Luminescence in trivalent rare earth activated Sr4Al2O7 phosphor

In this paper we report the combustion synthesis of trivalent rare-earth (RE³⁺ = Dy, Eu and Ce) activated Sr₄Al₂O₇ phosphor. The prepared phosphors were characterized by the X-ray powder diffraction (XRD) and photoluminescence (PL) techniques. Photoluminescence emission peaks of Sr₄Al₂O₇:Dy³⁺ phosphor at 474 nm and 578 nm in the blue and yellow region of the spectrum. The prepared Eu³⁺ doped phosphors were excited by 395 nm then we found that the characteristics emission of europium ions at 615 nm (⁵D₀?⁷F₂) and 592 nm (⁵D₀?⁷F₁). Photoluminescence (PL) peaks situated at wavelengths of 363 and 378 nm in the UV region under excitation at around 326 nm in the Sr₄Al₂O₇:Ce³⁺ phosphor.     

Conductivity and water uptake of Sr4(Sr2Nb2)O11·nH2O and Sr4(Sr2Ta2)O11·nH2O

The hydrated oxygen deficient complex perovskite-related materials Sr₄(Sr₂Nb₂)O₁₁·nH₂O and Sr₄(Sr₂Ta₂)O₁₁·nH₂O were studied at high water vapour pressures over a large temperature range by electrical conductivity measurements, thermogravimetry (TG), and X-ray powder diffraction (XRPD). In humid atmospheres both materials are known to exhibit protonic conductivity below dehydration temperatures, with peak-shaped maxima at about 500 °C. In this work we show that the peaks expand to plateaus of high conductivity from 500 to 700 °C at a water vapour pressure of 1 atm. However, in situ synchrotron XRPD of Sr₄(Sr₂Nb₂)O₁₁·nH₂O as a function of temperature shows that these observations are in fact coincident with melting and dehydration of a secondary phase Sr(OH)₂. The stability of Sr₄(Sr₂Nb₂)O₁₁·nH₂O and Sr₄(Sr₂Ta₂)O₁₁·nH₂O in humid atmospheres is thus insufficient, causing decomposition into perovskites with lower Sr content and SrO/Sr(OH)₂ secondary phases. This, in turn, rationalizes the observation of peaks and plateaus in the conductivity of these materials.     

Enhanced luminescent properties of long-persistent Sr2MgSi2O7:Eu, Dy phosphor prepared by the co-precipitation method

Sr₂MgSi₂O₇:Eu²⁺, Dy³⁺ phosphors were prepared by the (aminopropyl)-triethoxysilane (APTES) co-precipitation method. Effects of synthesis temperature on the crystal characteristics, luminescent properties and afterglow performance of Sr₂MgSi₂O₇:Eu²⁺, Dy³⁺ phosphors have been discussed in detail and compared with the corresponding commercial product. The experimental results indicated that the sample could be synthesized at a relatively lower temperature and had better performance on the above-mentioned properties using the co-precipitation method.     

Effects of dopant concentrations on phosphorescence properties of Eu/Dy-doped Sr3MgSi2O8

Long afterglow Sr₃MgSi₂O₈: Eu, Dy phosphor with high brightness was prepared by sintering at high temperature and weak reductive atmosphere. The luminescent properties of this photoluminescent pigment were studied systematically by investigating concentration effects. The analytical results indicated that the main emission peaks appear at 482 nm. The excitation and emission spectra of this phosphor show that both of them are broadband. This is ascribed to the 4f⁷→4f⁶5d¹ transition of Eu²⁺ in the pigment matrix, which is in good agreement with the calculated value of 470 nm, and implies that luminescent centers Eu²⁺ occupy the deca-coordinated Sr²⁺ sites with the host of Sr₃MgSi₂O₈.     

