Synthesis and characterization of Sr2CeO4 phosphor: Positive features of sol-gel technique

Nano crystalline powder of Sr₂CeO₄ has been synthesized by sol-gel technique. The luminescence properties of the material were compared with the one synthesized by conventional solid state reaction technique. The homogeneity and uniformity of the particle prepared by this method is much better than that prepared by the solid state reaction technique. This blue emitting phosphor was characterized by X-ray diffraction, scanning electron microscopy images and luminescent measurements. The emission spectrum of the material exhibits a broad band around ∼480 nm. The photoluminescence spectra of the Sr₂CeO₄ reveals that the strong blue emission is assigned to the Ce⁴⁺-O²⁻ charge transfer transition (CTT) of Sr₂CeO₄ and not related to lattice defect. The Commission International de l’Eclairage coordinates are x=0.16 and y=0.25.     

Enhancement of photoluminescence in Sr2CeO4 phosphors by doping with non-rare earth impurities

Non-rare earth impurity doped Sr 2 CeO 4:X (X=Zn,Hg,Al,Ag,Cr) phosphors are prepared by using the combustion method.The structural and photoluminescent properties of the as-prepared phosphors are investigated by X-ray diffraction (XRD) and photoluminescence at room temperature.Experimental results show that zinc addition and firing processing can effectively enhance the photoluminescence of Sr 2 CeO 4 phosphors.     

Synthesis and photoluminescent characteristics of Sr2CeO4 phosphors prepared via a microwave-assisted solvothermal process

Blue-light emitting Sr₂CeO₄ phosphors were successfully prepared via a microwave-assisted solvothermal method employing ethylene glycol as a solvent. The formation of Sr₂CeO₄ phase began when the solvothermally derived precursors were heated at 800 °C. With increase in heating temperatures, significantly enhanced excitation and emission intensities were observed because of an increase in the amount of Sr₂CeO₄. Heating at 1200 °C led to a substantial decrease in mission intensity due to thermal decomposition of Sr₂CeO₄ at elevated temperatures. The solvothermally derived Sr₂CeO₄ was found to exhibit higher emission intensity than the solid-state-reaction-derived phosphors. According to the deconvoluted emission spectra, two emission peaks are attributed to two metal-to-ligand charge-transfer states. Based on the deconvoluted results, a qualitative energy-level diagram of Sr₂CeO₄ was proposed. VUV-excited luminescence studies for Sr₂CeO₄ indicate that one peak at 193 nm is assigned to the charge-transfer transition between Sr²⁺ and O²⁻.     

Optical study of Sr2SnO4:Eu phosphor

This work presents the influence of europium dopant on optical properties of Sr₂SnO₄:Eu³⁺ powders fabricated by a facile low temperature method. Powders were obtained from the same amounts of Eu³⁺ doping into the different concentrations of Sr(NO₃)₂. Powders were examined by means of scanning electron microscopy (SEM), X-ray diffraction (XRD), and photoluminescence (PL). SEM measurements different Eu concentrations in fabricated powders was determined to found different morphologies. XRD analysis revealed the existence of crystalline Sr₂SnO₄ in the form of tetragonal and the diffraction intensity was remarkably changed. PL studies showed a red luminescence of Sr₂SnO₄:Eu³⁺ powders. The intensity of luminescence increased with better crystallinity. This approach provides economically viable route for large-scale synthesis of this kind of nanopowders.     

Eu3+掺杂的Sr2CeO4发光材料的光致发光研究

利用高温固相反应法制备了Eu³⁺掺杂的Sr₂CeO₄样品,并对其吸附水前后的光谱特性进行了研究.结果发现,对于刚制备的Sr_2-xEu_xCeO_4+x/2样品, 在Ce⁴⁺—O^2-的电荷迁移激发中,只有强激发带(～35700cm^-1)与Eu³⁺离子间存在能量传递,而弱激发带 (～29400cm^-1)只是引起Ce⁴⁺—O^2-的电荷迁移发射;在Sr_2-xEu_xCeO_4+x/2样品吸附水后,Eu³⁺的线状吸收跃迁强度显著增加, Ce⁴⁺—O^2-两个激发带均向Eu³⁺离子传递能量. Ce⁴⁺—O^2-强激发带通过交换作用向Eu³⁺离子传递能量,而弱激发带与Eu³⁺离子间的能量传递机理是非辐射多极子近场力的相互作用. 关键词： 2-xEu_xCeO_4+x/2')" href="#">Sr_2-xEu_xCeO_4+x/2 发光性质 能量传递 吸附水     

