringence and better NLO properties compared with those of the low-temperature phase (α-phase). This work demonstrates that phase-transition-induced structural modification can improve the mid-IR NLO properties, which would provide an effective avenue to obtain materials with good optoelectronic performance.

An Unprecedented [BO2]-Based Deep-Ultraviolet Transparent Nonlinear Optical Crystal by Superhalogen Substitution.

Solid-state structures with the superhalogen [BO2]- have thus far only been observed with a few compounds whose syntheses require high reaction temperatures and complicated procedures, while their optical properties remain almost completely unexplored. Herein, we report a facile, energy-efficient synthesis of the first [BO2]-based deep-ultraviolet (deep-UV) transparent oxide K9[B4O5(OH)4]3(CO3)(BO2)·7H2O (KBCOB). Detailed structural characterization and analysis confirm that KBCOB possesses a rare four-in-one three-dimensional quasi-honeycomb framework, with three π-conjugated anions ([BO2]-, [BO3]3-, and [CO3]2-) and one non-π-conjugated anion ([BO4]5-) in the one crystal. The evolution from the  traditional halogenated nonlinear optical (NLO) analogues to KBCOB by superhalogen [BO2]- substitution confers deep-UV transparency (< 190 nm), a large second-harmonic generation response (1.0 × KH2PO4 @ 1064 nm), and a 15-fold increase in birefringence. This study affords a new route to the facile synthesis of functional [BO2]-based oxides, paving the way for the development of next-generation high-performing deep-UV NLO materials.

Wide-Band Gap Binary Semiconductor P3N5 with Highly Anisotropic Optical Linearity and Nonlinearity.

Wide-band gap binary semiconductors find extensive use in advanced optoelectronic devices due to their exceptional electronic, optical, and defect properties. This paper systematically investigates the linear and nonlinear optical and defect properties of two P3N5 structures as wide-band gap binary semiconductors and evaluates their responses to external pressure modulation using first-principles calculations. The research demonstrates that the high-pressure phase of P3N5 has a broad UV solar-blind band gap (Eg ∼ 4.9 eV) and displays highly anisotropic optical linearity and nonlinearity, including a significant second harmonic generation effect (d24 ∼ 1.8 pm/V) and large birefringence (Δn ∼ 0.12), exhibiting a relatively small change in amplitude against pressure due to unique lattice incompressibility. This material enables birefringent phase-matched second harmonic coherent output at a much shorter wavelength (down to 286 nm) than currently known wide-band gap binary semiconductors such as SiC, GaN, AlN, Ga2O3, and Si3N4. An in-depth study of the defect properties of P3N5 in relation to its UV optical properties is also provided. These results are important references for utilizing the optoelectronic functions of this binary material system.

Record Second-Harmonic Generation and Birefringence in an Ultraviolet Antimonate by Bond Engineering.

Oxides have attracted considerable attention owing to their potential for nonlinear optical (NLO) applications. Although significant progress has been achieved in optimizing the structural characteristics of primitives (corresponding to the simplest constituent groups, namely, cations/anions/neutral molecules) comprising the crystalline oxides, the role of the primitives' interaction in determining the resultant functional structure and optical properties has long been underappreciated and remains unclear. In this study, we employ a π-conjugated organic primitive confinement strategy to manipulate the interactions between primitives in antimonates and thereby significantly enhance the optical nonlinearity. Chemical bonds and relatively weak H-bonding interactions promote the formation of cis- and trans-Sb(III)-based dimer configurations in (C5H5NO)(Sb2OF4) (4-HPYSOF) and (C5H7N2)(Sb2F7) (4-APSF), respectively, resulting in very different second-harmonic generation (SHG) efficiencies and birefringences. In particular, 4-HPYSOF displays an exceptionally strong SHG response (12 × KH2PO4 at 1064 nm) and a large birefringence (0.513 at 546 nm) for a Sb(III)-based NLO oxide as well as a UV cutoff edge. Structural analyses and theoretical studies indicate that polarized ionic bond interactions facilitate the favorable arrangement of both the inorganic and organic primitives, thereby significantly enhancing the optical nonlinearity in 4-HPYSOF. Our findings shed new light on the intricate correlations between the interactions of primitives, inorganic primitive configuration, and SHG properties, and, more broadly, our approach provides a new perspective in the development of advanced NLO materials through the interatomic bond engineering of oxides.

