Synthesis and characterization of MFe2O4 (M = Mg,Mn, Ni) nanoparticles

MFe₂O₄ nanoparticles were obtained in the presence of natural compounds as carboxymethylcellulose (CMC). The CMC polymer had a double function as a capping agent and as a protecting agent in the growth process of nanoparticles. The synthesized nanoparticles were characterized using thermal analysis (TG, DTA, DTG), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM) and a vibrating sample magnetometer (VSM). The XRD patterns indicate that all the samples were formed in single phase spinel structure. The results also show that the samples calcinated at 500 °C for 6 h have the best crystallinity and the calculated crystallite size was in the range of 6–13 nm. The thermal analysis and FTIR spectra indicate a core–shell structure of the MFe₂O₄ nanoparticles obtained.     

Luminescent properties of Eu3+ activated MLa2(MoO4)4 based (M = Ba,Sr and Ca) novel red-emitting phosphors

Eu³⁺-activated novel red phosphors, MLa₂(MoO₄)₄ (M = Ba, Sr and Ca) were synthesized by the conventional solid state method. The excitation and emission spectra indicate that these phosphors can be effectively excited by UV (395 nm) and blue (466 nm) light, and exhibit a satisfactory red performance at 614 nm. Upon excitation with a 466 nm light, our synthesized phosphors have stronger emission intensity than the sulfide red phosphors used in white LEDs. Due to high emission intensity and a good excitation profile, the Eu³⁺-doped CaLa₂(MoO₄)₄ phosphor may be a promising candidate in solid-state lighting applications.     

Photoluminescence properties of Y0.95 − xMxBO3:Eu3+ (M = Ca,Sr, Ba,Zn, Al, 0 ≤ x ≤ 0.1) in 100–450 nm regions

The single phases of Y_0.95 − xM_xBO₃:5%Eu³⁺ (M = Ca, Sr, Ba, Zn, Al, 0 ≤ x ≤ 0.1) were synthesized successfully by solid-state reaction. Their luminescent properties were studied under UV and VUV excitation. The results indicated that with the incorporation of Ca²⁺, Sr²⁺, Ba²⁺, Zn²⁺ or Al³⁺ into the host lattice of YBO₃:Eu³⁺, the high symmetry around Eu³⁺ was destroyed and the ratio of red emission(⁵D₀–⁷F₂) to orange one (⁵D₀–⁷F₁) increased, leading to a better chromaticity. Furthermore, the co-doping ions such as Ca²⁺, Zn²⁺ and Al³⁺ were beneficial to enhance the luminescent intensity of Eu³⁺. These phenomena were evaluated, and possible explanations were proposed.     

Investigation on the luminescence of RE3+ (RE = Ce,Tb, Eu and Tm) in KMGd(PO4)2 (M = Ca,Sr) phosphates

The VUV excited luminescent properties of Ce³⁺, Tb³⁺, Eu³⁺ and Tm³⁺ in the matrices of KMGd(PO₄)₂ (M = Ca, Sr) were investigated. The bands at about 165 nm and 155 nm in the VUV excitation spectra are attributed to host lattice absorptions of the two matrices. For Ce³⁺-doped samples, the Ce³⁺ 5d levels can be identified. As for Tb³⁺-doped samples, typical 4f–5d absorption bands in the region of 175–250 nm were observed. For Eu³⁺ and Tm³⁺-doped samples, the O²⁻–Eu³⁺ and O²–Tm³⁺ CTBs are observed to be at about 229 nm and 177 nm, respectively. From the standpoints of color purity and luminescent efficiency, KCaGd(PO₄)₂:Tb³⁺ is an attractive candidate of green light PDP phosphor.     

Influence of MO/MF2 modifiers (M = Ca,Sr, Ba) on spectroscopic properties of Eu3+ ions in germanate and borate glasses

Series of Eu³⁺-doped lead-free germanate and borate glasses were synthesized. The MO glass modifiers (M = Ca, Sr or Ba) were partially or totally substituted by MF₂ in chemical composition. In contrast to samples modified by CaO/CaF₂ or SrO/SrF₂, the germanate glass samples containing BaO and/or BaF₂ are fully amorphous, while the lead-free borate glasses are fully amorphous, independently from glass modifiers. Effect of glass modifiers on spectroscopic properties of Eu³⁺ were systematically investigated.For that reason, excitation and emission spectra of Eu³⁺ ions in examined systems were registered. Based on the emission spectra, ratio of integrated luminescence intensity of the ⁵D₀ → ⁷F₂ transition to that of the ⁵D₀ → ⁷F₁ transition (R factor) was calculated. Moreover, the luminescence decay curves were collected and the luminescence lifetimes of the ⁵D₀ excited state of Eu³⁺ ions were determined in function of MF₂ concentration.     

