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Deep-ultraviolet (deep-UV) nonlinear optical (NLO) crystals are key materials in creating tunable deep-UV lasers for frequency conversion technology. However, practical application of the sole usable crystal, KBe2 BO3 F2 , has been hindered by the high toxicity of beryllium and its layering tendency in crystal growth. Herein, we report a beryllium-free deep-UV NLO material NaSr3 (OH)(B9 O16 )[B(OH)4 ] (NSBOH), synthesized by a covalent bond modification strategy under hydrothermal conditions. Moisture-stable NSBOH exhibits strong second-harmonic generation (SHG) at 1064 nm (3.3 × KH2 PO4 ) and 532 nm (0.55 × β-BaB2 O4 ), both amongst the largest powder SHG responses for a deep-UV borate, with good phase-matchability and a short wavelength cutoff edge (below 190 nm). NSBOH possesses a 3D covalent anionic [B9 O19 ]∞ honeycomb-like framework with no layering. The Sr2+ and Na+ ions, residing in the cavities of the anionic framework, act as templates for the assembly and favorable alignment of NLO-active groups, resulting in an optimal balance between strong SHG activities and wide UV transparency. These merits indicate NSBOH is a very attractive candidate for deep-UV NLO applications.

Second-harmonic-generation (SHG) efficiency and birefringence are two crucial parameters for nonlinear optical (NLO) crystals. However, the simultaneous optimization of these two key parameters rem...

A double holmium-silver sulfate was obtained for the first time. The temperature intervals for the formation and stability of the compound were determined by differential scanning calorimetry. The crystal structure of AgHo(SO4)2 was determined by Rietveld method. The X-ray diffraction (XRD) analysis showed that the compound crystallizes in the monoclinic syngony, space group P21/m, with the unit cell parameters a = 4.71751 (4) A, b = 6.84940 (6) A and c = 9.89528 (9) A, β = 95.1466 (4)°, V = 318.448 (5) A3, Z = 2, RB = 1.55 %, T = 303 K. Two types of sulfate tetrahedra were found in the structure, which significantly affected the spectral properties in the infrared range. In the temperature range of 143−703 K, a negative thermal expansion along the b direction accompanied by a positive thermal expansion along the a and c directions was observed. It was established that negative thermal expansion is the result of the deformation of sulfate tetrahedra, which is affected by the movement of holmium and silver atoms. The excitation in the blue spectral range (457.9 nm) produces a luminescence in light blue (489 nm), green (545 nm) and red (654 nm) spectral ranges, and the latter two were of comparable intensity that is favorable for WLED sources. The observed luminescent band distribution is ascribed to the specific crystal field at Ho3+ ion sites rather than a variation of radiationless probability.

This research was financially supported by the NationalNatural Science Foundation of China (nos. 51432006,52002276), the Ministry of Education of China for theChangjiang Innovation Research Team (no. IRT14R23), theInnovation Program of Shanghai Municipal Education Com-mission, and the Ministry of Education and the StateAdministration of Foreign Experts Affairs for the 111 Project(no. B13025). M.G.H. thanks the Australian Research Coun-cil for support (DP170100411).

On the basis of density functional theory calculations, we explored the catalytic properties of various heteroatom-doped black and gray arsenene toward the oxygen reduction reaction (ORR), the oxygen evolution reaction (OER), and the hydrogen evolution reaction (HER). The calculation results show that pristine black (b-As) and gray arsenene (g-As) exhibit poor catalytic performance because of too weak intermediate adsorption. Heteroatom doping plays a key role in optimizing catalytic performance. Among the candidate dopants O, C, P, S, and Sb, O is the most promising one used in arsenene to improve the ORR and OER catalytic performance. Embedding O atoms could widely tune the binding strength of reactive intermediates and improve the catalytic activity. Single O-doped g-AsO1 can achieve efficient bifunctional activity for both the OER and the ORR with optimal potential gap. b-AsO1 and b-AsO2 exhibit the optimal OER and ORR catalytic performance, respectively. For the HER, double C-doped g-AsC2 could tune the adsorption of hydrogen to an optimal value and significantly enhance the catalytic performance. These findings indicate that arsenene could provide a new platform to explore high-efficiency electrocatalysts.

