真空紫外-紫外光激发下的Na2SO4:TbF3的发光性质

在空气中900℃温度下,对纯天然无水芒硝(Na2SO4)和TbF3的混合粉末加热20min,制备了Na2SO4:TbF3发光材料。并测量了在室温中真空紫外-紫外光下的发射和激发光谱。发射光谱有一系列由于Tb3+离子的5D4→7FJ(J=6,5,4,3,2)和5D3→7FJ(J=6,5,4,3,2)跃迁发射峰。通过监测542nm处发光得到的激发光谱,分别由Tb3+离子4fn→4fn-15d跃迁(187,193,218nm),基质吸收(165,200,240nm)引起的强激发谱和禁戒的f-f跃迁产生的弱激发谱组成。在TbF3的掺杂量在0.3%～2%范围内,随着Tb3+离子掺杂量的增加,真空紫外区的激发谱相应地逐渐增强。     

Structural transformations and phase stabilization in Na2S04-Na2C03 system

The phase diagram of the system Na2S04-Na2C03 has been determined from room temperature upto liquidus. At low temperature the system shows partial miscibility and the formation of a compound, burkeite. At high temperature, the whole system shows complete miscibility and crystalizes in hexagonal system, with P63/m mc structure. This high temperature phase is quenched from the melt at room temperature, and shows: 1. Shortening of the hexagonal c-axis in the mixed series as a result of substitutions of tetrahedron complex with plane group. 2. Heat treatment of single crystals of intermediate phase of Na2S04 with Cmcm structure shows topotaxial phase change to high phases. 3. Na2CO3 rich region show satellite reflections, which vanish with rise of temperature. 4. Quenched high phase is metastable and reverts to low phases after sometime.     

Spatially resolved Raman investigation on phase separations of mixed Na2SO4/MgSO4 droplets

By using spatially resolved Raman spectroscopy together with in situ microscopy, mixed Na₂SO₄/MgSO₄ aerosol particles with molar ratios of 1:1 and 2:1 deposited on a quartz substrate were carefully examined in their evaporation processes. Upon decreasing the relative humidity (RH), phase separations were found to occur for these droplets. For Na₂SO₄/MgSO₄ droplets with a molar ratio of 1:1, two paths of phase separation were identified, and a large amount of small crystals were observed to disorderly distribute in the residual solutions. By comparisons with the known Raman spectra of crystals, it was concluded that the scattered crystals in the two paths were anhydrous Na₂SO₄ in metastable phase III and the double salt of Na₂SO₄· MgSO₄ · 4H₂O, respectively. For Na₂SO₄/MgSO₄ aerosol droplets with a molar ratio of 2:1, only anhydrous Na₂SO₄ in metastable phase III precipitated with the decrease of RH. Copyright © 2008 John Wiley & Sons, Ltd.     

Na2SO3溶液的拉曼光谱研究和保存

使用抗坏血酸(VC)代替甲醛作为保护剂,通过拉曼光谱仪测试SO32-、SO42-的拉曼峰的转变过程来观察Na2 SO3溶液变质情况,测试简单,快速.实验表明,Na2 SO3溶于煮沸冷却后的蒸馏水,以VC作为保护剂,当浓度为0.5 mol/L时,可以持续暴露在空气中保存12h以内不变质;当通入N2或煮沸冷却后的蒸馏水或添...     

Temperature‐induced phase transformations in Na2WO4 and Na2MoO4 crystals

Temperature‐dependent Raman studies on Na₂WO₄and Na₂MoO₄ crystals were performed in order to obtain information on structural changes induced by temperature evolution. The stability of the cubic phase of Na₂WO₄and Na₂MoO₄crystals was assessed and our results indicate that this phase is stable in the 8–823 K and 15–773 K ranges for Na₂WO₄ and Na₂MoO₄, respectively. The crystal of Na₂WO₄ shows a phase transition occurring at 833 K. Na₂MoO₄undergoes a sequence of three phase transitions, which were observed at 783–803, 823–913 and 943–950 K. In both crystals, a strong first‐order phase transition occurs as indicated by the behavior of the Raman modes: the Raman bands split and new bands appear below 100 cm⁻¹. These transitions are connected with tilting and/or rotations of the WO₄and MoO₄ tetrahedra, which leads to a disorder at the WO₄and MoO₄ sites. Copyright © 2010 John Wiley & Sons, Ltd.     

