The red-emitting phosphors of Mn4+-activated A2MgWO6 (A = Ba,Sr, Ca) for light emitting diodes

A series of Mn⁴⁺ ions activated A₂MgWO₆ (A = Ba, Sr, Ca) phosphors, showing bright red emission peaks appeared around 700 nm under the excitation of 355 nm, were synthesized by the solid-state reaction. The crystal structures and photo-luminescent (PL) properties of these synthesized phosphors were deeply investigated with the aids of X-ray diffraction measurement (XRD), and the temperature dependent PL/decay curves in detail. The optimum doping concentration of Mn⁴⁺ ions in A₂MgWO₆ (A = Ba, Sr, Ca) lattices were studied through the relationship between the Mn⁴⁺ ions doping concentrations and the luminescent intensities. The thermal stability of the synthesized red-emitting phosphors was checked based on the temperature-dependent PL intensities ranging from 7 to 510 K. Comparative studies of the luminescent properties for Mn⁴⁺ ions in isostructural A₂MgWO₆ (A = Ba, Sr, Ca) lattice with double perovskite structure were studied. The results indicate that the synthesized red-emitting phosphors are the ideal choice for white light emitting diodes (W-LEDs).     

Effects of sintering aids on microstructure,mechanical, and optical properties of AlON ceramics synthesized by SPS

Aluminum oxynitride (AlON) ceramics doped with different sintering aids were synthesized by spark plasma sintering process. The microstructures, mechanical, and optical properties of the ceramics were investigated. The results indicate that the optimal amount of sintering aids is 0.06 wt% La₂O₃ + 0.16 wt% Y₂O₃ + 0.30 wt% MgO. The addition of La³⁺ and Mg²⁺ decreases the rate of grain boundary migration in ceramics, promotes pore elimination, and inhibits grain growth. The addition of Y³⁺ facilitates liquid-phase sintering of AlON ceramics. Moreover, the addition of Mg²⁺ effectively promotes twin formation in the ceramics, which hinders crack propagation and dislocation motion when the ceramics are loaded. Hence, the AlON ceramic doped with 0.06 wt% La₂O₃ + 0.16 wt% Y₂O₃ + 0.30 wt% MgO exhibits a relative density of 99.95%, an average grain size of 9.42 μm, and a twin boundary content of 10.3%, which contributes to its excellent mechanical and optical properties.     

Fabrication and investigation of a new highly humidity stable nanocrystalline perovskite,tetramethylammonium lead triiodide to be used in solar cells

Hybrid organic-inorganic halide perovskite (CH₃NH₃PbI₃) materials have broad applications such as photovoltaics, light emitting diodes, and sensors. However, one of the major drawbacks of these materials is their sensitivity to moisture. Here, a new promising perovskite nanocrystal: tetramethylammonium lead triiodide (CH3)₄NPbI₃ has been introduced and fabricated. Investigation of morphology, structural, optical and stability properties was done by XRD, SEM, TEM, and UV–visible. Characterization prove that the processed powder is in the form of nanoparticles and drop cast film has a hexagonal rod-like morphology. Crystal structure was determined by powder x-ray diffraction and electron diffraction pattern. It shows a perovskite crystal structure with a hexagonal unit cell. Powder XRD after 45 days suggests that this new compound is stable in humid conditions. Direct optical band gap of this material was obtained by transforming diffuse reflectance spectrum to Kubelka-Munk spectrum and it was 2.66 eV.     

Site-selective substitution and resulting magnetism in arc-melted perovskite ATiO3-δ (A = Ca,Sr, Ba)