The first alkali–metal selenate nonlinear optical (NLO) crystal, Na5(SeO4)(HSeO4)3(H2O)2, has been obtained by in situ hydrothermal synthesis. It crystallizes in the polar and noncentrosymmetric (NCS) space group Cc (no. 9). The structure of Na5(SeO4)(HSeO4)3(H2O)2 can be described as isolated [SeO4]/[HSeO4] tetrahedra connected by H-bonding interactions and Na–O bonds, forming a three-dimensional framework. Powder second-harmonic generation (SHG) measurements show that Na5(SeO4)(HSeO4)3(H2O)2 is phase-matching with a moderate SHG response of ca. 1.6 times that of benchmark KH2PO4. The UV-Vis-NIR spectrum shows that the title compound is a wide-band semiconductor with an optical band gap of 3.57 eV. The birefringence of Na5(SeO4)(HSeO4)3(H2O)2 was calculated to be 0.037 at 1064 nm. First-principles studies show that the [SeO4]/[HSeO4] tetrahedra are the main contributors to its NLO properties. This study indicates that pure selenate is a promising material system to afford NCS structures with SHG responses.

Ultraviolet (UV) nonlinear optical (NLO) materials possess important practical applications in modern laser science and technology, which are attracting significant research interest in the fields of chemistry, physics and materials science. A fluorinated mixed main-group metal sulfate and its defluorinated analogue, Rb2InF3(SO4) (RIFSO) and Rb3In(SO4)3 (RISO), were designed and synthesized for NLO study via mild hydrothermal methods. RIFSO unfortunately crystallizes in centric space group Pbcn (no. 60) while RISO crystallizes in acentric space group R3c (no. 161). Both of them exhibit a one-dimensional chain structure composed of corner-shared [InO2F4] or [InO6] and [SO4] groups. Optical property studies reveal that RISO displays a large band gap (5.75 eV) corresponding to a short UV cut-off edge (215 nm), a moderate second harmonic generation (SHG) efficiency of ∼0.5 times that of KH2PO4 (KDP) with phase-matchable performance, and high thermostability (>800 °C), demonstrating its potential as an applicable UV NLO material. First-principles calculations have been performed to further elucidate the structure–property relationship of RISO.

The first fluorinated lead vanadate selenite Pb2(V2O4F)(VO2)(SeO3)3 (PVOFS) was successfully synthesized via a mild hydrothermal method. This compound crystallizes in the chiral space group P212121 of the orthorhombic system and it is the first noncentrosymmetric structure in the PbII–VV–SeIV–O–F system. PVOFS is composed of five kinds of second-order Jahn–Teller susceptible asymmetric motifs, including three distinct types of vanadium-centered polyhedral units ([VO5F], [VO6] and [VO5]), [SeO3] pyramids and Pb2+ cations. It features a unique three-dimensional open framework structure displaying three types of tunnels (10-, 8- and 7-membered rings), which enriches the structural diversity for fluorinated vanadate selenite systems. Optical property studies revealed that PVOFS shows a second-harmonic generation response of 0.3 times that of the commercial KH2PO4 with phase matching behavior, a wide transparent region covering IR windows, an optical band gap of 2.35 eV, a high laser damage threshold of 61 times that of AgGaS2, and a large birefringence of 0.105 at 1064 nm. Theoretical calculations have been performed to clarify the correlation between the molecular structure and the optical properties of PVOFS.

Birefringent materials with strong optical anisotropy are indispensable for optoelectronic devices as polarizers, isolators, and beam splitters, etc. Here, a new ionic cocrystal KBr·H3C3N3O3 (I) wa...

Anomalous mechanical materials, with counterintuitive stress-strain responding behaviors, have emerged as novel type of functional materials with highly enhanced performances. Here we demonstrate that the materials with coexisting negative, zero and positive linear compressibilities can squeeze three-dimensional volume compressibility into one dimension, and provide a general and effective way to precisely stabilize the transmission processes under high pressure. We propose a “corrugated-graphite-like” structural model and discover lithium metaborate (LiBO2) to be the first material with such a mechanical behavior. The capability to keep the flux density stability under pressure in LiBO2 is at least two orders higher than that in conventional materials. Our study opens a way to the design and search of ultrastable transmission materials under extreme conditions., Anomalous mechanical behaviors provide an opportunity to regulate the functions of materials. Here the authors show that materials with coexisting negative, zero and positive compressibilities can “squeeze” volume compressibility into one direction, and thus stabilize transmission processes under pressure.

