Novel Fluorite-Structured Materials for Solid-State Refrigeration.

Refrigeration based on the electrocaloric effect can offer many advantages over conventional cooling technologies in terms of efficiency, size, weight, and power source. The discovery of ferroelectric and antiferroelectric properties in fluorite-based materials in 2011 has led to diverse applications related to memory (e.g., ferroelectric tunnel junctions, nonvolatile memory, and field-effect transistors) and energy fields (e.g., energy storage and harvesting, electrocaloric refrigeration, and infrared detection). Fluorite-based materials exhibit several properties not shared by most conventional materials (such as in terms of compatibility with complementary metal-oxide semiconductors and 3D nanostructures, deposition thickness at the nanometer scale, and simple composition). Here, the electrocaloric refrigeration properties of fluorite-based ferroelectric/antiferroelectric materials are reviewed by focusing on the advantages of ZrO2 - and HfO2 -based materials (e.g., relative to conventional perovskite- and polymer-based counterparts). Finally, the recent progress made in this research field are also discussed along with its future perspectives.

Core-Shell Structured Upconversion/Lead-Free Perovskite Nanoparticles for Anticounterfeiting Applications.

In view of their excellent luminescence properties, nanocrystalline metal halide perovskites have diverse optoelectronic applications, including those related to anticounterfeiting. However, high-quality optical anticounterfeiting typically requires multiple encryptions relying on several optical modes to ensure information security. Herein, an efficient anticounterfeiting strategy based on dual optical encryption is realized by combining up- and downconversion luminescence in a nanocomposite with NaYF4 : Er3+ ,Yb3+ as core and a CsMnCl3 as shell. The emission color of this nanocomposite depends on the penetration depth of incident radiation and can be changed by varying the excitation source (980 nm laser or UV light) to produce different luminescent patterns. This feature allows one to effectively improve the anticounterfeiting index and fabricate professional anticounterfeiting materials.

Solvatochromic Photoluminescent Effects in All-Inorganic Manganese(II)-Based Perovskites by Highly Selective Solvent-Induced Crystal-to-Crystal Phase Transformations.

The development of lead-free perovskite photoelectric materials has been an extensive focus in the recent years. Herein, a novel one-dimensional (1D) lead-free CsMnCl3 (H2 O)2 single crystal is reported with solvatochromic photoluminescence properties. Interestingly, after contact with N,N-dimethylacetamide (DMAC) or N,N-dimethylformamide (DMF), the crystal structure can transform from 1D CsMnCl3 (H2 O)2 to 0D Cs3 MnCl5 and finally transform into 0D Cs2 MnCl4 (H2 O)2 . The solvent-induced crystal-to-crystal phase transformations are accompanied by loss and regaining of water of crystallization, leading to the change of the coordination number of Mn2+ . Correspondingly, the luminescence changes from red to bright green and finally back to red emission. By fabricating a test-paper containing CsMnCl3 (H2 O)2 , DMAC and DMF can be detected quickly with a response time of less than one minute. These results can expand potential applications for low-dimensional lead-free perovskites.

A Highly Red-Emissive Lead-Free Indium-Based Perovskite Single Crystal for Sensitive Water Detection.

Low-dimensional luminescent lead halide perovskites have attracted tremendous attention for their fascinating optoelectronic properties, while the toxicity of lead is still considered a drawback. Herein, we report a novel lead-free zero-dimensional (0D) indium-based perovskite (Cs2 InBr5 ⋅H2 O) single crystal that is red-luminescent with a high photoluminescence quantum yield (PLQY) of 33 %. Experimental and computational studies reveal that the strong PL emission might originate from self-trapping excitons (STEs) that result from an excited-state structural deformation. More importantly, the in situ transformation between hydrated Cs2 InBr5 ⋅H2 O and the dehydrated form is accompanied with a switchable dual emission, which enables it to act as a PL water-sensor in humidity detection or the detection of traces of water in organic solvents.

Intrinsic Self-Trapped Emission in 0D Lead-Free (C4 H14 N2 )2 In2 Br10 Single Crystal.