Synthesis,characterization and magnetic properties of MFe2O4 (M=Co,Mg, Mn,Ni) nanoparticles using ricin oil as capping agent

We focused on obtaining MFe₂O₄ nanoparticles using ricin oil solution as surfactant and on their structural characterization and magnetic properties. The annealed samples at 500 °C in air for 6 h were analyzed for the crystal phase identification by powder X-ray diffraction using CuKα radiation. The particle size, the chemical composition and the morphology of the calcinated powders were characterized by scanning electron microscopy. All sintered samples contain only one phase, which has a cubic structure with crystallite sizes of 12–21 nm. From the infrared spectra of all samples were observed two strong bands around 600 and 400 cm⁻¹, which correspond to the intrinsic lattice vibrations of octahedral and tetrahedral sites of the spinel structure, respectively, and characteristic vibration for capping agent. The magnetic properties of fine powders were investigated at room temperature by using a vibrating sample magnetometer. The room temperature M–H hysteresis loops show ferromagnetic behavior of the calcined samples, with specific saturation magnetization (M_s) values ranging between 11 and 53 emu/g.     

Comparative analysis of crystal field effects and energy level scheme of six-fold coordinated Cr ion in the pyrochlores, Y2B2O7 (B=Ti, Sn)

The electronic energy levels of the six-fold coordinated Cr⁴⁺ ion in the pyrochlores Y₂B₂O₇ (B=Sn⁴⁺, Ti⁴⁺), have been computed using the exchange charge model of crystal field theory. The calculated Cr⁴⁺ energy levels and their trigonal splitting are in good agreement with experimental spectra. Calculations of the crystal field parameters show that the higher crystal field strength in Y₂Sn₂O₇ (in comparison with Y₂Ti₂O₇) arises from increased orbital overlap effects between the Cr⁴⁺ ion and the nearest oxygen ions, which are located at the 48f crystallographic position of the pyrochlore lattice. The increased overlap in Y₂Sn₂O₇ occurs despite the fact that the Cr⁴⁺-O^2- bond distance in Y₂Sn₂O₇ is longer than in Y₂Ti₂O₇. This is attributed to a lack of hybridization (covalent bonding) between the filled 2p orbital of oxygen ion occupying the 48f site of the pyrochlore lattice and the filled Sn⁴⁺ 4d¹⁰ orbital. As a result, a stronger crystal field is experienced by Cr⁴⁺ ions in Y₂Sn₂O₇, even if the Cr⁴⁺-O²⁻ distances are greater in this case, when compared to those in Y₂Ti₂O₇.     

Double-ceramic-layer thermal barrier coatings based on La2(Zr0.7Ce0.3)2O7/La2Ce2O7 deposited by electron beam-physical vapor deposition

Double-ceramic-layer (DCL) thermal barrier coatings (TBCs) of La₂(Zr_0.7Ce_0.3)₂O₇ (LZ7C3) and La₂Ce₂O₇ (LC) were deposited by electron beam-physical vapor deposition (EB-PVD). The composition, interdiffusion, surface and cross-sectional morphologies, cyclic oxidation behavior of DCL coating were studied. Energy dispersive spectroscopy and X-ray diffraction analyses indicate that both LZ7C3 and LC coatings are effectively fabricated by a single LZ7C3 ingot with properly controlling the deposition energy. The chemical compatibility of LC coating and thermally grown oxide (TGO) layer is unstable. LaAlO₃ is formed due to the chemical reaction between LC and Al₂O₃ which is the main composition of TGO layer. Additionally, the thermal cycling behavior of DCL coating is influenced by the interdiffusion of Zr and Ce between LZ7C3 and LC coatings. The failure of DCL coating is a result of the sintering of LZ7C3 coating surface, the chemical incompatibility of LC coating and TGO layer and the abnormal oxidation of bond coat. Since no single material that has been studied so far satisfies all the requirements for high temperature applications, DCL coating is an important development direction of TBCs.     

Luminescent properties of RE-activated CaAl2B2O7 (RE=Tb, Ce) in VUV-visible region

RE³⁺-activated α- and β-CaAl₂B₂O₇ (RE=Tb, Ce) were synthesized with the method of high-temperature solid-state reaction. Their VUV excitation and VUV-excited emission spectra are measured and discussed in the present article. The charge transfer band of Tb³⁺ and Ce³⁺ is respectively calculated to be at 151±2 and 159±3 nm. All the samples show an activator-independent excitation peak at about 175 nm and an emission peak at 350-360 nm ascribed to the host absorption and emission band, respectively.     