Synthesis of crystal MFe2O4 (M = Mg,Cu, Ni) microspheres

A low-temperature solvothermal synthetic method was developed for the large-scale synthesis of uniform-sized ferrite MFe₂O₄ (M = Mg, Cu, Ni) microspheres. The size of the as-prepared ferrite MFe₂O₄ microspheres could be controlled to be diameters 300–800 nm in diameter by adjusting some growth parameters, such as reaction time and concentration. The structures of the as-prepared ferrite MFe₂O₄ microspheres were characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and energy-dispersive X-ray analysis (EDAX). The magnetic properties of the ferrite MFe₂O₄ microspheres were also investigated.     

Structure and properties of the ordered double perovskites Sr2MWO6 (M = Co,Ni) by sol-gel route

The single-phase double perovskites Sr₂MWO₆ (M = Co, Ni) were prepared by sol-gel method. Crystal Structure, magnetic properties and the morphology of Sr₂CoWO₆ and Sr₂NiWO₆ were investigated. X-ray powder diffraction (XRD) analysis shows single phase structure for Sr₂MWO₆ (M = Co, Ni) without any traces of impurities and the crystal structure of all the samples belongs to the tetragonal I4/m space group. SEM image for Sr₂MWO₆ (M = Co, Ni) indicate that the grains are homogeneous and connect each other very well. The Neel temperature for Sr₂CoWO₆ and Sr₂NiWO₆ are 23 K and 59 K, respectively. Magnetic measurements showed that the magnetic moment in these double perovskites originates mainly from the interactions between Ni ions and Co ions.     

VUV–UV spectroscopic properties of RE (RE = Ce, Eu and Tb)-doped KMLn(PO4)2 (M = Ca, Sr; Ln = Y, La, Lu)

The VUV excited luminescent properties of Ce³⁺, Eu³⁺ and Tb³⁺ in the matrices of KMLn(PO₄)₂ (M²⁺ = Ca, Sr; Ln³⁺ = Y, La, Lu) were investigated. The bands at about 155 nm in the VUV–UV excitation spectra are attributed to the host lattice absorption, which indicates that the optical band gap of KMLn(PO₄)₂ is about 8.0 eV. Ce³⁺-doped samples show typical Ce³⁺ emission in the range of 300–450 nm, and the energy transfer from host lattice to Ce³⁺ is efficient. For Eu³⁺-doped samples, the O²⁻–Eu³⁺ CTBs are observed to be at about 228 nm except KSrLu(PO₄)₂:Eu³⁺ (247 nm). As for Tb³⁺-doped samples, typical 4f → 5d absorption bands in the region of 175–250 nm were observed.     

Nonlinear Optical Sulfides LiMGa8S14 (M = Rb/Ba,Cs/Ba) Created by Li+ Driven 2D Centrosymmetric to 3D Noncentrosymmetric Transformation

Cations that can regulate the configuration of anion group are greatly important but regularly unheeded. Herein, the structural transformation from 2D CS to 3D noncentrosymmetric (NCS, which is the prerequisite for second-order NLO effect) is rationally designed to newly afford two sulfides LiMGa₈S₁₄ (M = Rb/Ba, 1 ; Cs/Ba, 2 ) by introducing the smallest alkali metal Li⁺ cation into the interlamination of 2D centrosymmetric (CS) RbGaS₂. The unusual frameworks of 1 and 2 are constructed from C₂-type [Ga₄S₁₁] supertetrahedrons in a highly parallel arrangement. 1 and 2 display distinguished NLO performances, including strong phase-matchable second-harmonic generation (SHG) intensities (0.8 and 0.9 × AgGaS₂ at 1910 nm), wide optical band gaps (3.24 and 3.32 eV), and low coefficient of thermal expansion for favorable laser-induced damage thresholds (LIDTs, 4.7, and 7.6 × AgGaS₂ at 1064 nm), which fulfill the criteria of superior NLO candidates (SHG intensity >0.5 × AGS and band gap >3.0 eV). Remarkably, 1 and 2 melt congruently at 873.8 and 870.5 °C, respectively, which endows them with the potential of growing bulk crystals by the Bridgeman-Stockbarge method. This investigated system provides a new avenue for the structural evolution from layered CS to 3D NCS of NLO materials.     

