Polymorphism of Niobium Phosphate Bronze Na4Nb8P4O32 Deduced by the Packings of Structural Units: A Centrosymmetric Phase.

Herein, a new centrosymmetric phase Na4Nb8P4O32 (referred to as CS-Na4Nb8P4O32) was obtained by a molten salt method, which is a polymorph of niobium phosphate bronze Na4Nb8P4O32. CS-Na4Nb8P4O32 displays high structural similarity to the noncentrosymmetric Na4Nb8P4O32 phase (referred to as NCS-Na4Nb8P4O32): Distorted NbO6 octahedra are corner-coordinated to form ReO3-type layers, which are further joined together by isolated PO4 tetrahedra. However, two polymorphous phases adopt different packings of structural units, resulting in distinct symmetries. NbO3 layers and PO4 tetrahedra are reversely arranged along the crystallographic a direction in CS-Na4Nb8P4O32, thereby producing a centrosymmetric structure. The reverse packing cancels out all contributions of dipole moments originating from the distorted NbO6 octahedra; NCS-Na4Nb8P4O32 exhibits the C2-rotation distribution of NbO3 layers and PO4 tetrahedra, thus generating a noncentrosymmetric and polar structure. The C2-rotation packing of structural units brings a constructive addition of the dipole moments, further obtaining large calculated independent second harmonic generation susceptibilities. The study of structural evolution deduced by the packings of structural units in polymorphous Na4Nb8P4O32 might provide valuable insights into polymorphism and structural regulation.

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.

Antimony Doping Enabled Photoluminescence Quantum Yield Enhancement in 0D Inorganic Bismuth Halide Crystals.

Owing to the exterior self-trapped excitons (STEs) with adjustable fluorescence beams, low-dimensional ns2-metal halides have recently received considerable attention in solid-state light-emitting applications. However, the photoluminescence (PL) mechanism in metal halides remains a major challenge in achieving high efficiency and controllable PL properties because the excited-state energy of ns2 conformational ions varies inhomogeneously with their coordination environments. Here, a novel zero-dimensional (0D) lead-free bismuth-based Rb3BiCl6·0.5H2O crystal was reported as a pristine crystal to modulate the optical properties. By doping Sb3+ ions with 5s2 electrons into Rb3BiCl6·0.5H2O crystals, bright orange emission at room temperature was obtained with a photoluminescence quantum yield of 39.7%. Optical characterizations and theoretical studies show that the Sb3+ doping can suppress the strong exciton-phonon coupling, optimize the electronic energy band structure, improve the thermal activation energy, soften the structural lattice of the host crystals, deepen the STE states, and ultimately lead to strong photoluminescence. This work manifests a fruitful manipulation in ripening bismuth-based halides with high-efficiency PL properties, and the PL enhancement mechanisms will guide future research in the exploration of emerging luminescent materials.

Co-substitution design: a new glaserite-type rare-earth phosphate K2RbSc(PO4)2 with high structural tolerance.

An alkali rare-earth phosphate K2RbSc(PO4)2 was successfully obtained as a derivative of glaserite-type K3Na(SO4)2 by co-substitution of K(1)O12 → RbO12, K(2)O10 → KO7, NaO6 → ScO6 and SO4 → PO4, while maintaining the original anionic framework. K2RbSc(PO4)2 exhibits a layered [Sc(PO4)2]∞ framework built from ScO6 octahedra and PO4 tetrahedra, with K and Rb residing in the interlayers. Its isostructural lanthanide analogues K2RbEr(PO4)2 and K2RbLu(PO4)2, inspired by an elemental substitution strategy, were also prepared by a high-temperature solid state reaction. The successful substitution indicates that the skeleton of K2RbSc(PO4)2 is stable with high structural tolerance, which can provide a possibility for substitution of resident ions to obtain diverse structural types and applications.

Improvement on Magnetocaloric Effect through Structural Evolution in Gadolinium Borate Halides Ba2Gd(BO3)2X (X = F, Cl).

