Combustion synthesis of Sr3MgSi2O8:Eu and Sr2MgSi2O7:Eu phosphors

Sr₃MgSi₂O₈:Eu²⁺ and Sr₂MgSi₂O₇:Eu²⁺ phosphors find uses in applications such as plasma display panel (PDP), solid-state lighting, longafter glow. Preparation of these phosphors by a modified combustion synthesis is described in this paper. As-prepared samples did not show photoluminescence. After reducing the samples at 900 °C, characteristic Eu²⁺ emission was observed. Preparation of these phosphors by using similar methods helped clarifying various results obtained for Sr₃MgSi₂O₈:Eu²⁺ by different investigators.     

Phase transition in Cd2Ta2O7 and the Cd2Nb2−xTaxO7 series

A phase transition has been discovered in Cd₂Ta₂O₇ near 200 K which is about the same temperature at which Cd₂Nb₂O₇ is known to become ferroelectric. However, Cd₂Ta₂O₇ does not become ferroelectric below this transition. The nature of this transition was also studied in the Cd₂Nb_2?xTa_xO₇ series using low temperature X-ray, SHG, and DSC techniques.     

Impurity effect on the Raman spectra of Sr0.6Ba0.4Nb2O6

Impurity effect on the rare earth ion doped Sr_0.6Ba_0.4Nb₂O₆(SBN40) was studied at room temperature. Doping the rare earth ions of Pr³⁺ or Nd³⁺ changed the Raman profile: spectral broadening, central frequency shifts and relative intensity decreasing. Two reasons are considered according to the ferroelectric and optical properties of the rare earth ion doped SBN40: impurity-induced crystal disorder and the crystal structure change. SBN40 is the general disorder crystal and the disorder which is enhanced by doping the rare earth ion is especially strong along the x?y plane of the material compared with that along the polar C-axis.     

Phase transitions in ferroelectric ceramics of the type Sr0.5Ba0.5Nb2O6

The ferro-paraelectric transition in Sr_0.5Ba_0.5Nb₂O₆ ceramics has been studied by measuring the dielectric pennitivity as a function of temperature at different frequencies. Experimental results in the transition region over ?'_max fit the relation $\text{1}\text{?'}\text{?}\text{1}\text{?'}{}_{\max }\text{= A(T ?}\text{T}{}_0\text{)}{}^{\mathrm{\gamma }}$ with γ > 1, as is typical for dif transitions. In an interval of temperatures of 17°C over T₀ (=Tfor?'_max), γ = 2. Using the relaxors' model of Kirillov-Isupov an equation is inferred in order to compute the activation energy of the relaxors as a function of parameters directly obtained from the experimental results.     

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.     

Spectral properties of Nd:Ca2Nb2O7 crystal

This paper reports the spectral properties of Nd³⁺:Ca₂Nb₂O₇. The spectral parameters of Nd³⁺ in Nd³⁺:Ca₂Nb₂O₇ crystal have been investigated based on Judd-Ofelt theory. The spectral parameters were obtained. The parameters of line strengths Ω_λ are Ω₂=4.967×10⁻²⁰ cm², Ω₄=5.431×10⁻²⁰ cm², Ω₆=5.693×10⁻²⁰ cm². The radiative lifetime, the fluorescence lifetime and the quantum efficiency are 122 μs, 103 μs and 84.4%, respectively. The fluorescence branch ratios calculated: β₁=0.425, β₂=0.479, β₃=0.091, β₄=0.004. The emission cross section at 1068 nm is 6.204×10⁻²⁰ cm².     

Electronic structure of the Sr2MgSi2O7:Eu persistent luminescence material

The electronic structures of the distrontium magnesium disilicate (Sr₂MgSi₂O₇(:Eu²⁺)) materials were studied by a combined experimental and theoretical approach. The UV-VUV synchrotron radiation was applied in the experimental study while the electronic structures were investigated theoretically by using the density functional theory. The structure of the valence and conduction bands and the band gap energy of the material as well as the position of the Eu²⁺ 4f ground state were calculated. The calculated band gap energy (6.7 eV) agrees well with the experimental value of 7.1 eV. The valence band consists mainly of the oxygen states and the bottom of the conduction band of the Sr states. The calculated occupied 4f ground state of Eu²⁺ lies in the energy gap of the host though the position depends strongly on the Coulomb repulsion strength. The position of the 4f ground state with respect to the valence and conduction bands is discussed using the theoretical and experimental evidence available.     

Tuning the Mottness in Sr3Ir2O7 via Bridging Oxygen Vacancies

The electronic evolution of Mott insulators into exotic correlated phases remains puzzling, because of electron interaction and inhomogeneity. Introduction of individual imperfections in Mott insulators could help capture the main mechanism and serve as a basis to understand the evolution. Here we utilize scanning tunneling microscopy to probe the atomic scale electronic structure of the spin-orbit-coupling assisted Mott insulator Sr₃Ir₂O₇. It is found that the t...     