Influence of postdeposition annealing on structural properties and electrical characteristics of thin Tm2O3 and Tm2Ti2O7 dielectrics

We describe the structural properties and electrical characteristics of thin thulium oxide (Tm₂O₃) and thulium titanium oxide (Tm₂Ti₂O₇) as gate dielectrics deposited on silicon substrates through reactive sputtering. The structural and morphological features of these films were explored by X-ray diffraction, X-ray photoelectron spectroscopy, secondary ion mass spectrometry, and atomic force microscopy, measurements. It is found that the Tm₂Ti₂O₇ film annealed at 800 °C exhibited a thinner capacitance equivalent thickness of 19.8 Å, a lower interface trap density of 8.37 × 10¹¹ eV⁻¹ cm⁻², and a smaller hysteresis voltage of ∼4 mV than the other conditions. We attribute this behavior to the Ti incorporated into the Tm₂O₃ film improving the interfacial layer and the surface roughness. This film also shows negligible degrees of charge trapping at high electric field stress.     

Electronic band structure and Fermi surface of new low-temperature Ni-based superconductors: 3.3 K (Ni2P2)(Sr4Sc2O6) and 2.7 K (Ni2As2)(Sr4Sc2O6) from first principles

Based on first-principle FLAPW-GGA calculations, we have investigated structural and electronic properties of the recently synthesized tetragonal (space group P4/nmm) nickel-based pnictide oxide superconductors: 3.3 K (Ni₂P₂)(Sr₄Sc₂O₆) and 2.7 K (Ni₂As₂)(Sr₄Sc₂O₆). Optimized structural data, electronic bands, total and partial densities of states, and Fermi surface topology have been obtained and discussed in comparison with available experiments and with their Fe-based (Fe₂P₂)(Sr₄Sc₂O₆) and (Fe₂As₂)(Sr₄Sc₂O₆) analogs.     

Luminescent properties of green- or red-emitting Eu-doped Sr3Al2O6 for LED

Eu²⁺-doped Sr₃Al₂O₆ (Sr_3−xEu_xAl₂O₆) was synthesized by a solid-state reaction under either H₂ and N₂ atmosphere or CO atmosphere. When H₂ was used as the reducing agent, the phosphor exhibited green emission under near UV excitation, while CO was used as the reducing agent, the phosphor mainly showed red emission under blue light excitation. Both emissions belong to the d-f transition of Eu²⁺ ion. The relationship between the emission wavelengths and the occupation of Eu²⁺ at different crystallographic sites was studied. The preferential substitution of Eu²⁺ into different Sr²⁺ cites at different reaction periods and the substitution rates under different atmospheres were discussed. Finally, green-emitting and red-emitting LEDs were fabricated by coating the phosphor onto near UV- or blue-emitting InGaN chips.     

Comparison of magnetic properties in LixCoO2 and its decomposition products LiCo2O4 and Co3O4

Thermal stability of cathode material in the charged state is an important aspect for the safety of rechargeable batteries. It is well known that layered Li_xCoO₂ decomposes to a mixture of LiCoO₂ and Co₃O₄ at elevated temperatures. However, not many experimental evidences exist on intermediate phases those may form during the decomposition. Using magnetic measurements we show that it is possible to distinguish between the spinels LiCo₂O₄ and Co₃O₄ and thereby follow the decomposition of Li_xCoO₂. We characterize the magnetic behavior of thermally aged Li_xCoO₂ (x = 0.98, 0.76, 0.55) with increasing annealing time. Our results reveal the appearance of magnetic ordering in the thermally degraded products. The detailed analysis illustrates that the formation of Co₃O₄ is preceded by the formation of a meta stable LiCo₂O₄ phase.     