Low-dimensional lead halide perovskite materials recently have drawn much attention owing to the intriguing broadband emissions; however, the toxicity of lead will hinder their future development. Now, a lead-free (C4 H14 N2 )2 In2 Br10 single crystal with a unique zero-dimensional (0D) structure constituted by [InBr6 ]3- octahedral and [InBr4 ]- tetrahedral units is described. The single crystal exhibits broadband photoluminescence (PL) that spans almost the whole visible spectrum with a lifetime of 3.2 µs. Computational and experimental studies unveil that an excited-state structural distortion in [InBr6 ]3- octahedral units enables the formation of intrinsic self-trapped excitons (STEs) and thus contributing the broad emission. Furthermore, femtosecond transient absorption (fs-TA) measurement reveals that the ultrafast STEs formation together with an efficient intersystem crossing has made a significant contribution to the long-lived and broad STE-based emission behavior.

High-Frequency Ultrasonic Imaging with Lead-free (Na,K)(Nb,Ta)O3 Single Crystal.

Lead-free (Na,K)(Nb,Ta)O3 (KNNT) piezoelectric single crystal has been successfully grown using the top-seeded solution growth technique. The electromechanical coupling factors are very high ( k33 = 0.827, kt = 0.646), and the dielectric loss tangent is as low as 0.004. Acoustic impedance was calculated to be 26.5 MRayl. From the single crystal, a single element transducer was fabricated. The transducer achieved a 57.6% -6 dB bandwidth and 32.3 µm axial resolution at the center frequency of 45.4 MHz, which can identify the cornea of porcine eyeball with high resolution. Comparison between KNNT single crystal and lead-based single crystal was discussed. The results suggest that this single crystal transducer is an excellent candidate to replace lead-containing transducer for high-frequency ultrasonic imaging applications.

The Influence of Lanthanum Admixture on Microstructure and Electrophysical Properties of Lead-Free Barium Iron Niobate Ceramics.

This article presents the research results of lead-free Ba1-3/2xLax(Fe0.5Nb0.5)O3 (BFNxLa) ceramic materials doped with La (x = 0.00-0.06) obtained via the solid-state reaction method. The tests of the BFNxLa ceramic samples included structural (X-ray), morphological (SEM, EDS, EPMA), DC electrical conductivity, and dielectric measurements. For all BFNxLa ceramic samples, the X-ray tests revealed a perovskite-type cubic structure with the space group Pm3¯m. In the case of the samples with the highest amount of lanthanum, i.e., for x = 0.04 (BFN4La) and x = 0.06 (BFN6La), the X-ray analysis also showed a small amount of pyrochlore LaNbO4 secondary phase. In the microstructure of BFNxLa ceramic samples, the average grain size decreases with increasing La content, affecting their dielectric properties. The BFN ceramics show relaxation properties, diffusion phase transition, and very high permittivity at room temperature (56,750 for 1 kHz). The admixture of lanthanum diminishes the permittivity values but effectively reduces the dielectric loss and electrical conductivity of the BFNxLa ceramic samples. All BFNxLa samples show a Debye-like relaxation behavior at lower frequencies; the frequency dispersion of the dielectric constant becomes weaker with increasing admixtures of lanthanum. Research has shown that using an appropriate amount of lanthanum introduced to BFN can obtain high permittivity values while decreasing dielectric loss and electrical conductivity, which predisposes them to energy storage applications.

BaHf0.05Ti0.95O3 Ceramics from Sol-Gel and Solid-State Processes: Application to the Modelling of Piezoelectric Energy Harvesters.