Phase evolution and microstructural development in sol-gel derived MSnO3 (M = Ca, Sr and Ba)

Alkaline-earth stannates having the general chemical formula MSnO₃ (M = Ca, Sr and Ba) have been projected as potential electronic ceramics. In view of the information gaps in the reported research, a vigorous and systematic investigation on these exotic materials has been carried out. In this communication, the synthesis of CaSnO₃, SrSnO₃ and BaSnO₃ via sol-gel technique is reported. Infrared spectroscopy and X-ray analyses of various gel samples with different thermal history helped in identifying the reaction pathways and the stage where amorphous gel to crystalline phase transition occurred. Grains of submicron size with narrow size distribution and spherical morphology, were the most noticeable characteristics of sintered calcium metastannate derived by sol-gel method. In the case of barium analogue, a fascinating sugar cube structure (akin to that observed in solid-state reaction and the self-heat-sustained reaction derived samples) having improved density characteristics evolved at low sintering temperatures. This gradually transformed into a more familiar spherical granular motif with improved density characteristics as the sintering profiles were varied from 1200 °C/24 h to 1500 °C/2 h. This seems to be an inherent feature of this system, irrespective of the method of synthesis.     

Synthesis and luminescent properties of SrZnO2:Eu3+,M+ (M = Li,Na, K) phosphor

In this paper, a series of novel luminescent materials, SrZnO₂:Eu³⁺,M⁺ (M = Li, Na, K) have been synthesized by conventional solid-state reaction. X-ray diffraction (XRD) patterns and photoluminescence (PL) spectra were carried out to characterize their structural and luminescent properties. It was found that under ultraviolet excitation with a wavelength of 301 nm, SrZnO₂:Eu³⁺ gives a red luminescence that was attributed to the transitions from ⁵D₀ excited states to ⁷F_J (J = 0–4) ground states of Eu³⁺ ions. The feature and the high intensity of hypersensitive transition ⁵D₀ → ⁷F₂ indicate that Eu³⁺ prefers to occupy a low symmetry site. The incorporation of alkali metal ions greatly enhanced the luminescence intensity and slightly changed the excitation and emission peak position, probably due to the influence of the coordination conditions for Eu³⁺ ions.     

Crystal structure,oxygen permeability and stability of Ba0.5Sr0.5Co0.8Fe0.1M0.1O3−δ (M = Fe,Cr, Mn,Zr) oxygen-permeable membranes

Oxygen-permeable ceramic membrane materials of the Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCFO) and partially Fe-substituted Ba_0.5Sr_0.5Co_0.8Fe_0.1M_0.1O_3−δ (M = Cr, Mn, Zr) were synthesized by solid-state reaction method. These materials possess purely cubic perovskite structure with the exception of Ba_0.5Sr_0.5Co_0.8Fe_0.1M_0.1O_3−δ (M = Mn, Zr), in which minor impurities exist. Oxygen permeability across these dense membrane disks were measured under an air/He oxygen partial pressure gradient in the temperature range of 973–1123 K. The results demonstrated that the oxygen permeation fluxes of the Ba_0.5Sr_0.5Co_0.8Fe_0.1M_0.1O_3−δ (M = Fe, Cr, Mn, Zr) membranes increased in the following order: Fe (BSCFO) > Cr > Zr > Mn. The corresponding activation energies for oxygen permeation of Ba_0.5Sr_0.5Co_0.8Fe_0.1M_0.1O_3−δ (M = Fe, Cr, Zr) membranes were calculated to be similar (53 ± 4 kJ/mol), which was remarkably lower than that (99 ± 3 kJ/mol) of Ba_0.5Sr_0.5Co_0.8Fe_0.1M_0.1O_3−δ (M = Mn) membrane. In addition, good oxygen permeation stability of the Ba_0.5Sr_0.5Co_0.8Fe_0.1M_0.1O_3−δ (M = Cr) membrane was achieved at the temperature lower than 1123 K. The X-ray diffraction (XRD) and differential thermal analysis (DTA) experiments showed that the structural stability of BSCFO could be significantly improved when Fe ions in the BSCFO material were partially substituted by Cr, Mn or Zr ions.     