Nonlinear optical (NLO) materials have critically important applications in advanced laser technologies. However, achieving a good balance between the mutual competing NLO properties and band gap w...

9 pags., 3 figs., 3 tabs., The presence of extremely large negative linear compressibilities (NLC) in crystalline silver oxalate was discovered in a recent work by using first principles solid-state calculations. Although the minimum value of the NLC was found for a negative applied hydrostatic pressure, in this work the presence of NLC in this material for positive applied pressures is verified experimentally by means of high-pressure X-ray diffraction experiments performed at room temperature in the Beijing synchrotron radiation facility. The results of this study demonstrate with certainty that the compressibility of silver oxalate along [010] crystallographic direction is negative for applied pressures in the range from 0.0 to 0.85 GPa. Since the measured compressibility decreases largely as the pressure decreases, large negative values of the compressibility are expected for negative applied pressures in agreement with the results found using first principles methods. Furthermore, the analysis of the variation of the lattice parameters of the crystal structure of silver oxalate under pressure in the principal axes reference system revealed that this material also exhibits the largest negative area compressibility phenomenon found so far at zero pressure, the values of the compressibilities along two of the principal axes being − 16.7 and − 20.0 TPa−1. The negative compressibility phenomenon in silver oxalate can be rationalized in terms of a “chains of rotating parallelograms” structural model., This work was supported by the National Scientific Foundations of China (Grants 51702330, 51872297 and 11974360).

Zero thermal expansion (ZTE) materials, keeping size constant as temperature varies, are valuable for resisting the deterioration of the performance from environmental temperature fluctuation, but ...

An ultraviolet nonlinear optical (NLO) crystal La2SrB10O19 (LSB) has been firstly grown through the spontaneous crystallization using SrO-Li2O-B2O3-MoO3 flux. LSB crystallizes in noncentrosymmetric...

Combining a wide band gap, strong second-harmonic generation (SHG), and good phase matchability into a single structure is an ongoing challenge in the design of nonlinear optical (NLO) materials. H...

Structural modulations have been recently found to cause some unusual physical properties, such as superconductivity or charge density waves; however, thus-induced nonlinear optical properties are rare. We report herein two unprecedented incommensurately modulated nonlinear optical sulfides exhibiting phase matching behavior, A2 SnS5 (A=Ba, Sr), with the (3+1)D superspace groups P21 21 2(00γ)00s or P21 (α0γ)0, featuring different modulations of the [Sn2 S7 ]∞ belts. Remarkably, Ba2 SnS5 exhibits an excellent second harmonic generation (SHG) of 1.1 times that of the benchmark compound AgGaS2 at 1570 nm and a very large laser-induced damage threshold (LIDT) of 8×AgGaS2 . Theoretical studies revealed that the structural modulations increase the distortions of the Sn/S building units by about 44 or 25 % in A2 SnS5 (A=Ba, Sr), respectively, and enhance significantly the SHG compared with α-Ba2 SnSe5 without modulation. Besides, despite the smaller Eg , the A2 SnS5 samples exhibit higher LIDTs owing to their smaller thermal expansion anisotropies (Ba2 SnS5 (1.51)

Rational doping and compositional control remain significant challenges in designing luminescent metal halides to achieve highly efficient and tunable emission. Here, the air-stable lead-free Cs2In...

The first alkali metal rare earth iodate fluoride, Li2Ce(IO3)4F2, was synthesized using hydrothermal method. This compound features a novel [CeI4O12F2]∞ infinite layer with each [CeO6F2] dodecahedr...

La/Ce0.5Mn0.5 catalysts prepared by wetness impregnation of Ce0.5Mn0.5 with La(NO3)3 (aq) were used in catalytic oxidation of ethyl acetate. Characterization by X-ray diffractometer (XRD), H2 temperature programmed reduction (H2-TPR), X-ray photoelectron spectroscopy (XPS) and Raman shows that La/Ce0.5Mn0.5 catalysts have fluorite-like structure. La/Ce0.5Mn0.5 catalyst presents high activity with T90 (the temperature needed for 90% conversion) of 200 °C, where ethyl acetate is almost completely converted into CO2. The conversion at 195 °C maintains at 80% for at least 100 h. The intermediates were mainly ethanol and acetaldehyde whose amounts are below 250 × 10–6 and 16 × 10–6, respectively. In situ FTIR indicates that the modes of dissociative adsorption of ethyl acetate are related to surface oxygen species, which affect the stability of catalysts and the selectivity for ethanol and acetaldehyde intermediates.