Magnetic properties in perovskite titanates ATiO_3-δ (A = Ca, Sr, Ba) were investigated before and after arc melting. Crystal structure analysis was conducted by powder synchrotron X-ray diffraction with Rietveld refinements. Quantitative chemical element analysis was carried out by X-ray photoelectron spectroscopy. Magnetic measurements were conducted by vibrating sample magnetometer and X-ray magnetic circular dichroism (XMCD). The magnetic properties are found to be affected by impurities of 3d elements such as Fe, Co, and Ni. Depending on the composition and crystal structure, the occupation of the magnetic ions in perovskite titanates is selectively varied, which is interpreted to be the origin of the different magnetic behaviors in arc-melted perovskite titanates ATiO_3-δ (A = Ca, Sr, Ba). In addition, both formation of oxygen vacancies and the reduction of Ti⁴⁺ to Ti³⁺ during arc-melting also play a role as proven by XMCD. Nevertheless, preferential site occupation of magnetic impurities is dominant in the magnetic properties of arc-melted perovskite ATiO_3-δ (A = Ca, Sr, Ba).     

Effects of Mn,Cl co-doping on the structure and photoluminescence properties of novel walnut-shape MAPb0.95Mn0.05I3-xClx films

The novel walnut shape MAPb_0.95Mn_0.05I_3-xCl_x film was successfully synthesized by a one-step method followed by chlorobenzene anti-solvent treatment. The bandgap energy and PL intensity of perovskite film can be effectively tuned by Mn and Cl co-doping. The enlarged bandgap energy is mainly attributed to the synergistic effect of strengthened Pb-I interaction, Cl incorporation and smaller electronegativity of Mn dopants. The stronger coupling between the Mn d- and Pb p-bands, extended carriers diffusion length and more emitting defect-associated states caused by Mn and Cl co-doping are the main reasons for the enhancing PL intensity of MAPb_0.95Mn_0.05I_3-xCl_x walnut shape film. This work not only helps to in-depth understand the correlation between co-doping elements and optical properties of nanostructured perovskite films, but also provides important strategy for future designing the lower-toxic nanostructured perovskite materials with enhanced electrical and optical properties.     

Double perovskite Sr2FeReO6 oxides: Structural,dielectric, magnetic,electrical, and optical properties

Double perovskite Sr₂FeReO₆ (SFRO) powders were synthesized by so-gel process and annealed in argon atmosphere. Their structural, dielectric, magnetic, electrical, and optical properties were comprehensively investigated. It was found that the SFRO powders possessed a tetragonal crystal structure with I4/m space group and exhibited spherical shapes with some agglomeration due to the magnetic interactions between particles of the powders. Quantitative energy dispersive X-ray spectrometer data revealed the atomic ratio of Sr, Fe, Re, and O elements close to the nominal values of 2:1:1:6. X-ray photoemission spectroscopy spectra reveal two species of Re⁵⁺ and Re^6-7+ coexist in the SFRO powders. Sr, Fe, and O elements are present as Sr²⁺, Fe³⁺, and lattice oxygen, respectively. Dielectric property measurements revealed a Maxwell–Wagner type dielectric dispersion in the SFRO ceramics. Ferromagnetic behavior was verified by the observed magnetic hysteresis loops in the SFRO powders at 2 K and 300 K. The remanent magnetization and coercive field at 2 K were 8.23 emu/g and 3152 Oe, respectively, and the saturated magnetization was estimated to be 21.8 emu/g (or 2.0 μ_B/f.u.), smaller than the theoretical value of 3.0 μ_B/f.u. owing to the presence of the anti-site defects. Magnetic Curie temperature (T_C) was estimated to be 432.3 K. Intergranular tunneling magnetoresistance and hysteresis phenomena were observed in the SFRO powders at low temperatures, and the MR (2 K, 6 T) was measured to be −15% and −10% for MR (100 K, 6 T). Electrical transport and optical absorption measurements demonstrate the semiconducting nature of the SFRO with optical band gap of 1.39 eV. The electrical transport process follows the small polaron variable range hopping theory. The unique combination of high T_C ferromagnetism with the semiconductivity enables the SFRO to be a promising candidate for spintronic devices.     