Maxwell-Wagner characterization of dielectric relaxation in Ni0.8Zn0.2Fe2O4/Sr0.5Ba0.5Nb2O6 composite

Dense composites were prepared through incorporating the dispersed Ni_0.8Zn_0.2Fe₂O₄ ferromagnetic particles into Sr_0.5Ba_0.5Nb₂O₆ ferroelectric matrix. Extrinsic dielectric relaxation and associated high permittivities of the materials are reported in the composites. We used an ideal equivalent circuit to explain electrical responses in impedance formalism. A Debye-like relaxation in the permittivity formalism was also found. Interestingly, real permittivity (ε′) of the sample containing 30% Ni_0.8Zn_0.2Fe₂O₄ shows obvious independence of the temperature at 100 kHz. Dielectric relaxation and high-ε′ properties of the composites are explained in terms of the Maxwell-Wagner (MW) polarization model.     

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.     

Improved microstructure, dielectric and ferroelectric properties of microwave-sintered Sr0.5Ba0.5Nb2O6

Ferroelectric strontium barium niobate (SBN) powder was synthesized by solid-state route. The green compacts were sintered in the temperature range 1250–1350 °C in a microwave furnace. The microstructural study, using SEM, revealed the grain size in the range 2–20 μm without microcracks, depending on the sintering temperature. This is in contrast to that obtained conventionally for sintered SBN powder synthesized by solid-state route, which resulted in duplex microstructure with microcracks [N.S. VanDamme, A.E. Sutherland, L. Jones, K. Bridger, S.R. Winzer, J. Am. Ceram. Soc. 74 (1991) 1785; P.K. Patro, A.R. Kulkarni, C.S. Harendranath, Ceram. Int. 30 (2004) 1405; P.K. Patro, Trans. Ind. Inst. Metals 59 (2006) 229]. Impedance analyzer and ferroelectric hysteresis loop tracer were used for the dielectric, ferroelectric, I–V and electric fatigue measurements. Microwave sintering also resulted in a dielectric maxima ε_max 2590 at 1 kHz (compared to 1103 for conventional sintering [P.K. Patro, A.R. Kulkarni, C.S. Harendranath, Ceram. Int. 30 (2004) 1405]) for the sintering temperature of 1350 °C. Similar improvement in microwave-sintered SBN was observed depending on their sintering condition. The origin of this difference is attributed to microstructural differences arising from difference in heating method. In this paper the observed dielectric and ferroelectric behavior have been correlated with different sintering conditions and consequent microstructure due to microwave sintering.     

(1-x)Sr_3Sn_2O_7+xCa_3Mn_2O_7陶瓷合成及其光电性能

为了寻求新的非常规多铁性材料,采用固相合成方法制备具有Ruddlesden-Popper结构的(1-x)Sr_3Sn_2O_7+x Ca_3Mn_2O_7(SSO+CMO)(x=0,0.05,0.10,0.15,0.20)系列陶瓷.通过X射线衍射、紫外可见光谱和磁性测量,发现SSO+CMO陶瓷为单一正交相结构,空间群为A21am.随着掺杂量x的增加,样品的晶胞参数和体积相应地减小;在室温不同频率下,样品的介电常数和介电损耗随着频率增加而减小,且在x=0.1时有弱的铁磁性.     

Preparation and characterization of Cd2Nb2O7 thin films on Si substrates

Thin Cd₂Nb₂O₇ films were grown on single-crystal p-type SiO₂/Si substrates by the metallo-organic decomposition (MOD) technique. The films were investigated by X-ray diffraction, X-ray energy-dispersive spectroscopy, and field emission scanning electron microscopy, and showed a single phase (cubic pyrochlore), a crack-free spherical grain structure, and nanoparticles with a mean size of about 68 nm. A Cauchy model was also used in order to obtain the thickness and index of refraction of the stack layers (transparent layer/SiO₂/Si) by spectroscopic ellipsometry (SE). The dielectric constant (K) of the films was calculated to be about 25 from the capacitance-voltage (C-V) measurements.     

Investigation on Eu doped Sr2MgSi2O7 red-emitting phosphors for white-light-emitting diodes

In this work, we report the high temperature solid-state synthesis of red phosphors Sr₂MgSi₂O₇: Eu³⁺ with various Eu³⁺ concentrations. Their luminescent properties at room temperature are investigated. The X-ray diffraction patterns indicate that the red phosphors powder conforms to the tetragonal Sr₂MgSi₂O₇. Impurity structure appears when more than 20% Eu³⁺ is doped. The samples show a strong emission line at 615 nm and the intensity increases with the increase of Eu³⁺ concentration until concentration quenching occurs. Charge compensation assists in the reduction of the impurity structure and vacancies; hence the luminescent intensity is enhanced. The decay measurement indicates that the lifetime of Eu³⁺ emission is about 2-3 ms. Some of the Eu³⁺ can be reduced to Eu²⁺; this is also discussed.     