Structure and magnetic properties of sputtered FePtM (M = Ti,Nb) thin films

FePt thin films doped with various Ti and Nb concentrations ranging from 2.9 to 9.1 at.% were prepared by r.f. magnetron sputtering. The structural and magnetic properties of Ti- and Nb-doped FePt thin films were investigated. Structural studies revealed that the long range ordering in FePt thin films depends on the doping concentration and annealing temperature of FePt thin films. The addition of Ti and Nb is found to enhance the grain refining in FePt thin films. The effects of doping concentration on the magnetic properties of FePt thin films were studied and discussed.     

Spinel ferrite MFe2O4 (M = Ni,Co, or Cu) nanoparticles prepared by a proteic sol-gel route for oxygen evolution reaction

In this work, spinels of MFe₂O₄ (M = {Ni, Co, Cu}) were successfully prepared by proteic sol-gel method using commercial flavorless gelatin as a chelating agent. To break down aggregated particles, the samples were milled in alcohol at 400 rpm for 1 h. According to Rietveld refinements and transmission electron microscopy, the samples had crystallite and particle sizes in the range of 36–53 nm and 44–147 nm, respectively, confirming the as-prepared samples in a nanoscale. X-ray diffraction and Rietveld refinement confirmed that the samples are single phase. In addition, Mössbauer spectroscopy analysis and X-ray photoelectron spectroscopy revealed the mixed spinel composition. Besides, X-ray photoelectron spectroscopy showed surface oxygen vacancies, given by ratio areas between oxygen vacancies (O_V) and oxygen in the lattice (O_L), of 0.63, 0.27 and 0.10 for NiFe₂O₄, CuFe₂O₄ and CoFe₂O₄ powders, respectively. Magnetic measurements showed ferrimagnetic behavior for all samples. Toward oxygen evolution reaction (OER), copper-oxygenated groups on the CuFe₂O₄ nanoparticle surface may play an important role, once CuFe₂O₄ showed superior electrocatalytic performance, with overpotentials of 369 mV (CuFe₂O₄) < 386 mV (NiFe₂O₄) < 448 mV (CoFe₂O₄) at a current density of 10 mA cm^?2 and Tafel slopes of 76.3 mV dec^-1 (CuFe₂O₄), 85.7 mV dec^-1 (NiFe₂O₄) and 148.1 mV dec^-1 (CoFe₂O₄). All samples exhibited mechanical stability during the OER process.     

Synthesis and optical properties of metal (M) atom-doped polycarbosilane (PCS) with extended conjugation (M = Al,Dy, Er and Eu)

Optical properties of metal atom-doped polycarbosilane (PCS) which originated from σ-conjugation effect were studied. Al, Dy, Er and Eu were introduced into PCS by one-pot method to yield polyaluminocarbosilane (PACS), polydysprosiumcarbosilane (PDCS), polyerbiumcarbosilane (PErCS) and polyeuropiumcarbosilane (PECS), respectively. Effects of oxidation curing and ultraviolet (UV) radiation on the photoluminescence (PL) properties of the samples were investigated. PL spectra show strong blue light-emissions and the intensity of PCS is enhanced by adding metal atoms. PACS with extended σ-conjugation exhibits an obvious PL red-shift, high intensity, high quantum yield and excellent oxidation resistance as compared with those of others. As treated under UV lamp for 3 h in air, PACS retains good UV resistance performance, owing to the AlOx (x = 4, 5, or 6) groups which effectively extend the σ-conjugation. The obtained results are expected to have important applications in active sources for electroluminescence (EL) devices, especially suitable for blue emission.     