Efficient broadband emission related to self-trapped excitons (STEs) in low-dimensional perovskites and double perovskite crystals has attracted great attention due to its potential applications in optoelectronics. However, the formation of STEs in three-dimensional (3D) all-inorganic perovskites (CsPbX3) is difficult. Here, through the doping of ytterbium (Yb3+) or erbium (Er3+) ions into CsPbX3 crystals, we observed broadband luminescence from STEs with a linewidth exceeding 200 nm and a Stokes shift of ∼1 eV. Theoretical concepts and experimental studies demonstrated that Pb2+ in perovskites could be replaced by impure ions. Also, the Pb2+ substitution defects and adjacent halide vacancies introduced defect states lying just below the conduction band minimum of perovskites. Strong electron–phonon coupling between trapped carriers and deformation of the [PbX6] cage reduced the energy gap of perovskites. Our study sheds new light on the permanent defects induced in extrinsic STEs in 3D inorganic perovskites. It also provides a clear microscopic picture of the underlying mechanism of formation of STEs.

An electrochemical sensor for doxycycline hyclate (DC) detection with high sensitivity and good selectivity is reported. The sensor was fabricated by electro-polymerization of molecularly imprinted polymers (MIPs) in the presence of DC onto multi-walled carbon nanotubes modified glassy carbon electrode (MWCNTs/GCE). The MWCNTs can significantly increase the current response of the sensor, leading to enhanced sensitivity. The MIPs provide selective recognition sites for DC detection. The experimental parameters, such as the polymer monomer concentration, supporting electrolyte pH, the time for electro-polymerization and the incubation time of the sensor with DC were optimized. Under optimized experimental conditions, the sensor displayed a linear range of 0.05 μmol/L–0.5 μmol/L towards DC detection, with the detection limit of 1.3 × 10−2 μmol/L. The sensor was successfully applied for recovery test of DC in human serum samples.

By a simple cross-substitution of A-site Li/Na in tetragonal tungsten bronze (TTB) structures, we successfully synthesized a new niobate compound, Pb2.15(Li0.25Na0.75)0.7Nb5O15, with a superstructure. This compound exhibits a strong second harmonic generation (SHG) up to ∼47 × KDP. The large SHG response is related to strengthened local distortion, manifesting cross-substitution as a possibly general route to improve the NLO effect in stiff and low symmetric structures.

New paracelsian-like beryllophosphates M4LiBe4P7O24 and their tetrahedra-substituted phases M4Li(Li3P)P7O24 (MLiP2O6, M = Cs, Rb) were designed and synthesized. Interestingly, two types of phases undergo a structural transformation, resulting from the radius and valence charge number of tetrahedral cations. Four crystals demonstrate potential deep-UV NLO properties such as short absorption edges (

Increasing the energy band gap under the premise to maintain a large nonlinear optical (NLO) response is a challenging issue for the exploration and molecular design of mid-infrared nonlinear optical crystals. Utilizing a charge-transfer engineering method, we designed and synthesized a rare earth chalcogenide, KYGeS4. With an NLO effect as large as that in AgGaS2, KYGeS4 breaks through the limitation of energy band gap, i.e., the “3.0 eV wall”, in NLO rare earth chalcogenides, and thus exhibits an excellent comprehensive NLO performance. First-principles electronic structure analysis demonstrates that the large band gap in KYGeS4 is ascribed to the decreased covalency of Y–S bonds by transferring charge from [YS7] to [GeS4] polyhedra. The charge-transfer engineering strategy would have significant implications for the exploration of good-performance NLO crystals.

Self-trapped excitons (STEs) have recently been observed in several metal halide perovskites (MHPs), especially in low-dimensional ones. Despite studies that have shown that factors like dopant, chemical composition, lattice distortion, and structural and electronic dimensionality may all affect the self-trapping of excitons, a general understanding of their mechanism of formation in MHPs is lacking. Here, we study the intrinsic and defect-induced self-trapping of excitons in three-, two-, and one-dimensional MHPs. We find that whether the free excitons could be trapped is simply determined by the competition of the energy-gap decrease and deformation-energy increase along with the lattice distortion. Both introducing halogen defects into the lattice and decreasing the dimensionality can tip the balance between them and thus facilitate the self-trapping of free excitons. This general picture of the mechanism of formation of STEs provides important insights into the design and development of high-performance white-light devices and solar cells with MHPs.