Synthesis and magnetic properties of hard magnetic (CoFe2O4)-soft magnetic (Fe3O4) nano-composite ceramics by SPS technology

CoFe₂O₄/Fe₃O₄ nano-composite ceramics were synthesized by Spark Plasma Sintering. The X-ray diffraction patterns show that all samples are composed of CoFe₂O₄ and Fe₃O₄ phases when the sintering temperature is below 900 °C. It is found that the magnetic properties strongly depend on the sintering temperature. The two-step hysteresis loops for samples sintered below 500 °C are observed, but when sintering temperature reaches 500 °C, the step disappears, which indicates that the CoFe₂O₄ and Fe₃O₄ are well exchange coupled. As the sintering temperature increases from 500 to 800 °C, the results of X-ray diffractometer indicate the constriction of crystalline regions due to the ion diffusion at the interfaces of CoFe₂O₄/Fe₃O₄ phases, which have great impact on the magnetic properties.     

Spectral properties of Nd-doped Sr3Ga2Ge4O14 crystal

Neodymium doped strontium gallogermanate crystals were grown successfully by the Bridgman technique. The linear thermal expansion coefficients for the c- and a-axes were measured as 5.8 × 10⁻⁶ °C⁻¹ and 6.5 × 10⁻⁶ °C⁻¹. Absorption spectra, and fluorescence spectra, as well as fluorescence decay curves of Nd³⁺-doped Sr₃Ga₂Ge₄O₁₄ crystal, have been recorded at room temperature and used to calculate the absorption and stimulated emission cross-sections. Based on the Judd-Ofelt theory, three intensity parameters were obtained. The luminescent quantum efficiency of the ⁴F_3/2 level was determined to be approximately 73.8% for this material. Compared with other Nd³⁺-doped laser crystals, Nd³⁺-doped Sr₃Ga₂Ge₄O₁₄ crystal displays special laser properties due to its disorder structure.     

Synthesis,magnetic and electrical transport properties of magnetoresistance material Sr2FeMoO6 by microwave sintering

Magnetoresistance material Sr₂FeMoO₆ with double perovskite structure was synthesized by microwave sintering method using SrCO₃, Fe₂O₃ and MoO₃ as raw materials, with MnO₂ for microwave absorber. The phase structure, magnetic and electrical transport properties were investigated by X-ray powder diffraction (XRD) and vibrating-sample magnetometer. XRD analysis shows that the as-synthesized sample is Sr₂FeMoO₆ with tetragonal crystal structure and I4/mmm space group. The unit cell parameters are a=0.5587 nm, c=0.7894 nm, volume=0.2464 nm³. The calculated grain size of the sample is 31.62 nm, which is obtained by the Scherrer formula using the diffraction data. Magnetism testing results show that the sample Sr₂FeMoO₆ is ferromagnetic with the magnetic transition temperature of about 380 K. Under 1.0 T magnetic field, the saturation and spontaneous magnetization of Sr₂FeMoO₆ is 1.25 μ_B/f.u. and 1.00 μ_B/f.u. at room temperature. The magnetoresistance ratio of the sample is 28%. Electrical transport properties testing results indicate that the sample exhibits typical semiconductor behavior. The conductive mechanism of Sr₂FeMoO₆ is highly dependent on temperature: within the temperature range of 100–300 K, the mechanism is attributed to the small polaron variable-range hopping model; while it is ascribed to the adiabatic small polaron model within the temperature range of 80–100 K.     

Analysis of the magnetic structure of the manganese oxychalcogenides R2Mn2Se2O (R=LaO,BaF) by density functional calculations

We examined the magnetic structures of R₂Mn₂Se₂O (R=LaO, BaF) by evaluating the spin exchange interactions on the basis of density functional theory (DFT) calculations, and compared how they differ from that of the Fe analog (BaF)₂Fe₂Se₂O. In R₂Mn₂Se₂O (R=LaO, BaF), the two-dimensional antiferromagnetic square lattice defined by J₁ is spin-frustrated, and the exchange J₁ is stronger for (LaO)₂Mn₂Se₂O than for (BaF)₂Mn₂Se₂O by a factor of 1.5 explaining why the magnetic susceptibility maximum occurs at a higher temperature for (LaO)₂Mn₂Se₂O than for (BaF)₂Mn₂Se₂O (360 vs. 210 K). We identified two probable reasons why the specific heat anomaly at the long-range antiferromagnetic ordering temperature T_N is significantly weaker for R₂Mn₂Se₂O than for (BaF)₂Fe₂Se₂O.     