Sr2CeO4电荷迁移发光的光谱结构规律研究

利用高温固相反应法分别合成了不同物相形成机理的Sr₂CeO₄，Sr ₂CeO₄∶Ca ²⁺和Sr₂CeO₄∶Ba²⁺样品，并对其光谱特性进行 了研究.结果发现，对于由SrO和CeO₂直接反应生成的Sr₂CeO₄(Ⅰ)，激发主峰位于256nm 左右；而对于SrCeO₃和SrO反应生成的Sr₂CeO₄(Ⅱ)，激发主峰位于279nm左右.在Sr₂CeO₄(Ⅰ)中掺入Ca²⁺，其激发光谱随着Ca²⁺离子浓度的增加逐渐接近于Sr₂CeO_{4(Ⅱ)的激发光谱.激发主峰带应属于CeO6八面体终端Ce⁴⁺—O^2-键的电荷迁移 带.对于激发光谱中340nm左右的弱激发峰，其峰值波长不受形成机理及Ca²⁺掺杂的影响，只是其强度 随着 激发主峰的红移而增加，它可能属于CeO6八面体平面上Ce⁴⁺—O2-键的电荷 迁移带.形成机理及Ca²⁺掺杂对发射光谱没有影响.Ca²⁺在Sr 2CeO 4(Ⅱ)与Ba²⁺在Sr2CeO4(Ⅰ)和(Ⅱ)中均难 于替代Sr²⁺的位置.}     

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.     

Acetate reduction synthesis of Sr2Si5N8:Eu phosphor and its luminescence properties

A novel synthesis method was developed for the efficient red phosphor, Eu²⁺-activated Sr₂Si₅N₈, by employing the strontium acetate as both the reducing agent and strontium source. The phase purity of final product was strongly dependent on the heating rate of the precursors. Sr₂Si₅N₈:Eu²⁺ (2 at%) phosphor presented a broadband excitation spectrum in the range 300–500 nm, matching well with the blue emission (400/460 nm) of current InGaN light-emitting diodes (LEDs). The red emission peaking at 619 nm gave the relatively high (about 155%) intensity compared with the Y₃Al₅O₁₂ (YAG) (P46-Y3) standard phosphor. In addition, the saturated chromatic coordinates (0.638, 0.359) allowed it a promising candidate as a red phosphor in white LEDs application for illumination or display.     

A red oxide phosphor, Sr2ScAlO5:Eu2+ with perovskite-type structure, for white light-emitting diodes

Sr2ScAlO5:Eu2+,a red oxide phosphor with a perovskite-type structure,has been synthesized through a solid-state reaction and its luminescence properties have been investigated.An absorption band centering at 450 nm is observed from the diffuse reflection spectra and the excitation spectra,indicating that the phosphor can match perfectly with the blue light of InGaN light-emitting diodes.A broad red emission band at 620 nm is found from the emission spectra,originating from the 4f 6 5d-4f 7 transition of the Eu 2+ ions.The best doping content of Eu in this material is about 5%.Sr2ScAlO5:Eu2+is a highly promising red phosphor for use in white light-emitting diodes.     

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.     