A Conjugated Phenylene Nanocage with a Guest-Adaptive Deformable Cavity.

The highly strained, phenylene-derived organic cages are typically regarded as very rigid entities, yet their deformation potential and supramolecular properties remain underexplored. Herein, we report a pliable conjugated phenylene nanocage by synergistically merging rigid and flexible building blocks. The anisotropic cage molecule contains branched phenylene chains capped by a calix[6]arene moiety, the delicate conformational changes of which endow the cage with a remarkably deformable cavity. When complexing with fullerene guests, the cage showcases excellent guest-adaptivity, with its cavity volume capable of swelling by as much as 85 %.

Synthesis, Structure, and Optical Properties of a 0D Hybrid Organic-Inorganic Metal Halide (C5N2H14Cl)GeCl3.

Due to the flexible structural tunability and excellent photoelectric performance, hybrid organic-inorganic metal halides (OIMHs) have attracted intensive attention and become a hot topic in the field of materials. It is important and necessary to explore new OIMHs and study their structure-property relationship. In this work, a new lead-free OIMH, (C5N2H14Cl)GeCl3, is synthesized by the combination of hydrothermal and solution methods. This compound features a zero-dimensional structure composed of inorganic [GeCl3]- trigonal pyramids surrounded by isolated Cl- anions and organic (C5N2H14)2+ cations. Preliminary characterization and first-principles calculations are performed to study its basic optical properties. Interestingly, (C5N2H14Cl)GeCl3 shows weak blue emission under ultraviolet excitation, and the intrinsic mechanism is discussed.

Giant Optical Anisotropy in a Covalent Molybdenum Tellurite via Oxyanion Polymerization.

Large birefringence is a crucial but hard-to-achieve optical parameter that is a necessity for birefringent crystals in practical applications involving modulation of the polarization of light in modern opto-electronic areas. Herein, an oxyanion polymerization strategy that involves the combination of two different types of second-order Jahn-Teller distorted units is employed to realize giant anisotropy in a covalent molybdenum tellurite. Mo(H2O)Te2O7 (MTO) exhibits a record birefringence value for an inorganic UV-transparent oxide crystalline material of 0.528 @ 546 nm, which is also significantly larger than those of all commercial birefringent crystals. MTO has a UV absorption edge of 366 nm and displays a strong powder second-harmonic generation response of 5.4 times that of KH2PO4. The dominant roles of the condensed polytellurite oxyanions [Te8O20]8- in combination with the [MoO6]6- polyhedra in achieving the giant birefringence in MTO are clarified by structural analysis and first-principles calculations. The results suggest that polymerization of polarizability-anisotropic oxyanions may unlock the promise of birefringent crystals with exceptional birefringence.

Unusual Pressure-Induced Self-Trapped Exciton to Free Exciton Transfer in Chiral 2D Lead Bromide Perovskites.

The phenomenon of pressure-induced emission alterations related to complex excitonic dynamics in 2D lead halide perovskites (LHPs) has gained considerable attention for understanding their structure-property relationship and obtaining inaccessible luminescence under ambient conditions. However, the well-known pressure-induced emissions are limited to the formation of self-trapped excitons (STEs) due to the structural distortion under compression, which goes against the advantage of the highly pure emission of LHPs. Here, the pressure-induced detrapping from STEs to free excitons (FEs) accompanied by the dramatic transition from broadband orangish emission to narrow blue emission has been achieved in chiral 2D LHPs and R- and S-[4MeOPEA]2PbBr4, (4MeOPEA = 4-methoxy-α-methylbenzylammonium). The combined experimental and calculated results reveal that the distortion level of PbBr6 octahedra of R- and S-[4MeOPEA]2PbBr4 exhibits an unusually significant reduction as the applied pressure increases, which leads to decreased electron-phonon coupling and self-trapped energy barrier and consequently enables the detrapping of STEs to FEs. This work illustrates the dramatic exciton transfer in 2D LHPs and highlights the potential for realizing highly efficient and pure light emissions by manipulating the structural distortion via strain engineering.