The nitrogen-rich compounds are promising candidates for high-energy-density applications, owing to the large difference in the bonding energy between triple and single/double nitrogen bonds. The exploration of stable copper–nitrogen (Cu–N) compounds with high-energy-density has been challenging for a long time. Recently, through a combination of high temperatures and pressures, a new copper diazenide compound (P63/mmc-CuN2) has been synthesized (Binns et al 2019 J. Phys. Chem. Lett. 10 1109–1114). But the pressure-composition phase diagram of Cu–N compounds at different temperatures is still highly unclear. Here, by combining first-principles calculations with crystal structure prediction method, the Cu–N compounds with different stoichiometric ratios were searched within the pressure range of 0–150 GPa. Four Cu–N compounds are predicted to be thermodynamically stable at high pressures, Pnnm-CuN2, two CuN3 compounds with the P-1 space group (named as I-CuN3 and II-CuN3) and P21/m-CuN5 containing cyclo-N5 −. Finite temperature effects (vibrational energies) play a key role in stabilizing experimentally synthesized P63/mmc-CuN2 at ∼55 GPa, compared to our predicted Pnnm-CuN2. These new Cu–N compounds show great promise for potential applications as high-energy-density materials with the energy densities of 1.57–2.74 kJ g−1.

Regulation of the radical pair spin states allows effective optimization of the electrocatalytic CO2 reduction reaction. This study for the first time reports an experimental observation of significantly boosting the catalytic activity of tin nanoparticle catalysts by an external magnetic field for electrocatalytic CO2 reduction to formate/formic acid. We reveal that enhancing the amount of singlet radical pairs via magnetic field-facilitated triplet → singlet spin evolution can significantly increase the catalytic activity toward an efficient overall electrochemical CO2 reduction reaction. When a common Sn nanoparticle electrode was used as an example, in a constant applied magnetic field (about 0.9 T), the yield of formic acid can be nearly doubled compared to that of zero magnetic field. This finding suggests the merits of radical pair spin states in the electron-transfer process and paves the way toward high formate production in electrocatalytic reduction of CO2.

The discovery of nonlinear optical (NLO) materials for generating coherent mid-infrared (mid-IR) light is critically important in developing laser technologies. However, all of the commercialized m...

Negative thermal expansion (NTE) with a giant volume shrinkage of 4.4%, comparable to the strongest NTE (ΔV = −4.8%) in PbTiO3-based perovskites, was achieved in a lead-free BiCoO3 derivative. The ...

Finding new low-dimensional metal halides with broad-band emission is attracting interest in single-component phosphor for white light-emitting diodes (WLEDs). The full-spectrum white light still r...

Constructing heterojunctions and designing advanced structures to mimic natural photosynthesis is an effective route to convert CO2 into high-energy chemicals with high efficiency and selectivity. Compared to a single-component catalyst, those of two components with different function can corporately facilitate the formation of C2 products. Herein, a type-II heterojunction based on graphitic carbon nitride (g-C3N4)/ZnTe was constructed for the first time for photoelectrochemical CO2 reduction, yielding an impressive ethanol generation rate of 17.1 μmol cm−2 h−1 at -1.1 V (vs. Ag/AgCl). The heterojunction accelerates separation of photo-generated electron-hole pairs and transfer of electrons from ZnTe to g-C3N4 driving by an interfacial internal electric field (IEF) formed between the two semiconductors. Moreover, the combination of ZnTe of high CO2 adsorption capacity serves as CO-producing site with g-C3N4 featuring of abundant pyridinic N subsequently accomplishes the C-C coupling process via adsorbing CO and proton-coupled electron transfer. A pipelined mechanism to rationalize the selective reduction of CO2 to ethanol is proposed and discussed.