Crystallization,spectroscopic and dielectric properties of CuO-added magnesium aluminosilicate-based glasses

In this work, CuO-doped MgO-Al₂O₃-SiO₂ based glasses have been synthesized successfully through conventional melt quenching method, and the effect of CuO content on the structure and properties of the glasses was investigated. The results revealed that CuO could act as a glass modifier to depolymerize the silicate network and its effect was superior to that of MgO. In addition, the main crystalline phase was α-cordierite (Mg₂Al₄Si₅O₁₈), indicating that copper ions did not participate in the formation of the crystalline phase, and still existed in the interstitial position. The characteristic absorption band around 750 nm owing to the ²B_1g→²B_2g transition of Cu²⁺ ions appeared on the optical transmittance spectra, which confirmed the existence of Cu²⁺ ions in the tetragonally distorted octahedral sites. The luminescence center was caused by Cu⁺ ions, and the luminescence lifetime decreased with the addition of CuO. The dielectric constant and dielectric loss increased with the increase of CuO content, indicating an increase in the insulation performance. Finally, the obtained chromaticity coordinate parameters indicate that the prepared CuO-doped magnesium aluminosilicate-based glasses can be applied in optical and electrical fields.     

Band gap modulation and improved magnetism of double perovskite Sr2KMoO6 (K = Fe,Co, Ni,Mn) doped BaTiO3 ceramics

BaTiO₃ ceramics doped with double perovskite Sr₂KMoO₆ (BT-SKM) are fabricated via solid-state reaction technology. The effects of SKM dopants on the structure, band gap and electrical/magnetic properties of BT are systematically studied. XRD and Raman spectra analysis show polycrystalline perovskite structure of the samples, which confirms the structural changes. With the addition of SKM dopants, the grain size of the samples decreases significantly. The band gaps of doped BT samples reduce, and the minimum band gap of BT-SCM is 1.77 eV, which is apparently reduced compared with the band gap of pure BT of 3.22 eV. However, the ferroelectric properties are weakened in samples doped with SKM. This ascribes to the introduction of more oxygen vacancies by dopants, which impedes the switching of domains, resulting in deterioration of ferroelectric properties. Furthermore, ferromagnetism of BT-SNM is observed, which may be attributed to the long-range exchange interaction between Ni²⁺ ions and oxygen vacancies. These results reveal the potential applications of these perovskite oxides in photovoltaic and memory devices.     

Investigation of structural,optical, electrical,and magnetic properties of Fe-doped La0.7Sr0.3MnO3 manganites

In this work, the physical properties of nanocrystalline samples of La_0.7Sr_0.3Mn_1−xFe_xO₃ (0.0 ≤ x ≤ 0.20) perovskite manganites synthesized by the reverse micelle (RM) technique were explored in detail. The phase purity, crystal structure, and crystallite size of the samples were determined using X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy. All the samples had rhombohedral crystal structure and crystallite size increased with increase in Fe content in La_0.7Sr_0.3MnO₃. The scanning electron micrographs (SEMs) exhibited smooth surface morphology and nonuniform shape of the particles. The optical properties studied using UV-visible absorption spectroscopy revealed a decrease in the absorbance and optical band gap with an increase in Fe content in La_0.7Sr_0.3MnO₃ compound. The temperature-dependent resistivity measurements revealed semiconducting nature of x = 0 and 0.1 samples up to the studied temperature range, while a metal-to-insulator transition was observed at higher Fe doping. Magnetic studies revealed weak ferromagnetism in all the samples and a reduction in the maximum magnetization with an increase in Fe content. A close correlation between electrical transport and magnetic properties was observed with the doping of Fe ion in La_0.7Sr_0.3MnO₃ at Mn site. These results advocate strong interactions associated with the double exchange mechanism among Fe³⁺ and Mn³⁺ ions.     

Solvothermal syntheses and structures of A3AgSn3Se8 (A = Rb,K)

Two new compounds A₃AgSn₃Se₈(A = K, Rb) were synthesized solvothermally and characterized by X-ray single crystal diffraction. These compound are composed of building block [Sn₃Se₈]⁴⁻ formed by three edge-sharing SnSe₄ tetrahedra, and these building blocks are further connected by tetrahedral coordinated Ag⁺ to form infinite chains, alkali cations are located between the chains. Their optical properties were also studied.     