A Non-π-Conjugated Molecular Crystal with Balanced Second-Harmonic Generation, Bandgap, and Birefringence.

Traditional nonlinear optical (NLO) crystals are exclusively limited to ionic crystals with π-conjugated groups and it is a great challenge to achieve a subtle balance between second-harmonic generation, bandgap, and birefringence for them, especially in the deep-UV spectrum region (Eg  > 6.20 eV). Herein, a non-π-conjugated molecular crystal, NH3 BH3 , which realizes such balance with a large second-harmonic generation response (2.0 × KH2 PO4 at 1064 nm, and 0.45 × ß-BaB2 O4 at 532 nm), deep-UV transparency (Eg > 6.53 eV), and moderate birefringence (Δn = 0.056@550 nm) is reported. As a result, NH3 BH3 exhibits a large quality factor of 0.32, which is evidently larger than those of non-π-conjugated sulfate and phosphate ionic crystals. Using an unpolished NH3 BH3 crystal, effective second-harmonic generation outputs are observed at different wavelengths. These attributes indicate that NH3 BH3 is a promising candidate for deep-UV NLO applications. This work opens up a new door for developing high-performance deep-UV NLO crystals.

Birefringent Dispersion Optimization to Achieve Superior Nonlinear Optical Phase Matching in Deeper Solar-Blind UV Band from KH2PO4 to BeH3PO5.

Nonlinear-optical (NLO) crystals require birefringent phase matching (BPM), particularly in the solar-blind ultraviolet (UV) (200-280 nm) and deep-UV (100-200 nm) regions. Achieving BPM requires optimization of optical dispersion along with having large birefringence. This requirement is especially critical for structures with low optical anisotropy, including classical phosphate UV-NLO crystals like KH2PO4 (KDP). However, there is a scarcity of in-depth theoretical analysis and general design strategies based on structural chemistry to optimize dispersion. This study presents findings from a simplified dielectric model that uncover two vital factors to micro-optimize transparent optical dispersion: effective mass (m*) of excited states and effective number (N*) of photo-responsive states. Smoothing of dispersion occurs as m* increases and N* decreases. First-principles analysis of deep-UV KBe2BO3F2-family structures is used to confirm the conciseness and validity of the model. It further proposes substituting K+ with Be2+ to decrease N* and increase m* while enlarging bandgap. This will lead to improved dispersion and an overall enhancement of KDP's BPM capability. The existing BeH3PO5 (BDP) is predicted to improve the shortest BPM wavelength for second-harmonic generation, from 251 nm in KDP to 201 nm in BDP. BDP's extension into the broader UV solar-blind waveband fully supports the proposed optimization strategy.

A Machine Learning Study on High Thermal Conductivity Assisted to Discover Chalcogenides with Balanced Infrared Nonlinear Optical Performance.

Exploration of novel nonlinear optical (NLO) chalcogenides with high laser-induced damage thresholds (LIDT) is critical for mid-infrared (mid-IR) solid-state laser applications. High lattice thermal conductivity (κL ) is crucial to increasing LIDT yet often neglected in the search for NLO crystals due to lack of accurate κL data. A machine learning (ML) approach to predict κL for over 6000 chalcogenides is hereby proposed. Combining ML-generated κL data and first-principles calculation, a high-throughput screening route is initiated, and ten new potential mid-IR NLO chalcogenides with optimal bandgap, NLO coefficients, and thermal conductivity are discovered, in which Li2 SiS3 and AlZnGaS4 are highlighted. Big-data analysis on structural chemistry proves that the chalcogenides having dense and simple lattice structures with low anisotropy, light atoms, and strong covalent bonds are likely to possess higher κL . The four-coordinated motifs in which central cations show the bond valence sum of +2 to +3 and are from IIIA, IVA, VA, and IIB groups, such as those in diamond-like defect-chalcopyrite chalcogenides, are preferred to fulfill the desired structural chemistry conditions for balanced NLO and thermal properties. This work provides not only an efficient strategy but also interpretable research directions in the search for NLO crystals with high thermal conductivity.