A new Gd3+-containing borate Ba2Gd(BO3)2F has been successfully grown via the high-temperature solution method using BaF2-NaF-B2O3 flux. Ba2Gd(BO3)2F crystallizing in the orthorhombic space group Pnma is with lattice parameters a = 7.571(4) Å, b = 10.424(5) Å, c = 8.581(4) Å, α = ß = γ = 90°, and Z = 2. Its three-dimensional framework was constructed from interesting pinwheel-like [Gd(BO3)F]∞ layers bridged by sharing [BO3]3-, which is different from the [Gd(BO3)]∞ layer in the model structure Ba2Gd(BO3)2Cl. The magnetic measurements indicated that Ba2Gd(BO3)2F has a larger magnetocaloric effect with -ΔSm,max = 27.82 J·kg-1·K-1at 2 K and 9 T than that of Ba2Gd(BO3)2Cl under the same conditions. Moreover, thermal stability, infrared spectrum (IR), and ultraviolet-visible-near-infrared diffuse reflectance spectrum were carried out to characterize the title compounds. The first-principles computations also looked into the electronic band structures, densities of states, and refractive indices.

Synthesis, Crystal Structure, and Nonlinear-Optical Properties of a Diamond-Like Chalcogenide Cu2GeS3.

Metal sulfides with diamond-like (DL) structures generally exhibit excellent mid-IR nonlinear-optical (NLO) properties. Here, Cu2GeS3 (CGS) as a member of the DL chalcogenides was synthesized by a high-temperature solid-state method, and the optical properties were carefully studied experimentally and theoretically. The results revealed that CGS has a large second harmonic generation (0.8 × AgGaSe2) and a moderate birefringence of 0.067 at 1064 nm. In addition, the linear and NLO properties of the A2MS3 (A = Cu, Li; M = Ge, Si) series of compounds were evaluated and compared with the help of first-principles calculations.

Mixed Alkali Metal and Alkaline Earth Metal Scandium Borate Birefringence Material with Layered Structure and Short Ultraviolet Cutoff Edge.

Birefringent crystals are essential in the domains of linear and nonlinear optics that need light wave polarization control. Rare earth borate has become a popular study material for ultraviolet (UV) birefringence crystals due to its short cutoff edge in the UV area. RbBaScB6O12, a two-dimensional layered structure compound with the B3O6 group, was effectively synthesized through spontaneous crystallization. The UV cutoff edge of RbBaScB6O12 is shorter than 200 nm, and the experimental birefringence is 0.139 @ 550 nm. Theoretical research indicates that the large birefringence originates from the synergistic impact of the B3O6 group and the ScO6 octahedron. RbBaScB6O12 is an outstanding candidate material for birefringence crystals in the UV and even deep UV regions due to its short UV cutoff edge and significant birefringence.

Atomic Substitution to Tune ScO6 Distortion in Ba2MSc2(BO3)4 (M = Na, K, Ba) to Acquire a Large Birefringence.

Replacing alkali metals (K, Na atoms) by an alkaline-earth metal (Ba atom), α-Ba3Sc2(BO3)4 (high-temperature phase) is successfully obtained by a molten salt method, taking Ba2K1.6Na0.4Sc2(BO3)4 as the parent template. Although both of them exhibit similar layered structures composed of ScO6 and BO3 units, α-Ba3Sc2(BO3)4 shows largely distorted ScO6 octahedra (Δd = 0.56) forced by the uniform tension of a larger space effect from BaO12 polyhedrons, rather than regular ScO6 octahedra like in Ba2K1.6Na0.4Sc2(BO3)4. Experimental measurements and calculated analyses elucidate that distorted ScO6 octahedra in α-Ba3Sc2(BO3)4, displaying a second-order Jahn-Teller (SOJT) effect, enlarge the experimental birefringence up to 0.14@550 nm, while Ba2K1.6Na0.4Sc2(BO3)4 with regular ScO6 octahedra only shows Δn = 0.11 under the same condition. In addition, other optical and thermal properties of the two title compounds were characterized. The experimental results indicate that Ba2K1.6Na0.4Sc2(BO3)4 and α-Ba3Sc2(BO3)4 are promising birefringent materials.

ε-La(IO3)3: A Polar Iodate with High Thermal Stability and a Large Second-Harmonic-Generation Response Obtained by a Supercritical Hydrothermal Method.