Structural properties and electrical characteristics of high-k Tm2Ti2O7 gate dielectrics

In this article, the structural and electrical characteristics of high-k Tm₂Ti₂O₇ gate dielectrics deposited on Si (1 0 0) by means of reactive cosputtering were reported. The Tm₂Ti₂O₇ dielectrics annealed at 800 °C exhibited excellent electrical properties such as high capacitance value, small density of interface state, almost no hysteresis voltage, and low leakage current. This phenomenon is attributed to a rather well-crystallized Tm₂Ti₂O₇ structure and composition and a smooth surface observed by X-ray diffraction, X-ray photoelectron spectroscopy, and atomic force microscopy, respectively. This film also shows almost negligible charge trapping under high constant voltage stress.     

Theoretical investigation of optical spectra and covalent effect of Cr in Y2Ti2O7 and Y2Sn2O7

In this work, the complete matrix of optical spectral levels in trigonal symmetry of 3d² (3d⁸) ions are established on basis of strong field coupling mechanism by using two spin–orbit coupling parameters model. The contribution of the spin–orbit coupling of ligand to the optical spectra has been included in these formulas. As an application, the optical spectra of Cr⁴⁺ in Y₂Ti₂O₇ and Y₂Sn₂O₇ have been studied by the complete diagonalization (energy matrix) method. The covalent effect has been studied and the difficulty about Dq parameter in explanation of optical spectra of Cr-doped Y₂Ti₂O₇ and Y₂Sn₂O₇ is removed. The theoretical results are in good agreement with observed data.     

The low temperature synthesis of Euand Dy activated Sr3Al2O6 nanophosphors by microwave method

The Eu²⁺and Dy³⁺ activated Sr₃Al₂O₆ (S3A2O-ED) nanophosphors were synthesized by a new microwave method. The S3A2O-ED sample calcined in microwave oven at around 650 °C for 20 min possesses a cubic Sr₃Al₂O₆ single phase. The sample showed small size (80–100 nm) and spherical shape. The excitation and emission spectra indicated that excitation broad band chiefly sited in visible range and the nanophosphors emitted strong light at 611 nm under around 473 nm excitation. Comparing with conventional method, the microwave synthesis of S3A2O-ED greatly decreased the calcining temperature and time. However, the brightness of S3A2O-ED nanophosphors was reduced. The change of luminescent intensity in S3A2O-ED nanophosphors could be attributed to the effect of surface energy.     

Preparation and magnetic properties of BaFe12O19/Ni0.8Zn0.2Fe2O4 nanocomposite ferrite

Nanocomposite of hard (BaFe₁₂O₁₉)/soft ferrite (Ni_0.8Zn_0.2Fe₂O₄) have been prepared by the sol–gel process. The nanocomposite ferrite are formed when the calcining temperature is above 800 °C. It is found that the magnetic properties strongly depend on the presintering treatment and calcining temperature. The “bee waist” type hysteresis loops for samples disappear when the presintering temperature is 400 °C and the calcination temperature reaches 1100 °C owing to the exchange-coupling interaction. The remanence of BaFe₁₂O₁₉/Ni_0.8Zn_0.2Fe₂O₄ nanocomposite ferrite with the mass ratio of 5:1 is higher than a single phase ferrite. The specific saturation magnetization, remanence magnetization and coercivity are 63 emu/g, 36 emu/g and 2750 G, respectively. The exchange-coupling interaction in the BaFe₁₂O₁₉/Ni_0.8Zn_0.2Fe₂O₄ nanocomposite ferrite is discussed.