Crystallography, Charge Transfer, and Two-Photon Absorption Relations in Molecular Cocrystals for Two-Photon Excited Fluorescence Imaging.

Two-photon excited fluorescence imaging requires high-performance two-photon absorption (TPA) active materials, which are commonly intramolecular charge transfer systems prepared by traditional chemical synthesis. However, this typically needs harsh conditions and new methods are becoming crucial. In this work, based on a collaborative intermolecular charge transfer (inter-CT) strategy, three centimeter-sized organic TPA cocrystals are successfully obtained. All three cocrystals exhibit a mixed stacking arrangement, which can effectively generate inter-CT between the donor and acceptor. The ground and excited state characterizations compare their inter-CT ability: 1,2-BTC > 2D-BTC > 1D-BTC. Transient absorption spectroscopy detects TCNB•-, indicating that the TPA mechanism arises from molecular polarization caused by inter-CT. Meanwhile, 1,2-BTC exhibits the highest excited-state absorption and the longest excited-state lifetime, suggesting a stronger TPA response. First-principles calculations also confirm the presence of inter-CT interactions, and the significant parameter Δµ which can assess the TPA capability indicates that inter-CT enhances the TPA response. Besides, cocrystals also demonstrate excellent water solubility and two-photon excited fluorescence imaging capabilities. This research not only provides an effective method for synthesizing TPA crystal materials and elucidates the connection between inter-CT ability and TPA property but also successfully applies them in the fields of multi-photon fluorescence bioimaging.

Secondary-Bond-Driven Construction of a Polar Material Exhibiting Strong Broad-Spectrum Second-Harmonic Generation and Large Birefringence.

Considerable effort has been invested in the development of non-centrosymmetric (NCS) inorganic solids for ferroelectricity-, piezoelectricity- and, particularly, optical nonlinearity-related applications. While great progress has been made, a persistent problem is the difficulty in constructing NCS materials, which probably stems from non-directionality and unsaturation of the ionic bonds between metal counter-cations and covalent anionic modules. We report herein a secondary-bond-driven approach that circumvents the cancellation of dipole moments between adjacent anionic modules that has plagued second-harmonic generation (SHG) material design, and which thereby affords a polar structure with strong SHG properties. The resultant first NCS counter-cation-free iodate, VO2 (H2 O)(IO3 ) (VIO), a new class of iodate, crystallizes in a polar lattice with ∞ 1 [ ${{}_{{\rm { \infty }}}{}^{{\rm { 1}}}{\rm { [}}}$ VO2 (H2 O)(IO3 )] zigzag chains connected by weak hydrogen bonds and intermolecular forces. VIO exhibits very large SHG responses (18 × KH2 PO4 @ 1200 nm, 1.5 × KTiOPO4 @ 2100 nm) and sufficient birefringence (0.184 @ 546 nm). Calculations and crystal structure analysis attribute the large SHG responses to consistent polarization orientations of the ∞ 1 [ ${{}_{{\rm { \infty }}}{}^{{\rm { 1}}}{\rm { [}}}$ VO2 (H2 O)(IO3 )] chains controlled by secondary bonds. This study highlights the advantages of manipulating the secondary bonds in inorganic solids to control NCS structure and optical nonlinearity, affording a new perspective in the development of high-performance NLO materials.