A typical piezoelectric energy harvester is a bimorph cantilever with two layers of piezoelectric material on both sides of a flexible substrate. Piezoelectric layers of lead-based materials, typically lead zirconate titanate, have been mainly used due to their outstanding piezoelectric properties. However, due to lead toxicity and environmental problems, there is a need to replace them with environmentally benign materials. Here, our main efforts were focused on the preparation of hafnium-doped barium titanate (BaHfxTi1-xO3; BHT) sol-gel materials. The original process developed makes it possible to obtain a highly concentrated sol without strong organic complexing agents. Sol aging and concentration can be controlled to obtain a time-stable sol for a few months at room temperature, with desired viscosity and colloidal sizes. Densified bulk materials obtained from this optimized sol are compared with a solid-state synthesis, and both show good electromechanical properties: their thickness coupling factor kt values are around 53% and 47%, respectively, and their converse piezoelectric coefficient d33∗ values are around 420 and 330 pm/V, respectively. According to the electromechanical properties, the theoretical behavior in a bimorph configuration can be simulated to predict the resonance and anti-resonance frequencies and the corresponding output power values to help to design the final device. In the present case, the bimorph configuration based on BHT sol-gel material is designed to harvest ambient vibrations at low frequency (<200 Hz). It gives a maximum normalized volumetric power density of 0.03 µW/mm3/Hz/g2 at 154 Hz under an acceleration of 0.05 m/s2.

(1-C5H14N2Br)2MnBr 4 : A Lead-Free Zero-Dimensional Organic-Metal Halide With Intense Green Photoluminescence.

Low-dimensional organic-inorganic hybrid materials have attracted tremendous attentions due to their fascinating properties as emerging star materials for light-emitting applications. Taking advantage of their rich chemical composition and structural diversity, here, a novel lead-free organic-manganese halide compound, (1-mPQBr)2MnBr4 (1-mPQ = 1-methylpiperazine, 1-C5H14N2) with zero-dimensional structure has been rationally designed and successfully synthesized through solvent-evaporation method. Systematical characterizations were carried out to investigate the structure, thermal and photophysical properties. The (1-mPQBr)2MnBr4 was found to crystallized into an orthorhombic crystal (P212121) with lattice parameters of a = 8.272(6) Å, b = 15.982(10) Å and c = 17.489(11) Å. The structure consists of isolated [MnBr4]2- clusters and free Br- ions as well as [C5H14N2]2+ molecules. Thermal analysis indicates that it is stable up to 300°C. Upon ultraviolet photoexcitation, the (1-mPQBr)2MnBr4 exhibits intense green emission centered at 520 nm with a narrow full width at half-maximum of 43 nm at room temperature, which should be assigned to the spin-forbidden internal transition (4T1(G) to 6A1) of tetrahedrally coordinated Mn2+ ions. The superior photoluminescence properties coupled with facile and efficient synthesis method of this material suggest its considerable promise to be utilized as light-emitting materials.

Preparation and Dielectric Properties of K1/2Na1/2NbO3 Ceramics Obtained from Mechanically Activated Powders.

Alkaline based materials have been considered as a replacement for environmentally harmful Pb(Zr,Ti)O3 (PZT) electro-ceramics. In this paper, the K1/2Na1/2NbO3 (KNN) ceramics were prepared in a three stage process: first Nb2O5, Na2CO3, and K2CO3 were milled in a high energy mill (shaker type) for different periods, between 25 h and 100 h, consecutively a solid state reaction was carried out at 550 °C. Finally, the uniaxially pressed samples were sintered at 1000 °C. The reaction temperature is lower for mechanically activated powders than in the case of the conventional solid-state method. The ceramic samples, prepared from the mechanically activated powders, were investigated by dielectric spectroscopy. The influence of the duration of the mechanical activation on the properties of the ceramic materials, e.g., ceramic microstructures, phase transition temperatures, character of the temperature dependences of dielectric permittivity, are discussed.

Perovskite-Derivative Valleytronics.

Halide perovskites are revolutionizing the renewable energy sector owing to their high photovoltaic efficiency, low manufacturing cost, and flexibility. Their remarkable mobility and long carrier lifetime are also valuable for information technology, but fundamental challenges like poor stability under an electric field prevent realistic applications of halide perovskites in electronics. Here, it is discovered that valleytronics is a promising route to leverage the advantages of halide perovskites and derivatives for information storage and processing. The synthesized all-inorganic lead-free perovskite derivative, Cs3 Bi2 I9 , exhibits strong light-matter interaction and parity-dependent optically addressable valley degree of freedom. Robust optical helicity in all odd-layer-number crystals with inversion symmetry breaking is observed, indicating excitonic coherence extending well beyond 11 layers. The excellent optical and valley properties of Cs3 Bi2 I9 arise from the unique parallel bands, according to first principles calculations. This discovery points to new materials design principles for scalable valleytronic devices and demonstrates the promise of perovskite derivatives beyond energy applications.