Na2GdPO4F2:Tb3+绿色荧光粉阴极射线发光研究

通过高温固相法制备Na2GdPO4F2:Tb3+绿色荧光粉,并利用X射线粉末衍射(XRD)和阴极射线光谱分别对其物相、阴极射线发光性能进行研究.结果表明,Tb3+作为绿色发光中心进入到Na2Gd2PO4F2的晶格中取代Gd3+的格位,在低电子束(0.5～5 kV)激发下主要表现出Tb3+的特征跃迁(5D3.4→7FJ,J=6～2),其中以5D4→7F5跃迁(546 nn)为主.样品Na2Gd0.95Tb0.05PO4F2在阴极射线激发下的色坐标为(0.240 3,0.438 6).随着电压、电流和掺杂量的增加,荧光粉的发光强度逐渐提高,其中,最佳的样品为Na2Gd0.9Tb0.1PO4F2.     

Pb9AI2F24 fluoride: Phase transition and electrical characterization of the high-temperature form

A phase transition at T _r = 365±10K is shown for Pb₉Al₂F₂₄ by high-temperature X-ray diffraction, microcalorimetric and electrical measurements. The AC conductivity data of Pb₉Al₂F₂₄ are analyzed in the Z*, M* and ε* formalisms and the conductivity relaxation parameters are determined. The high-temperature form of Pb₉Al₂F₂₄ is characterized by an activation energy (Δσ)_h.t. close to that of the (x = 0.12) limiting composition of the Pb_1-.xAl_xF_{2 2+x} solid solution with fluorite-type structure.     

Ionic conduction and dielectric relaxation in polycrystalline Na2SO4

In the present paper, the ionic conductivity and the dielectric relaxation properties of Na₂SO₄ have been investigated by means of impedance spectroscopy measurements over wide ranges of frequencies and temperatures. The frequency dependent impedance data has been modeled by appropriate equivalent circuit representing the bulk and grain boundary properties of the sample. Phase transition temperatures and the activation energies of conduction in different phases have been determined from the temperature dependence of the dc conductivity. Dielectric relaxation has been studied using the complex electric modulus and permittivity formalisms. From the electric modulus formalism it is concluded that the relaxation mechanism is independent of temperature. The dielectric permittivity spectra were analyzed by the Cole–Cole formula where different parameters are determined.     

Effect of temperature and high pressure on Mn2+ EPR spectra in K3H(SO4)2 fast-proton conductor

The linewidth ΔH _pp and spin-Hamiltonian parameters under temperature and high hydrostatic pressure by X-band continuous wave electron paramagnetic resonance in the K₃H(SO₄)₂ crystal were studied. Spin-Hamiltonian parameters, direction cosines and coordination of Mn²⁺ ion were determined at room temperature. The pressure at 300?MPa leads to the change of hydrogen bond potential and the transition from double well to single well potential moves about 10?K towards a higher temperature.     

A Modified Technique for the Preparation of SO2 from Sulphates and Sulphides for Sulphur Isotope Analyses

Abstract

A modified technique for the conversion of sulphates and sulphides to SO₂ with the mixture of V₂O₅—SiO₂ for sulphur isotopic analyses is described. This technique is more suitable for routine analysis of large number of samples. Modification of the reaction vessel and using manifold inlet system allows to analyse up to 24 samples every day. The modified technique assures the complete yield of SO₂, consistent oxygen isotope composition of the SO₂ gas and reproducibility of δ³⁴S measurements being within 0.10‰. It is observed, however, oxygen in SO₂ produced from sulphides differs in δ¹⁸O with respect to that produced from sulphates.     