NaMoO3(IO3)(H2O): water molecule introduction induces strong second harmonic generation response, widened band gap and large anisotropy.

Exploring feasible tactics to induce the formation of non-centrosymmetric (NCS) structures, especially from centrosymmetric (CS) structures, is essential for the development of nonlinear optical crystals with more potential. An NCS alkali metal-containing molybdenum iodate hydrate, namely, NaMoO3(IO3)(H2O), was designed based on the CS matrix NaMoO3(IO3) via introducing a water molecule into the structure. The introduction of one crystalline water molecule results in the rearrangement of Λ-shaped cis-[MoO4(IO3)2] units, and the proper array of the cis-[MoO4(IO3)2] units in NaMoO3(IO3)(H2O) results in its strong SHG response of 4.6 × KH2PO4. In addition, NaMoO3(IO3)(H2O) exhibits a wider optical bandgap of 3.44 eV and a larger birefringence of 0.231 than its matrix. Furthermore, the framework of NaMoO3(IO3)(H2O) is highly similar to that of α-KMoO3(IO3), with water molecules assisting Na+ cations in occupying the position of K+. However, due to the extra hydrogen bond of water molecules, the [MoO3(IO3)]∞ layers in NaMoO3(IO3)(H2O) retain a parallel-stacking arrangement, different from the antiparallel arrangement of layers in α-KMoO3(IO3) with a centric structure. This study confirms the feasibility of applying a water molecule to adjust the orientation of basic building block units to assemble an NCS structure based on CS crystals.

Realizing Persistent Zero Area Compressibility over a Wide Pressure Range in Cu2 GeO4 by Microscopic Orthogonal-Braiding Strategy.

Zero area compressibility (ZAC) is an extremely rare mechanical response that exhibits an invariant two-dimensional size under hydrostatic pressure. All known ZAC materials are constructed from units in two dimensions as a whole. Here, we propose another strategy to obtain the ZAC by microscopically orthogonal-braiding one-dimensional zero compressibility strips. Accordingly, ZAC is identified in a copper-based compound with a planar [CuO4 ] unit, Cu2 GeO4 , that possesses an area compressibility as low as 1.58(26) TPa-1 over a wide pressure range from ≈0 GPa to 21.22 GPa. Based on our structural analysis, the subtle counterbalance between the shrinkage of [CuO4 ] and the expansion effect from the increase in the [CuO4 ]-[CuO4 ] dihedral angle attributes to the ZAC response. High-pressure Raman spectroscopy, in combination with first-principles calculations, shows that the electron transfer from in-plane bonding dx 2 -y 2 to out-of-plane nonbonding dz 2 orbitals within copper atoms causes the counterintuitive extension of the [CuO4 ]-[CuO4 ] dihedral angle under pressure. Our study provides an understanding on the pressure-induced structural evolution of copper-based oxides at an electronic level and facilitates a new avenue for the exploration of high-dimensional anomalous mechanical materials.

Photoluminescence and Nonlinear Optical Properties of Two Terpyridine-Based Hybrid Zn/Cd Halides.

Hybrid metal halides (HMHs) with low-dimensional structures have attracted increasing attention due to their striking optical properties. Herein, two new zero-dimensional HMHs have been fabricated by CdCl2/ZnCl2 and 4'-(4-pyridyl-phenyl)-2,2':6',2″-terpyridine (Tpy), including (TpyH3)[CdCl4][Cl] (Tpy-Cd) and (TpyH3)[ZnCl4][Cl] (Tpy-Zn). Their structures are consisted of a [TpyH3]3+ organic cation, an inorganic [ZnCl4] or [CdCl4] tetrahedron, and one isolated Cl- anion. Tpy-Cd crystallizes to a noncentrosymmetric structure and possesses a moderate second harmonic response of 0.72 × KH2PO4, while Tpy-Zn features a centrosymmetric space group. Though Tpy-Cd and Tpy-Zn crystallize into space groups of completely different symmetry due to distinct connection mode and molecular distortion, they display quite similar photoluminescence of bright green light emission under ultraviolet excitation, nearly identical in Stokes shift, photoluminescence quantum yield, decay lifetime, and energy. The photoluminescence quantum yields of green light emission were measured to be nearly 25%, outperforming most of the Cd/Zn low-dimensional HMHs.