Double perovskite oxides Sr2MMoO6 (M = Fe and Co) as cathode materials for oxygen reduction in alkaline medium

The oxygen reduction reaction (ORR) was studied on Sr₂MMoO₆ (M = Fe and Co) double perovskites, prepared by a solid-state reaction, in 0.5 M NaOH at 25 °C with a rotating disk electrode (RDE). The two oxide powders were characterized by X-ray diffraction, scanning electron microscopy and BET analysis. The electrochemical techniques considered are linear voltammetry, steady state polarization and ac impedance spectroscopy. The electrocatalysts (SFMO/C, SCMO/C) consisting of the double perovskite oxides and carbon (Vulcan XC-72) were mixed and spread out into a thin layer on a glassy carbon substrate. At room temperature, a significantly electrocatalytic activity is observed for both electrocatalysts. Compared to SFMO/C, the SCMO/C electrocatalyst was found to show a relatively high electrocatalytic activity for O₂ reduction, which agrees well with the results obtained using the ac impedance spectroscopy.     

Synthesis of nanocrystalline MMoO4 (M = Ni,Zn) phosphors via a citrate complex route assisted by microwave irradiation and their photoluminescence

Nanocrystalline MMoO₄ (M = Ni, Zn) phosphors, which have wolframite-type structure, were successfully synthesized at low temperatures via a modified citrate complex route assisted by microwave irradiation. The citrate complex precursors were heat-treated from 300 to 600 °C for 3 h. Crystallization of the MMoO₄ (M = Ni, Zn) nanocrystallites were detected at 500 °C, and entirely completed at a temperature of 600 °C. The nanoparticles presented primarily dispersed and homogeneous morphology with particle size of 20–40 nm. The nanocrystalline MMoO₄ (M = Ni, Zn) phosphors prepared at 600 °C exhibited broad luminescence in green and blue wavelength region, respectively.     

A2−αAα′BO4-type oxides as cathode materials for IT-SOFCs (A = Pr,Sm; A′ = Sr; B = Fe,Co)

Preparation, thermal expansion, electrical conductivity and polarization of A_2−αA _α′BO₄-type oxides (A = Pr, Sm, A′ = Sr, B = Fe, Co) were investigated systematically to evaluate their potential as cathode materials for IT-SOFCs. Within 0.8 ≤ α ≤ 1.5, A_2−αSr_αBO_4−δ (A = Pr, Sm, B = Fe, Co) could be obtained as a single K₂NiF₄-structural phase. Thermal expansion coefficients (TECs) of the specimens increase with increasing Sr²⁺ content, TECs of cobaltites are much higher than that of ferrites. The electrical conductivity of cobaltites is in the order of 10² S cm^− 1 near 800 °C, which is acceptable for the cathode of IT-SOFC. Polarization measurements showed that Sm_0.5Sr_1.5CoO_4−δ exhibited the lowest cathodic overpotential at 700–900 °C (72 mV at 500 mA/cm² at 800 °C), being a high potential candidate of cathode material for IT-SOFCs.     

Li doping effects on the upconversion luminescence of Yb3+/Er3+-doped ABO4 (A = Ca,Sr; B = W,Mo) phosphors

ABO₄ (A = Ca, Sr; B = W, Mo):Er³⁺/Yb³⁺/Li⁺ phosphors tri-doped with different concentrations of Li⁺ ion ranging from 0 to 22.5 mol% were prepared by using a solid-state reaction method. And their upconversion (UC) luminescence properties were in estimated under a 975 nm laser-diode excitation. The four kinds of phosphors (CaWO₄, CaMoO₄, SrWO₄, and SrMoO₄) tri-doped with Er³⁺, Yb³⁺ and Li⁺ ions showed strong green UC emission peaks at 530 nm and 550 nm and weak red UC emission. The intensity of green UC emission of Li⁺ doped samples was several higher than that of Li⁺ un-doped samples due to the reduction of lattice constant and the local crystal field distortion around rare-earth ions. The optimum doping concentration of Li⁺ ions was investigated and the effects of Li⁺ concentration for UC emission intensity were studied in detail.     

Crystal field effect on the effective magnetic moment in A2CoWO6 (A = Ca,Sr and Ba)

We prepared the antiferromagnetic Ca₂CoWO₆, Sr₂CoWO₆ and Ba₂CoWO₆ double perovskites with different crystalline structures, which are monoclinic, tetragonal and cubic respectively. The crystalline structures affect the Co–O distances and the local octahedral symmetry. Magnetic susceptibility experiments were performed as a function of magnetic field and temperature. We found that the effective magnetic moment (μ_eff) of Co²⁺ is partially unquenched and we also found that μ_eff increases with the average of Co–O distances that form the CoO₆ octahedra. A simple model associates the behavior of the effective magnetic moment with crystal field effects.