On the low temperature phase transition in (NH4)2SO4

An ultracryostat and multidecameter were used to determine the temperature dependence of the dielectric constant ?′ and dielectric loss ?″ over a wide range of frequencies of single crystals and polycrystalline samples of (NH₄)₂SO₄ in the region of the low temperature phase transition. A sharp increase was observed in the values of ?′ and ?″ at about ?50°C. In addition, a dielectric dispersion was detected and found to be more pronounced in the high temperature phase. This dispersion was attributed to piezoelectric resonance. The observed sudden increase in the values of the dielectric constant and dielectric loss below ? 50°C was attributed to the ferroelectric nature of the low temperature phase of (NH₄)₂SO₄.A DTA thermogram showed a sharp peak at ? 50°C which indicated that the phase transition is one of first order type. A TMA thermogram showed that this transformation was associated with a rapid increase in the expansion coefficient. Such an increase in the lattice parameter might be attributed to the enhanced rotation of electric dipoles associated with the distorted NH₄⁺ and SO₄^2? ions. The distortion of both the ammonium and sulfate ions in addition to their expected orientational motion are suggested to be responsible for the ferroelectric behaviour of ammonium sulfate below ?50°C.A transition to a metastable hexagonal state at about ?40°C is thought to occur, and this transformation is found to be irreversible.     

Order-disorder transition in Na3Bi(PO4)2

Na₃Bi(PO₄)₂ exhibits several phase transitions at about 575, 820 and 905°C. The structure was determined at ambient temperature (α-form) and above the first transition (β-form). The α-form cell is monoclinic with a = 19.86(1), b = 5.353(6), c = 13.96(3) Å, β = 110.64(7)°, Z = 8, space group P2₁/ _c ; the structure was solved from 3769 independent reflections to an R value, calculated on intensities, of 0.069. The β-form cell is orthorhombic with a = 18.71(3), b = 7.18(2), c = 5.429(7) Å, Z = 4, space group Pnam; the structure was solved to an R value, calculated on structure factors, of 0.055 using intensities of 858 unique reflections measured on a single crystal at 650°C. Both structures are related to that of glaserite. At high temperature, one of the PO₄ tetrahedra is statistically disordered over two positions related by the m-mirror. Below the transition, ordering of this ion leads to a unit cell of lower symmetry. At the transition, two individuals grow on the two sides of the m-mirror which disappears; thus, at ambient temperature, the crystals are systematically twinned. Above the second transition, the unit cell is hexagonal.     

Synthesis and thermoluminescence characterization of Na6Mg(SO4)4:RE (RE = Ce,Tb) phosphors

Thermoluminescence (TL) properties of sulfate-based phosphors activated by different rare earths have received tremendous attention to the field of radiation dosimetry. Those TL materials based on CaSO₄ have been widely applied for medical and environmental dosimetry. Taking this fact into account we have synthesized Na₆Mg(SO₄)₄ doped with Ce and Tb by wet chemical method. The prepared phosphor was characterized by XRD, FTIR, photoluminescence (PL) and thermoluminescence. For TL study, the phosphor is irradiated with γ-rays from ⁶⁰Co source. For studying luminescence properties, the prepared phosphor was annealed at different temperatures and effects of these annealing temperatures on Na₆Mg(SO₄)₄ samples are investigated and quantified. The changes in the glow curve and PL emission spectrum are also investigated as a function of annealing temperature and the annealing temperature was optimized. For calculation of trapping parameters various methods such as peak shape (PS) method, initial rise (IR) method, various heating rate (VHR) method, and computerized glow curve deconvolution (CGCD) are employed.     