Hydrogen, as green chemistry and renewable energy, could be applied for a wide variety of renewable energy storage and conversion devices. Therefore, searching for highly efficient catalysts for the hydrogen evolution reaction (HER) is particularly important. In this work, by tuning the molar ratio of Co/Fe, we produced a series of ultra-thin two-dimensional (2D) Cox-Fey-P nanosheets on carbon cloth (Cox-Fey-P/CC) as HER electrocatalysts. The structure of Cox-Fey-P nanosheets directly depends on the amounts of iron incorporation in the Cox-Fey-P, while the optimal amount of incorporated iron motivates the electrocatalytic performance of Cox-Fey-P for HER, i.e. the Co0.515-Fe0.168-P exhibits excellent catalytic performance with a small overpotential of 41 mV at a high current density of 100 mA cm−2, and a Tafel slope as low as 31.5 mV dec−1 in 0.5 M H2SO4. Detailed investigation reveals the obviously increased electrochemically active surface area, faster electrons transfer, and the strong electronic interaction between the heterogeneous elements after incorporating Fe might boost the excellent HER performance.

We report the discovery of the zero thermal expansion (ZTE) effect in BPO4, a famous deep-ultraviolet (DUV) optical material with cristobalite-like structure. It is revealed that BPO4 has a linear ZTE coefficient of −0.16(5) MK–1 along the c-axis as temperature increases from 13 to 300 K, which originates from the subtle counterbalance between the rotation-induced expansion and contraction effects among BO4 and PO4 groups. BPO4 is a unique DUV cristobalite-like material exhibiting the linear ZTE behavior.

In search of new nonlinear optical crystals for the mid-IR range, we grew single crystals of LiGa0.55In0.45Te2 (LGITe) of optical quality; its structure was established, and the spectroscopic prope...

Overall water splitting is an attractive technology to produce clean hydrogen and oxygen. In this study, we constructed amorphous NixCoy layered double hydroxide (LDH) hybridized with three-dimensional NiCo2O4 to fabricate a core–shell nanowire array on Ni foam (NiCo2O4@NixCoy LDH/NF) as a highly efficient electrocatalyst for overall water electrolysis. By tuning the Ni/Co molar ratio in NixCoy LDH, extremely low overpotentials of 193 mV for oxygen evolution reaction (OER) and 115 mV for hydrogen evolution reaction (HER) at a current density of 10 mA cm–2 can be achieved for the NiCo2O4@Ni0.796Co LDH/NF. Detailed investigations verify that the hybrid structure can increase intrinsic activity of the NiCo2O4@Ni0.796Co LDH/NF and enhance the charge-transfer rate. Moreover, a strong electronic interaction between the heterogeneous elements Ni and Co at the interface of the NiCo2O4 and NixCoy LDH might ultimately influence the catalytic performance.

Efficient hydrogen production by a photovoltaic-electrolysis cell (PV-EC) system requires a low electrolyzer overpotential and a high coupling efficiency between both the components. Herein, Ni5 P4 is proposed as a cost-effective bifunctional electrocatalyst for hydrogen evolution and hydrazine oxidation in a reformed electrolyzer. Experiments indicate that the electrolytic overpotential could be significantly reduced by replacing the oxygen evolution reaction with the hydrazine oxidation reaction at the anode. Furthermore, a scenario for hydrogen production is demonstrated by utilizing a stable and low-cost perovskite solar cell (PSC) with a carbon back electrode to drive a reformed electrolyzer. Importantly, a single PSC can drive three reformed electrolyzers in series for hydrogen production by carefully matching the operating point of the electrolyzer with the maximum power point of the photovoltaic device, thereby, yielding a H2 evolution rate of 1.77 mg h-1 for the whole PV-EC system. This can be a potential starting point for hydrogen production using a single PSC-driven electrolysis system.

The exploration of a large negative thermal expansion (NTE) over a wide temperature range has been an important subject in materials science, since the overall coefficient of thermal expansion (CTE) of ordinary materials can be effectively tailored by the introduction of NTE materials. Here, we successfully achieved a large NTE within a broad temperature range in a new Pb/Bi-based ferroelectric of (1 − x)PbTiO3–xBi(Zn1/2V1/2)O3 by means of improving the ferroelectricity of PbTiO3. The present system exhibits an unusual enhanced tetragonality, large spontaneous polarization (PS), and high Curie temperature (TC). Specifically, the x = 0.1 compound exhibits an enhanced CTE of −2.10 × 10−5 °C−1 from room temperature (RT) up to its TC of 600 °C, which is in contrast to that of pristine PbTiO3 (−1.99 × 10−5 °C−1, RT−490 °C). More intriguingly, a large volume shrinkage (ΔV ≈ −1%) has also been observed during the ferroelectric-to-paraelectric phase transition. According to experimental and theoretical studies, the large NTE is attributed to the enhanced PS derived from the strong hybridization of Pb/Bi–O and Ti/Zn/V–O through the substitution of the polar Bi(Zn1/2V1/2)O3 perovskite. The present study demonstrates that a large NTE within a wide temperature range can be achieved in PbTiO3-based ferroelectrics by improving its ferroelectricity through introducing isostructural polar perovskites.