Ultrashort Phase-Matching Wavelength and Strong Second-Harmonic Generation in Deep-UV-Transparent Oxyfluorides by Covalency Reduction.

The development of urgently-needed ultraviolet (UV)/deep-UV nonlinear optical (NLO) materials has been hindered by contradictory requirements of the microstructure, in particular the need for a strong second-harmonic generation (SHG) response as well as a short phase-matching (PM) wavelength. We herein employ a "de-covalency" band gap engineering strategy to adjust the optical linearity and nonlinearity. This has been achieved by assembling two types of transition-metal (TM) polyhedra ([TaO2 F4 ] and [TaF7 ]), affording the first tantalum-based deep-UV-transparent NLO materials, A5 Ta3 OF18 (A = K (KTOF), Rb (RTOF)). Experimental and theoretical studies reveal that the highly ionic bonds and strong electropositivity of tantalum in the two oxyfluorides induce record short PM wavelengths (238 (KTOF) and 240 (RTOF) nm) for d0 -TM-centered oxides, in addition to strong SHG responses (2.8 × KH2 PO4 (KTOF) and 2.6 × KH2 PO4 (RTOF)), and sufficient birefringences (0.092 (KTOF) and 0.085 (RTOF) at 546 nm). These results not only broaden the available strategies for achieving deep-UV NLO materials by exploiting the currently neglected d0 -TMs, but also push the shortest PM wavelength into the short-wavelength UV region.

Evolution of Structures and Optical Properties in a Series of Infrared Nonlinear Optical Crystals LixAg1-xInSe2 (0 ≤ x ≤ 1).

In this work, a number of new infrared nonlinear optical (NLO) crystals of LixAg1-xInSe2, in which the ratio x of Li/Ag varies in a wide range from 0 to 1, are investigated. Structural analysis reveals that the space group of LixAg1-xInSe2 evolved from I4̅2d in AgInSe2 to Pna21 in LiInSe2 as x increases from low values (0, 0.2, 0.37) to large values (0.55, 0.78, 0.81, 1). Compared to other Li/Ag coexisting chalcogenides such as LixAg1-xGaS2 and LixAg1-xGaSe2, the structural distortions in LixAg1-xInSe2 are much more prominent. This may explain the limited crystallization region in the phase graph of the tetragonal structure LixAg1-xInSe2. The fundamental optical absorption edges in these LixAg1-xInSe2 compounds are determined from the direct electronic transitions and the band gaps Eg gradually increase as the lithium content increases, consistent with the first-principles calculations. The composition x = 0.78 is calculated to have a good set of optical properties with a large NLO coefficient (dpowder = 28.8 pm/V) and moderate birefringence (Δn ∼ 0.04). Accordingly, the Li0.78Ag0.22InSe2 crystal is grown by the modified Bridgman-Stockbarger method, and it exhibits a wide transparency range from 0.546 to 14.3 µm at the 2% transmittance level.

Unexpected giant negative area compressibility in palladium diselenide.

Negative area compressibility (NAC) is a counterintuitive 'squeeze-expand' behavior in solids that is very rare but attractive due to possible pressure-response applications and coupling with rich physicochemical properties. Herein, NAC behavior is reported in palladium diselenide with a large magnitude and wide pressure range. We discover that, apart from the rigid flattening of layers that has been generally recognized, the unexpected giant NAC effect in PdSe2 largely comes from anomalous elongation of intralayer chemical bonds. Both structural variations are driven by intralayer-to-interlayer charge transfer with enhanced interlayer interactions under pressure. Our work updates the mechanical understanding of this anomaly and establishes a new guideline to explore novel compression-induced properties.