Dielectric and Energy Storage Properties of Ba(1-x)CaxZryTi(1-y)O3 (BCZT): A Review.

The Ba(1-x)CaxZryTi(1-y)O3 (BCZT), a lead-free ceramic material, has attracted the scientific community since 2009 due to its large piezoelectric coefficient and resulting high dielectric permittivity. This perovskite material is a characteristic dielectric material for the pulsed power capacitors industry currently, which in turn leads to devices for effective storage and supply of electric energy. After this remarkable achievement in the area of lead-free piezoelectric ceramics, the researchers are exploring both the bulk as well as thin films of this perovskite material. It is observed that the thin film of this materials have outstandingly high power densities and high energy densities which is suitable for electrochemical supercapacitor applications. From a functional materials point of view this material has also gained attention in multiferroic composite material as the ferroelectric constituent of these composites and has provided extraordinary electric properties. This article presents a review on the relevant scientific advancements that have been made by using the BCZT materials for electric energy storage applications by optimizing its dielectric properties. The article starts with a BCZT introduction and discussion of the need of this material for high energy density capacitors, followed by different synthesis techniques and the effect on dielectric properties of doping different materials in BCZT. The advantages of thin film BCZT material over bulk counterparts are also discussed and its use as one of the constituents of mutiferroic composites is also presented. Finally, it summarizes the future prospects of this material followed by the conclusions.

Lead-Free Antiferroelectric Silver Niobate Tantalate with High Energy Storage Performance.

Antiferroelectric materials that display double ferroelectric hysteresis loops are receiving increasing attention for their superior energy storage density compared to their ferroelectric counterparts. Despite the good properties obtained in antiferroelectric La-doped Pb(Zr,Ti)O3 -based ceramics, lead-free alternatives are highly desired due to the environmental concerns, and AgNbO3 has been highlighted as a ferrielectric/antiferroelectric perovskite for energy storage applications. Enhanced energy storage performance, with recoverable energy density of 4.2 J cm-3 and high thermal stability of the energy storage density (with minimal variation of ≤±5%) over 20-120 °C, can be achieved in Ta-modified AgNbO3 ceramics. It is revealed that the incorporation of Ta to the Nb site can enhance the antiferroelectricity because of the reduced polarizability of B-site cations, which is confirmed by the polarization hysteresis, dielectric tunability, and selected-area electron diffraction measurements. Additionally, Ta addition in AgNbO3 leads to decreased grain size and increased bulk density, increasing the dielectric breakdown strength, up to 240 kV cm-1 versus 175 kV cm-1 for the pure counterpart, together with the enhanced antiferroelectricity, accounting for the high energy storage density.

Growth and Characterization of Lead-free Piezoelectric Single Crystals.

Lead-free piezoelectric materials attract more and more attention owing to the environmental toxicity of lead-containing materials. In this work, we review our first attempts of single crystal grown by the top-seeded solution growth method of BaTiO3 substituted with zirconium and calcium (BCTZ) and (K0.5Na0.5)NbO3 substituted with lithium, tantalum, and antimony (KNLSTN). The growth methodology is optimized in order to reach the best compositions where enhanced properties are expected. Chemical analysis and electrical characterizations are presented for both kinds of crystals. The compositionally-dependent electrical performance is investigated for a better understanding of the relationship between the composition and electrical properties. A cross-over from relaxor to ferroelectric state in BCTZ solid solution is evidenced similar to the one reported in ceramics. In KNLSTN single crystals, we observed a substantial evolution of the orthorhombic-to-tetragonal phase transition under minute composition changes.