Functional regulation is a key issue for the application of two-dimensional (2D) hybrid organic–inorganic perovskites in the fields of ferroelectrics, photovoltaics and fluorescence, etc. Here, we report a homologous series of new 2D hybrid organic–inorganic halide perovskites [(C4H3S)CH2NH3]2PbX4 ((C4H3S)CH2NH3+ = thienylmethylamine; X = Cl−, Br−, I−). Their properties, including crystal structures of high (HT) and low (LT) temperature phases, phase transition temperature (Tc), and dielectric constants (e′), exhibit significant discrepancies as the halogen atoms vary from Cl, to Br to I. This effect was unveiled to be divergent hydrogen bond interactions between the organic (C4H3S)CH2NH3+ cation and the inorganic [PbX6]2− octahedron layer resulting from the different radii of the halogen ions. Meanwhile, the direct band gaps of [(C4H3S)CH2NH3]2PbX4 were tuned from 3.78 eV (X = Cl−), to 3.08 eV (X = Br−) to 1.99 eV (X = I−) with a large regulative magnitude of 1.79 eV. The photoluminescence (PL) emission wavelength increases from 505, to 526 to 545 nm for Cl, Br and I, respectively, with the full width at half-maximum (FWHM) significantly decreasing from 123, 69 to 41 nm due to the quantum size effect and structural distortion. However, the average lifetimes of the three compositions reveal the reverse trend of Br > Cl > I, which is attributed to the changes of hydrogen bonds between organic [(C4H3S)CH2NH3]+ cations and inorganic PbX6 octahedral layers and thermal vibration. These results display the high tunability and versatility of the 2D hybrid organic–inorganic perovskites.

Lead halide perovskite have primarily considered as photovoltaic materials for luminescent materials and optoelectronic devices. However, lead halide perovskite is assumed to be unsuitable for photoelectrochemical conversion due to the instability in humid environment or polar solvents. Here, we report graphene oxide (GO) wrapped organic-inorganic lead halide perovskite quantum dots (GO/CH3NH3PbBr3 hybrid) for photoelectrochemical conversion CO2 into solar fuels in nonaqueous media. Graphene oxide can protect CH3NH3PbBr3 QDs from erosion by organic solvent, and also serve as electron transport medium to separate photoinduced electrons and holes. Therefore, a photocurrent increase by ca. 200% can be obtained in the GO/CH3NH3PbBr3 hybrid due to improved electron extraction and transport. The GO/CH3NH3PbBr3 electrode exhibits an effective CO2 reduction capacity to CO, yielding 1.05 μmol cm−2 h−1.

A new sodium-rich fluorooxoborate Na3B7O11F2 features a novel large [B14O24F4] ring formed by two corner-shared [B7O13F2] groups, each containing two [B3O7] anionic units linked by a [BO2F2] group. The band gap of the title compound calculated by using first-principles calculations is 7.69 eV (∼161 nm) and the birefringence refractive index is 0.083 at 193 nm. The results show that it is potentially an excellent deep-ultraviolet (DUV) birefringence crystal.

As a great deal of attention has been paid to studying the role of fluorine in functional oxyfluorides, the question that arises now is how to orderly incorporate the fluorine atoms into the compounds. Herein, we proposed an original strategy using [SeO3]2− anionic groups as the effective structure directing blocks to orderly introduce the F atoms into the [MOxFy] (M = metal) polyhedra. Accordingly, two ordered metal selenite fluorides with [MO3F3]7− groups, M2(SeO3)F2 (M = Zn, Cd), were successfully synthesized through hydrothermal reactions. The stabilities of these structures were confirmed by thermogravimetry tests, bond valence sum analysis and phonon spectrum calculations.