Low voltage electron induced cathodoluminescence degradation and surface characterization of Sr3(PO4)2:Tb phosphor

Tb³⁺-doped Sr₃(PO₄)₂ phosphor was prepared by a sol-gel combustion method. A trigonal structure having Sr and O atoms occupying two different lattice sites were obtained. Scanning Auger nanoprobe was used to analyze the morphology of the particles. Photoluminescence (PL) and cathodoluminescence (CL) properties of Sr₃(PO₄)₂:Tb powder phosphors were evaluated and compared. In addition, the CL intensity degradation of Sr₃(PO₄)₂:Tb was evaluated when the powders were irradiated with a beam of electrons in a vacuum chamber maintained at an O₂ pressure of 1 × 10⁻⁶ Torr or a background pressure of 1 × 10⁻⁸ Torr O₂. The surface chemical composition of the degraded powders, analyzed by X-ray photoelectron spectroscopy (XPS), suggests that new compounds (metal oxides) of strontium and phosphorous were formed on the surface. It is most likely that these compounds contributed to the CL intensity degradation of the Sr₃(PO₄)₂:Tb phosphors. The CL properties and possible mechanism by which the new metal oxides were formed on the surface due to a prolonged electron beam irradiation are discussed.     

Sr2RuO4正常态的c方向的磁阻的研究

研究了典型的层状钙钛矿结构超导单晶Sr₂RuO₄在c方向的磁阻(Δρ/ρ₀)(H∥ab,J∥c)的变化.实验发现,磁阻表现出强烈的各向异性,并且随着温度T的降低,磁阻效应越明显;当在平面ab内旋转磁场H的方向时,磁阻成周期性变化;实验表明,磁场沿(110)方向时,出现磁阻的极大值.分别从Sr₂RuO₄的费米面的各向异性、载流子散射率、c方向能带色散的各向异性等方面来解释这些输运性质. 关键词： 2RuO₄')" href="#">Sr₂RuO₄ 磁阻     

A white light emitting phosphor Sr1.5Ca0.5SiO4:Eu, Tb, Eu for LED-based near-UV chip: Preparation, characterization and luminescent mechanism

In this work, we report preparation, characterization and luminescent mechanism of a phosphor Sr_1.5Ca_0.5SiO₄:Eu³⁺,Tb³⁺,Eu²⁺ (SCS:ETE) for white-light emitting diode (W-LED)-based near-UV chip. Co-doped rare earth cations Eu³⁺, Tb³⁺ and Eu²⁺ as aggregated luminescent centers within the orthosilicate host in a controlled manner resulted in the white-light phosphors with tunable emission properties. Under the excitation of near-UV light (394 nm), the emission spectra of these phosphors exhibited three emission bands: one broad band in the blue area, a second band with sharp lines peaked in green (about 548 nm) and the third band in the orange-red region (588-720 nm). These bands originated from Eu²⁺ 5d→4f, Tb³⁺⁵D₄→⁷F_J and Eu³⁺⁵D₀→⁷F_J transitions, respectively, with comparable intensities, which in return resulted in white light emission. With anincrease of Tb³⁺ content, both broad Eu²⁺ emission and sharp Eu³⁺ emission increase. The former may be understood by the reduction mechanism due to the charge transfer process from Eu³⁺ to Tb³⁺, whereas the latter is attributed to the energy transfer process from Eu²⁺ to Tb³⁺. Tunable white-light emission resulted from the system of SCS:ETE as a result of the competition between these two processes when the Tb³⁺ concentration varies. It was found that the nominal composition Sr_1.5Ca_0.5SiO₄:1.0%Eu³⁺, 0.07%Tb³⁺ is the optimal composition for single-phased white-light phosphor. The CIE chromaticity calculation demonstrated its potential as white LED-based near-UV chip.     