The synthesis conditions at high temperature and high pressure are favorable for exploring new compounds with novel structures and properties. In this work, a new polar iodate, ε-La(IO3)3, is obtained by a supercritical hydrothermal method with high temperature and high pressure (T = 400 °C and P ≈ 25 MPa). Different from the known phases, ε-La(IO3)3 crystallizes in the chiral space group P21, which features a three-dimensional framework with multiple IO3- groups stacked along different directions around the LaOx polyhedra. ε-La(IO3)3 possesses high thermal stability up to 525 °C and exhibits a wide band gap of about 4.05 eV. Attributed to its noncentrosymmetric arrangement, ε-La(IO3)3 is second-harmonic-generation (SHG)-active and the powder SHG response is measured to be 11.1 × KH2PO4 at 1064 nm in the 26-50 µm particle size range. This work has enriched the structural diversity of iodates and would further promote the materials' exploration under a supercritical hydrothermal method.

(C5N2H14)GeBr4: A 2D Organic Germanium Bromide Perovskite with Strong Orange Photoluminescence Properties.

Hybrid organic-inorganic metal halide (OIMH) perovskites are regarded as potential photoluminescent (PL) materials and have attracted intensive attention. Here, we select 1-methylpiperazine as an organic component and successfully obtain a two-dimensional (2D) Ge-based OIMH perovskite, (1-mpz)GeBr4. It features a 2D layered structure composed of distorted [GeBr6]4- octahedra with organic (C5H14N2)2+ located between the layers. (1-mpz)GeBr4 exhibits strong orange color under ultraviolet (UV) light and possesses good PL stability for over 2 months. The photoluminescence quantum efficiency is measured to be 7.15% at room temperature, which is the largest among all reported low-dimensional Ge-based perovskites. Experimental measurements, combined with first-principles calculations, reveal that its PL property is attributed to self-trapped excitons (STEs) from [GeBr6]4- groups. From the deduced structure-property relationship, Ge-based OIMH PL perovskites with good stability and high PL efficiency can be expected.

Investigation into Structural Variation from 3D to 1D and Strong Second Harmonic Generation of the AHgPS4 (A+ = Na+, K+, Rb+, Cs+) Family.

The continuous exploration of multinary chalcogenide semiconductors has provided a variety of new functional materials. In this paper, four new quaternary chalcogenides AHgPS4 (A+ = Na+, K+, Rb+, Cs+) have been prepared by solid-state syntheses. These findings complement the lack of research on this quaternary system. Influenced by the size effect of cations and the coordination mode of Hg, the four compounds crystallize in four different space groups [NaHgPS4, P4̅n2; KHgPS4, Pnn2; RbHgPS4, P21/n; CsHgPS4, P212121] and show an interesting evolution from a 3D framework structure to a 1D chain structure. Moreover, all of these compounds feature noncentrosymmetric (NCS) structures except for RbHgPS4. The materials exhibit wide band gaps of 2.7 eV < Eg < 3.0 eV. The NCS- related second-harmonic-generation (SHG) property of NaHgPS4 and KHgPS4 was also studied. They display strong powder SHG responses (3.14 × AgGaS2 for NaHgPS4; 4.15 × AgGaS2 for KHgPS4), which indicate their intriguing potential as IR nonlinear-optical materials. Moreover, first-principles theoretical calculations were performed to understand the structure-property relationships of these materials.

NaGaI3O9F: a new alkali metal gallium iodate combined with IO3- and IO3F2- units.

Birefringent crystals are very important optical materials, which are widely used in the fields of optics and communication. In this work, we reported a new iodate, NaGaI3O9F, synthesized by mild hydrothermal method. NaGaI3O9F crystallized in the monoclinic space group P21/c (No. 14) and it featured a novel ∞[Ga2(IO3F)2(IO3)4]2- layer stacked with Na+ cations located in the void maintaining charge balance. Notably, IO3- and IO3F2- anionic units appeared in the same time in the A-Ga-I-O/F (A = alkali metal) system. According to the experimental characterization and theoretical calculations, NaGaI3O9F showed a wide bandgap (4.27 eV) and large birefringence (Δnexp∼ 0.203, Δncal = 0.197 at 1064 nm), indicating its potential use as a birefringent material.