Tunable lanthanum doping in double perovskite films for read-only memory

Rare earth ions have been widely investigated as active dopants in various materials because of their distinct optical, electronic and magnetic properties. Halide perovskite resistive memories have attracted increasingly recent interest for tremendous potential applications in computation and data storage techniques. However, it remains unexplored to implement rare earth doping strategy for tuning perovskite resistive memories. Here, we report read-only memories based on environment-friendly and air-stable lead-free double perovskite Cs₂AgBiBr₆ films, which is particularly tuned by rare earth La³⁺ doping. We use the vacuum sublimation and solution processing method to obtain Cs₂AgBiBr₆ crystals doped by different content La³⁺, and fabricate resistive memory devices based on doped Cs₂AgBiBr₆ films. The simplest sandwich-like structure composed of ITO/La-doped-Cs₂AgBiBr₆/Ag only is designed in cross-bar array architecture with high-integration and simple operation. The resistive memory device of La-doped Cs₂AgBiBr₆ films demonstrate a typical write-once-read-many-times (WORM) behavior with low onset voltage of 1 V and long retention time of 12000 s. In particular, the ON/OFF ratio of the La-doped Cs₂AgBiBr₆ film is 100 times higher than that of the undoped Cs₂AgBiBr₆ film. This study provides a new insight to design and manipulate memory devices based on lead-free halide perovskite materials through doping effect of rare earth ions.     

Band gap engineering and microwave dielectric properties evolution of mixed (Sr,La, Ce) TiMgO3 titanate–aluminate system

A mixed perovskite titanate-aluminate [(1-x)(Sr_0.6La_0.2Ce_0.2Ti_0.8Mg_0.2O₃)-xNdAlO₃ for x = 0.1 to 0.4] solid solution was successfully synthesized. X-ray diffraction patterns (XRD) and Rietveld refinement results indicated a stable perovskite phase with a cubic structure, in which Nd³⁺ occupies the A-site randomly while Al³⁺ occupies the B-site. No additional reflection spots (superlattice reflections) were detected in the HRTEM pattern (see SAED), confirming the cubic symmetry. All samples showed small Urbach tails, mainly due to compositional disorder. Microstructural analysis based on atomic force microscopy (AFM) showed no traces of impurity phases. For x = 0.4, excellent microwave dielectric properties (MWD) are obtained with a quality factor (Q × f) of 37,131 GHz at f = 5.2801 GHz, relative permittivity (ε_r) of 43, and temperature coefficient of resonant frequency (τ_f) of +1.3 ppm/°C. Variations in ε_r, Q × f values, and τ_f may be related to changes in relative density (ρ_rel), ion polarizability, optical band gap, and tolerance factor, respectively.     

Structural,dielectric and ferromagnetic behavior of (Zn,Co) co-doped SnO2 nanoparticles

Nanoparticles of basic composition Sn_0.94Zn_0.05Co_0.01O₂, Sn_0.92Zn_0.05Co_0.03O₂ and Sn_0.90Zn_0.05Co_0.05O₂ were synthesized by chemical precipitation method. The incorporation of Co and Zn in SnO₂ lattice introduced significant changes in the physical properties of all the three nanocrystals. The average particle size estimated from TEM data decreased from 15.71 to 6.41  nm with enhancement in concentration of oxygen vacancies as Co content is increased from 1 to 5 wt%. Increasing Co content enhanced the Sn:O atomic ratio as a result concentration of oxygen vacancies increased. The dielectric study revealed strong doping dependence. The dielectric parameters (ε′, tanδ and σ_ac) increased with increasing Co content and attained maximum values for 5% (Zn, Co) co-doped SnO₂ nanoparticles. The dielectric loss (ε′′) exhibited dispersion behavior and the Debye’s relaxation peaks observed in dielectric loss factor (tanδ), whose intensities increased with increasing Co content. The variation of dielectric properties and ac conductivity revealed that the dispersion is due to Maxwell-Wagner interfacial polarization and hopping of charge carriers between Sn⁺²/Sn⁺³ and Co⁺²/Co⁺³. The large dielectric constant of all samples made them interesting materials for device application. Magnetization measurements (M (H) loops) revealed enhancement in saturation magnetization with doping which is due to the formation of large amount of induced defects and oxygen vacancies in the samples. The present study clearly reveals doping dependent properties and the oxygen vacancies induced ferromagnetism in Zn, Co co-doped SnO₂ nanoparticles having applications in ultra-high dielectric materials, high frequency devices and spintronics.     