Two chalcogenide crystalline compounds, [enH2][Ag4Sb2S6] (en = ethylenediamine) and [enH][Ag2SbS3], have been successfully synthesized by mild ionothermal and solvothermal means. [enH][Ag2SbS3] crystallizes in the noncentrosymmetric (NCS) and polar space group Pc, and its linear and nonlinear optical (NLO) properties have been investigated for the first time. Second harmonic generation (SHG) measurements revealed that [enH][Ag2SbS3] affords powder SHG performance values of 2.5 × KDP @1064 nm and 0.2 × AgGaS2 @2100 nm. Additional particle size vs. SHG efficiency measurements indicate that [enH][Ag2SbS3] is phase-matchable. The calculated birefringence Δn is 0.177 at 1064 nm, which is sufficiently large (the largest value among NCS thioantimonates) to achieve phase matching. [enH2][Ag4Sb2S6] crystallizes in the centrosymmetric space group P21/c and its structure features a double-layered variant honeycomb-like anionic network parallel to the ac plane separated by [enH2]2+ cations. The optical band gaps of [enH2][Ag4Sb2S6] and [enH][Ag2SbS3] are found to be 2.37 and 2.53 eV, respectively. Theoretical studies using density functional theory have been implemented to further elucidate the relationship between the band structure and NLO properties in [enH][Ag2SbS3].

Efficient mid-infrared (mid-IR) nonlinear optical (NLO) crystals are highly desirable, but it remains extremely challenging to simultaneously achieve a strong second harmonic generation (SHG) response, a large optical bandgap, and wide optical transparency in a material. In this work, a fluorinated homovalent substitution strategy has been put forward and this strategy leads to a new cerium fluorinated iodate, CeF2(IO3)2, which is designed based on a noncentrosymmetric cerium iodate, Ce(IO3)4. The introduction of F− ions enables CeF2(IO3)2 to achieve the simultaneous enhancement of an enlarged phase-matchable SHG response that is 8 times that of KH2PO4 (KDP) and a bandgap of 2.90 eV compared to its parent Ce(IO3)4 (0.9× KDP and 2.17 eV, respectively). CeF2(IO3)2 exhibits a high thermal stability (∼430 °C) and a wide transparent region (0.43–6.46 μm), suggesting that CeF2(IO3)2 is a promising mid-IR NLO material. First-principles simulations reveal that the difference in linear and nonlinear optical properties between Ce(IO3)4 and CeF2(IO3)2 is mainly attributed to the divergent anisotropies of the cerium-centered polyhedra.

Understanding the behavior of H2-He binary mixtures at high pressure is of great importance. Two more recent experiments [J. Lim and C. S. Yoo, Phys. Rev. Lett. 120, 165301 (2018) and R. Turnbull et al., ibid. 121, 195702 (2018)] are in conflict, regarding the miscibility between H2 and He in solids at high pressure. On the basis of first-principles calculations combined with the structure prediction method, we investigate the miscibility for solid H2-He mixtures at pressures from 0 GPa to 200 GPa. It is found that there is no sign of miscibility and chemical reactivity in H2-He mixtures with any H:He ratio. Moreover, instead of H2-He mixtures, the calculated Raman modes of the N-H mixtures can better explain the characteristic peaks observed experimentally, which were claimed to be the H-He vibrational modes. These calculation results are more in line with the experimental findings by Turnbull et al. [Phys. Rev. Lett. 121, 195702 (2018)].

Intense search for new nonlinear optical crystals for the mid-infrared region is in progress, and Li-containing quaternary chalcogenides are expected to improve transparency range, stability, phase-matching conditions, and other parameters in comparison with commercially available AgGaS2, AgGaSe2, and ZnGeP2. Single crystals of Li2In2GeSe6 up to 8 mm in size were obtained by the Bridgman–Stockbarger growth technique, and their high quality was confirmed by exciton luminescence. A monoclinic structure and direct band-to-band electronic transitions were established, and the thermal expansion was shown to be virtually isotropic. Defect-related absorption and luminescence were revealed, and the way to lower them was suggested. The electronic structure, density of states, and some optical properties were calculated from the first principles for Li2In2GeSe6. The calculated nonlinear coefficients and rather large birefringence indicate a strong phase-matching ability. These investigations demonstrate that Li2In2...