AZn4(OH)4(C3N3O3)2 (A = Mg, Zn): Two Zn-Based Cyanurate Crystals with Various Cation Coordination and Large Birefringence.

Cyanurate crystals have recently become a research hot spot in birefringent materials owing to the large structural and optical anisotropy of the planar π-conjugated (HxC3N3O3)x-3 (x = 0-3) groups. In this study, two new Zn-based cyanurate crystals, Zn5(OH)4(C3N3O3)2 and MgZn4(OH)4(C3N3O3)2, were synthesized by the hydrothermal method. In Zn5(OH)4(C3N3O3)2, the d10 Zn2+ cations have three different coordinating environments, which has never been found in cyanurates. These compounds have wide band gaps (∼5 eV) and large birefringence (∼0.32 at 400 nm), indicating their potential as ultraviolet birefringence crystals.

Deep-Ultraviolet Nonlinear-Optical van-der-Waals Beryllium Borates*.

Van-der-Waals (vdW) deep-ultraviolet (DUV) nonlinear-optical (NLO) materials hold great potential to extend DUV NLO applications to two dimensions, but they are rare in nature. In this study, we propose a design principle to realize vdW DUV NLO materials via structural evolution from the non-vdW (BO3 )-(BeO3 F) layers in KBe2 BO3 F2 (KBBF) to the vdW (BO3 )-(BeO4 H) layers in berborite Be2 BO5 H3 (BBH) and the vdW (BO4 )-(BeO4 ) layers in beryllium metaborate BeB2 O4 (BEBO). Based on first-principles calculations, the fundamental NLO properties of BBH and BEBO demonstrate that a balanced DUV NLO performance can be achieved in these two systems. Importantly, BBH, a layered material existing in nature, can achieve an available DUV phase-matched output with strong second harmonic generation (SHG) for 177.3/193.7 nm DUV lasers, which is almost identical to that of KBBF. Remarkably, BEBO shows an excellent DUV SHG capacity and an even shorter phase-matching wavelength than KBBF. Therefore, the newly discovered vdW BBH and BEBO, once verified by experiments, could provide an ideal platform to study DUV NLO effects in three dimensions and two dimensions.

LiZn(OH)CO3 : A Deep-Ultraviolet Nonlinear Optical Hydroxycarbonate Designed from a Diamond-like Structure.

Deep-ultraviolet (DUV) nonlinear optical (NLO) crystals are vital to the development of science and state-of-the-art technologies, but the exploration of these important optoelectronic materials is hindered by the contradiction between NLO effect and energy band gap as well as the sensitivity of DUV phase-matching to optical birefringence. Here, in order to achieve a well balanced DUV NLO performance, we chose the diamond-like structure as the prototype template for the material design and synthesized the first DUV NLO hydroxycarbonate crystal, LiZn(OH)CO3 . This compound simultaneously has a wide band gap (>6.53 eV), a strong second harmonic generation response (ca. 3.2×KDP), and a large birefringence (0.147 at 1064 nm) with the shortest DUV phase-matching capability (λPM <190 nm) among all reported NLO carbonates. Moreover, the structure-property relationships in LiZn(OH)CO3 are analyzed by first-principles calculations. This work presents an effective design strategy to push the NLO performance of hydroxycarbonates into the DUV region.

Nonlinear Optical Oxythiophosphate Approaching the Good Balance with Wide Ultraviolet Transparency, Strong Second Harmonic Effect, and Large Birefringence.

Ultraviolet (UV) transparency, second harmonic effect and optical birefringence are three vital but mutually restrictive factors in the application of UV nonlinear optical (NLO) materials. It is difficult for traditional phosphates to achieve a good balance among these factors. In this communication, we propose that the structural evolution of the NLO motif from traditional phosphate to oxythiophosphate would enhance the birefringence and second harmonic generation (SHG) effect while maintaining wide UV transparency, which is confirmed by first-principles calculations and preliminary experimental measurements. Following this strategy, we predict that, compared with the well-known NLO phosphate KH2 PO4 , the oxythiophosphate KH2 PO3 S exhibits better balance for the UV NLO performance, including wide UV transparency (UV cutoff down to 203 nm), strong SHG effect (ca. 0.9 pm V-1 ), and large birefringence (ca. 0.1 at 1 µm) with short phase-matching SHG output wavelength (≈214 nm).