Luminescent and morphological study of Sr2CeO4 blue phosphor prepared from oxalate precursors

Luminescent and morphological studies of Sr₂CeO₄ blue phosphor prepared from cerium-doped strontium oxalate precursor are reported. Powder samples were prepared from 5 and 25 mol% Ce³⁺-doped strontium oxalate as well as from a mechanical mixture of strontium oxalate and cerium oxalate at a 4:1 ratio, respectively. All the samples were characterized by XRD, IR, PLS, and SEM. The luminescent and structural properties of the Sr₂CeO₄ material are little affected by the SrCO₃ remaining from precursors. The Sr₂CeO₄ material consists in one-dimensional chains of edge-sharing CeO₆ octahedra that are linked together by Sr²⁺ ions. The carbonate ion might be associated with oxygen ions of the linear chain, and also with the oxygen atoms located in the equatorial position, which consequently affects the charge transfer bands between O²⁻ and Ce⁴⁺. As observed by SEM, the morphological changes are related to each kind of precursor and thermal treatment, along with irregular powder particles within the size range 0.5-2 μm.     

Luminescent properties of Eu-activated Sr3B2SiO8: A red-emitting phosphor for white light-emitting diodes

Single-phased Sr₃B₂SiO₈:Eu³⁺ phosphor was prepared by a solid-state method at 1020 °C. The luminescence spectra showed that Sr₃B₂SiO₈:Eu³⁺ phosphor can be effectively excited by near ultraviolet light (393 nm) and blue light (464 nm). When excited at 393 or 464 nm Sr₃B₂SiO₈:Eu³⁺ exhibited the main emission peaks at 611 and 620 nm, which resulted from the supersensitive ⁵D₀→⁷F₂ transition of Eu³⁺. The luminescence intensity of Sr₃B₂SiO₈:Eu³⁺ at 611 and 620 nm reached the maximum when the doping content of Eu³⁺ was 4.5 mol%. Its chromaticity coordinates (0.646, 0.354) were very close to the NTSC standard values (0.67, 0.33). Thus, Sr₃B₂SiO₈:Eu³⁺ is considered to be an efficient red-emitting phosphor for long-UV InGaN-based light-emitting diodes.     

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.     

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.     

Luminescence of Ce and Pr doped Sr2Mg(BO3)2 under VUV-UV and X-ray excitation

This report presents the luminescence properties of Ce³⁺ and Pr³⁺ activated Sr₂Mg(BO₃)₂ under VUV-UV and X-ray excitation. The five excitation bands of crystal field split 5d states are observed at about 46 729, 44 643, 41 667, 38 314 and 29 762 cm⁻¹ (i.e. 214, 224, 240, 261 and 336 nm) for Ce³⁺ in the host lattice. The doublet Ce³⁺ 5d→4f emission bands were found at about 25 840 and 24 096 cm⁻¹ (387 and 415 nm). The influence of doping concentration and temperature on the emission characteristics and the decay time of Ce³⁺ in Sr₂Mg(BO₃)₂ were investigated. For Pr³⁺ doped samples, the lowest 5d excitation band was observed at about 42017 cm⁻¹ (238 nm), a dominant band at around 35714 cm⁻¹ (280 nm) and two shoulder bands were seen in the emission spectra. The excitation and emission spectra of Ce³⁺ and Pr³⁺ were compared and discussed. The X-ray excited luminescence studies show that the light yields are ∼3200±230 and ∼1400±100 photons/MeV of absorbed X-ray energy for the samples Sr_1.86Ce_0.07Na_0.07Mg(BO₃)₂ and Sr_1.82Pr_0.09Na_0.09Mg(BO₃)₂ at RT, respectively.     

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.     

Synthesis and characterization of Mn-doped Zn2SiO4/SiO2 phosphor particles in core-shell structure

Manganese-doped zinc silicate (Zn₂SiO₄:Mn) is a kind of phosphor material that has a photo-luminescent (PL) and cathode-luminescent (CL) properties with intensive green light emission at 520 nm. The particles consisting of SiO₂@Zn₂SiO₄:Mn (SiO₂ core-Zn₂SiO₄:Mn shell) were synthesized via colloidal process and forced precipitation. After drying, the Zn/Mn precipitates were coated on the surface of SiO₂ particles. The Zn/Mn precipitates reacted with SiO₂ and transformed to Zn₂SiO₄:Mn by suitable calcination. The microstructure, crystalline phase, and luminescent characteristics of the products were studied. Besides, a CL device consisting of the core-shell powder was characterized.