Low temperature sintering and dielectric properties of (Ba0.85Ca0.15)(Ti0.9Zr0.1)O3−xCu2+ ceramics obtained by the sol-gel technique

Lead-free Cu²⁺-modified (Ba_0.85Ca_0.15)(Ti_0.9Zr_0.1)O₃ (BCZT−xCu²⁺) piezoelectric ceramics was synthesized by sol-gel method. The effects of Cu²⁺ additions on sintering characteristics, the phase structure, microstructure, electrical properties and complex impedance characteristic were investigated systematically. The XRD patterns exhibited a pure perovskite structure without impurity phase in all samples. SEM micrographs, temperature dependence of dielectric constant and polarization-electric field (P-E) hysteresis loops indicated that a small amount of Cu²⁺ addition affected the properties obviously. The results revealed that the addition of Cu²⁺ significantly improved the sinterability of BCZT ceramics which resulted in a reduction of sintering temperature from 1440 °C to 1230 °C. The TG-DSC was analyzed to verify the reaction process of BCZT−Cu²⁺ materials. (Ba_0.85Ca_0.15)(Ti_0.9Zr_0.1)O₃ ceramics with x=0.020 Cu²⁺ exhibited good electrical properties: ε_m=12,112, T_c=360 K, ε_r=2614, tan δ=0.026, K_p=0.47 and d₃₃=382 pC/N. The results indicated that Cu²⁺-modified BCZT ceramics could be a promising candidate for commercial purposes.     

Microstructure and dielectric properties of high entropy Ba(Zr0.2Ti0.2Sn0.2Hf0.2Me0.2)O3 perovskite oxides

A series of high entropy Ba(Zr_0.2Ti_0.2Sn_0.2Hf_0.2Me_0.2)O₃ (Me=Y³⁺,Nb⁵⁺,Ta⁵⁺,V⁵⁺,Mo⁶⁺,W⁶⁺) perovskite oxides were synthesized by using a solid state reaction method. Three multiple-cation solid solutions formed pure phase compounds, and only two compounds were sintered into ceramics. Microstructure analysis showed the influence of configurational entropy on phase stability and grain growth. Dielectric measurements showed that the high entropy ceramics possessed decent temperature stability of permittivity from 25 °C to 200 °C, low dielectric loss (<0.002) from 20 Hz to 2 MHz, high resistance and moderate breakdown strength (290 kV/cm, 370 kV/cm). Evidence strongly confirmed that controlling configurational entropy could be a feasible perspective to set up highly tunable perovskite structures and explore novel species of dielectric materials.     

A KMnF3 perovskite structure with improved stability,low bandgap and high transport properties

Here, a new promising perovskite structure of KMnF₃ has been fabricated and characterized, which yields bandgap of 1.6?eV with fascinating moisture-resistance and phase stability. Investigation of structural, optical, stability and transport properties have done by XRD, SEM, UV–vis–NIR spectroscopy, photoluminescence and electrical conductivity test. Such examination indicated the high carrier mobility (18?cm²/V?s) and density (10¹⁴/cm³) even after a long interval between each excitation. These transport properties are comparable to that of the organic perovskite, indicating the importance of KMnF₃ for solar device applications.     