Na4CdGe2S7: A Sodium-Rich Quaternary Wide-Band-Gap Chalcogenide with Two-Dimensional [Ge2CdS7]∞ Layers.

Metal chalcogenides are an attractive subject for investigation owing to their wide application in optoelectronics. Here, we report the discovery of a new cadmium-based quaternary sulfide, Na4CdGe2S7, with a wide band gap of 3.35 eV. The microscopic structure of this compound features two-dimensional [Ge2CdS7]∞ layers, which were first found in metal sulfides. Remarkably, the Na/S ratio in Na4CdGe2S7 exceeds 50%, suggesting that it would have ecofriendly electrical applications, such as in sodium-ion-battery electrodes and fast ion conductors.

Selenite bromide nonlinear optical materials Pb2GaF2(SeO3)2Br and Pb2NbO2(SeO3)2Br: synthesis and characterization.

Two new isomorphic selenite bromides, namely Pb2GaF2(SeO3)2Br (1) and Pb2NbO2(SeO3)2Br (2), were synthesized by mild hydrothermal reactions. These two compounds crystallize in the noncentrosymmertric space group P21 and feature 3D structures consisting of 1D (Pb2Br)3+ cationic chains and 2D [Ga2F4(SeO3)4]6-/[Nb2O4(SeO3)4]6- anonic chains. As measured, 1 and 2 are phase-matchable and exhibit excellent second harmonic generation (SHG) responses of 4.5 and 1.4 times that of KDP, respectively. Moreover, these two compounds possess wide bandgaps (>3.17 eV) and high thermal stabilities (>425 °C), indicating their suitable performances as potential nonlinear optical materials. First-principle density functional theory (DFT) calculations were also performed to explicate the structure-property relationship.

Nonlinear-Optical Crystal Rb3YB6O12 with Condensed B5O10 Blocks That Exhibits an Intriguing Structural Arrangement and a Short Ultraviolet Absorption Edge.

Nonlinear-optical (NLO) crystals, which can regulate the laser wavelength through a cascading second-harmonic-generation technique, have been widely utilized in the field of optoelectronics. In this work, we grew the NLO borate crystal Rb3YB6O12 (RYBO) using the spontaneous crystallization method. RYBO crystallizes in a chiral trigonal space group of R32 with a new type of structural arrangement built from Y-O short chains and B5O10 groups. It is significantly different from the known structure of chemical analogues Rb3REB6O12 (RE = Nd, Eu) not only in the halved unit cell parameter but also in the Y-O connection manner. The NLO response of RYBO is about 0.8KDP, 8-fold larger than that of KB5O8·4H2O with the same B5O10 groups because of the coexistence of two NLO-active units of the distorted YO6 octahedra and B5O10 anions. Thanks to the short ultraviolet (UV) cutoff, RYBO may have potential NLO applications in the UV and even deep-UV spectral regions.

Realizing Deep-Ultraviolet Second Harmonic Generation by First-Principles-Guided Materials Exploration in Hydroxyborates.

Aimed to realize deep-ultraviolet (DUV, λ < 200 nm) second harmonic generation (SHG), herein we systematically investigate the scheme for screening DUV nonlinear optical (NLO) material "genes" from traditional borates to hydroxyborates. From our first-principles studies, it is demonstrated that, if hydroxyborate motifs can be aligned by reasonable layered crystallization types, the resulting crystal will exhibit potential DUV NLO capability. Guided by the theoretical analysis and prediction, two targeted hydroxyborates AEB8O15H4 (AE = Sr, Ca) screened from the inorganic crystal structure database are obtained in the experiments. Preliminary measurements including powder absorption spectra and SHG effects are in good agreement with our simulation results, well confirming our prediction that AEB8O15H4 (AE = Sr, Ca) would be promising DUV NLO crystals with large band gaps, strong SHG effects, and sufficient birefringence. This work is the first demonstration that hydroxyborates can realize DUV SHG, which may bring new opportunities for finding DUV NLO materials in the hydroxyborate system.