Two different effects of Na on Cu2ZnSnS4 thin-film solar cells

Among the many factors that affect the characteristics of Cu₂ZnSnS₄ thin-film solar cells, the effects of Na were examined. Because Na passivates the grain boundaries (GBs) in absorber layers, the GBs increase the minority carrier collection by providing a current pathway through which the minority carriers can reach the buffer and window layer and be collected. However, excess Na remains in the absorber layer and diffuses into the buffer and window layers, where it acts as an impurity and traps the minority carriers, thus decreasing the solar cell characteristics. By applying an annealing process after the deposition of the buffer and window layers, the Na content could be decreased. By improving the solar cell characteristics, a power conversion efficiency of 6.43% was achieved.     

Optically stimulated luminescence in doped K3Na(SO4)2 phosphors

Optically stimulated luminescence (OSL) in Cu and Eu doped K₃Na(SO₄)₂ is reported for the first time. The Cu-doped sample shows OSL sensitivity which gets enhanced by co-doping with Mg²⁺ ions. The Cu-doped and quenched sample shows better sensitivity which is almost double than that of the slowly cooled sample, whereas the sensitivity of Mg co-doped sample remains nearly same irrespective of the thermal treatment. The Cu-doped sample shows TL peak around 200 °C and moderate OSL sensitivity. Doping of Mg shifts the TL peak to around 160 °C and is correlated with good OSL sensitivity. Eu-doped sample does not show OSL sensitivity. However, relatively good OSL sensitivity is observed in Aluminium co-doped and slowly cooled sample, which is about 15% of the commercial Al₂O₃:C(Landuer Inc.). A near fully optically sensitive TL peak around 155 °C is observed. The dose response is linear and practically no OSL fading is observed in first five days of storage in slowly cooled sample. This study on conventional sulphate-based TL phosphors will be useful in developing OSL phosphors for radiation dosimetry.     

Raman spectra of mirabilite,Na2SO4·10H2O and the rediscovered metastable heptahydrate,Na2SO4·7H2O

Salt crystallisation in pores is known to cause serious damage to masonry. Sodium sulphate, often regarded as one of the most damaging salts, has a rich hydrate chemistry including one rediscovered metastable hydrate and a new high pressure octahydrate plus five known polymorphs of the anhydrous phase. The difficulty in working with these hydrates lies in their strong tendency to dehydrate or to convert to the stable phase, in the case of the heptahydrate. We present Raman spectra and a table of peak wavenumbers for randomly oriented crystals of mirabilite and the metastable heptahydrate, sufficient to distinguish between these phases that have SO₄ν₁ values of 989.3 and 987.6 cm⁻¹, respectively. Mirabilite has a Raman spectrum very similar to the free sulphate anion in solution, which is probably due to the mobility of oxygen atoms within the sulphate tetrahedron. The oxygen atoms in the heptahydrate sulphate groups have no partial occupancy, and predicted peak splitting is observed in the region 400–1200 cm⁻¹. Copyright © 2010 John Wiley & Sons, Ltd.     

Thermoluminescence and optically stimulated luminescence in various phases of doped Na2SO4

The dependence of optically stimulated luminescence (OSL) and thermoluminescence (TL) response due to crystal phase in Cu and Cu,Mg-doped Na₂SO₄ was studied. Study shows that the slowly cooled samples which crystallize in phase V show good OSL sensitivity whereas the quenched samples of Na₂SO₄ which crystallize in phase III irrespective of doping show no OSL sensitivity. However, during storage when phase III samples get converted to phase V, samples show OSL sensitivity comparable to freshly prepared samples in phase V. Hence, it is observed that TL–OSL properties of doped Na₂SO₄ are phase dependent .This study will be helpful in developing OSL phosphors in which phase plays an important role in deciding the desired properties.     

ESR evidence of a low-temperature phase transition in Na2SeO4

Abstract

ESR investigations of gamma-irradiated sodium selenate in powder form indicates a number of paramagnetic species as SeO^? ₂, SeO^? ₃, and SeO^? ₄, identified on the basis of their g-tensors. The appearance of a new ESR line at g = 2.0260 attributed to the SeO^? ₄ radical indicates a possible structural phase transition at 165 K in Na₂SeO₄.