The effect of nickel doping on the microstructure and conductivity of Ca(Ti,Al)O3–δ for solid oxide fuel cells

The ABO₃ type perovskite oxide-based ceramic membranes are one of the most important classes of materials for high-temperature solid oxide fuel cell applications. The acceptor-doped calcium titanate (CaTiO₃) perovskite has attracted considerable attention as an oxide ion-conducting membrane due to its potentially high ionic conductivity and excellent stability. Nonetheless, the ionic conductivity of the material must still be improved. Following the strategy of the substitution of dopants on the B-site, the current work is focused on exploring the effect of Al and Ni additions on electrical properties, by studying the nominal compositions CaTi_0.7Al_0.3–xNi_xO_3−δ (x = 0, 0.1, 0.2 and 0.3). The materials were synthesized by the sol–gel method and studied as a function of phase composition, microstructure, and electrical properties. The results demonstrate an increase of both total and specific grain boundary conductivity with increasing Ni content, while predominant p-type behavior is shown under oxygen-rich atmosphere.     

Structural,electrical, dielectric and magnetic behavior of Gd1−xBixFe1−yZryO3 nanoparticles for advanced technological applications

Gd_1−xBi_xFe_1−yZr_yO₃ nanoparticles were synthesized via micro-emulsion route with different molar concentrations of Bi⁺³ (x) and Zr⁺⁴ (y). The values of x and y were kept in the range 0.00, 0.15, 0.30, 0.45 and 0.60. The characterizations were done by the thermo-gravimetric analysis (TGA), X-ray Diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). The average particle size was ~50 nm. The effect of Bi³⁺ and Zr⁴⁺ contents on electrical, dielectric and magnetic parameters were studied. The DC resistivity measurements showed at certain Bi³⁺ and Zr⁴⁺ contents, more than two fold increase in electrical resistivity from 68×10⁸ Ω cm to 150×10⁸ Ω cm. The magnetic measurements showed the paramagnetic nature of Gd_1−xBi_xFe_1−yZr_yO₃ nanoparticles. The electrical and magnetic properties of these nanoparticles suggested that these materials are potential candidates for the fabrication of telecommunication and switching devices.     

Preparation,Characterization and Properties of Novel Cationic Gemini Surfactants with Rigid Amido Spacer Groups

A series of novel cationic gemini surfactants with rigid amido groups inserted as the spacers, named C ₁₂ ‐PPDA‐C ₁₂ , C ₁₄ ‐PPDA‐C ₁₄ and C ₁₆ ‐PPDA‐C ₁₆ , were synthesized by a two‐step reaction with dimethyl terephthalate, N,N‐dimethyl propylene diamine and alkyl bromide as raw materials. The chemical structures of the prepared compounds were confirmed by IR, ¹H and ¹³C NMR and element analysis. Surface activity properties of the synthesized compounds were investigated by surface tension, electrical conductivity and fluorescence. Increasing the number of carbon atoms in the hydrophobic alkyl chain, decreased the critical micelle concentration (CMC), surface tension at the CMC and the minimum surface area. Other relevant properties including foaming ability and emulsion stability were investigated. The results indicated that the synthesized gemini surfactants possess good surface properties, emulsifying properties and steady foam properties.     

Structure,electrical properties and reaction mechanism of (Ba0.85Ca0.15)(Zr0.1Ti0.9)O3 ceramics synthesized by the molten salt method

Lead-free (Ba_0.85Ca_0.15)(Zr_0.1Ti_0.9)O₃ (BCZT) ceramics with excellent electrical properties were successfully synthesized by a molten salt method (MSS). The submicron BCZT powders with pure perovskite phase were obtained by adjusting the KCl-NaCl content that was used as the eutectic salt. The effects of salt content and reaction temperature on the structure and properties of the BCZT materials were systematically investigated. Comparing with BCZT ceramics prepared by solid state method (SS), the reaction temperature of BCZT ceramics synthesized by MSS decreased approximately 200 °C. Moreover, BCZT ceramics sintered at 1360 °C with 50% eutectic salt showed the most outstanding electrical properties, which are as follows: d₃₃ = 604 pC/N, k_p = 57%, P_s = 17.11 µC/cm², P_r = 9.98 µC/cm², ε_m = 15872, ε_r = 2654 and tan δ = 0.013. In addition, this work revealed a possible reaction course processes and mechanism about MSS. The results provide a new design to optimize the performance of BCZT lead-free piezoelectric ceramics.