Synthesis and characterizations of Ho2O3 modified SrBi2Nb2O9 ceramics

SrBi_2-xHo_xNb₂O₉ (0?≤?x?≤?0.5) ceramics were prepared through the conventional solid state route. The materials have been characterized by XRD, FTIR and SEM. All compounds were pure and well crystallized. In FTIR spectroscopy, the spectral patterns of the crystal structures of these polyphases are unique and smoothly different from each other. The grains were prone to become spherical with increasing x. Dielectric and electrical properties of these materials as a function of temperature at different frequencies have been carried out. The dielectric constant and dielectric loss were found to decrease with an increase of the holmium concentration at room temperature. Reaching up to x?=?0.4 and x?=?0.5, the diffusivity increases, leading to the occurrence of relaxor behavior. 20–30 at.% Ho doping on the Bi-site can fill up the oxygen vacancies and decrease the AC conductivity. However, at higher doping levels greater than 40% holmium oxide, the AC conductivity is found to increase.     

PbBi2Nb2O9的晶体结构

PbBi₂Nb₂O₉晶体属正交晶系,空间群为D_2h²³-Fmmm。晶胞参数为a=5.4806(7)?,b=5.4791(7)?,c=25.416(3)?,α=β=γ=90.0(0)°,V=763.2(2)?³。单位晶胞内化学式数z=4,D_obs=7.91g/cm³,D_x=8.26g/cm³ 关键词：     

Single crystal growth and electronic structure of Rh-doped Sr3Ir2O7

Ruddlesden-Popper iridate Sr₃Ir₂O₇ is a spin–orbit coupled Mott insulator. Hole doped Sr₃Ir₂O₇ provides an ideal platform to study the exotic quantum phenomena that occur near the metal–insulator transition(MIT) region. Rh substitution of Ir is an effective method to induce hole doping into Sr₃Ir₂O₇ . However, the highest doping level reported in Sr3(Ir1-xRhx)2O7 single crystals was only around 3%,...     

Structural distortion and phase transition studies of Aurivillius type Sr1−xPbxBi2Nb2O9 ferroelectric ceramics

In this paper, effects of lead doping on the lattice response and phase transitions of Sr_1−xPb_xBi₂Nb₂O₉ (x=0.0-0.5 in steps of 0.1) ferroelectric ceramics are reported. It is observed that structure attains more tetragonality with doping of lead up to 40%. Increased orthorhombic distortion is observed for undoped SBN and 50 at.% lead substituted SBN. Phase transitions for all samples were studied using Curie temperature measurements and are explained in terms of lattice response of these ceramics. Sample with x=0.5 shows decreased tetragonal strain and Curie temperature. Relationship of polarization with lattice response is discussed.     

Incommensurate modulations in Ba2TiSi2O8, Sr2TiSi2O8, and Ba2TiGe2O8

Incommensurate modulations in Ba₂TiSi₂O₈, Sr₂TiSi₂O₈, and Ba₂TiGe₂O₈ are compared based on their corresponding electron diffraction patterns. The dependence of the modulations on chemical composition provides a suitable model system for the investigation of incommensurations in framework structures using high-resolution transmission electron microscopy (HRTEM). A widening of quantitative HRTEM utilising the concept of rigid units is proposed allowing for a determination of atomistic displacements responsible for the formation of incommensurately modulated structures.     

A novel blue-emitting long-lasting proyphosphate phosphor Sr2P2O7:Eu,Y

By introducing the Y³⁺ into Sr₂P₂O₇:Eu²⁺, we successfully prepared a kind of new phosphor with blue long-lasting phosphorescence by the high-temperature solid-state reaction method. In this paper, the properties of Sr₂P₂O₇:Eu²⁺,Y³⁺ were investigated utilizing XRD, photoluminescence, luminescence decay, long-lasting phosphorescence and thermoluminescence (TL) spectra. The phosphor emitted blue light that was related to the 4f⁶5d¹-⁸S_7/2 transition of Eu²⁺. The bright blue phosphorescence could be observed by naked eyes even 8 h after the excitation source was removed. Two TL peaks at 317 and 378 K related to two types of defects appeared in the TL spectrum. By analyzing the TL curve the depths of traps were calculated to be 0.61 and 0.66 eV. Also, the mechanism of LLP was discussed in this report.