Sr和Ba替代对Eu掺杂Ca2.955Si2O7的结构和发光特性的影响研究

利用固相反应法制备了Sr和Ba替代的Ca_2.955-xM_xSi₂O₇: 0.045Eu²⁺ (M= Sr, Ba, x= 0.1-0.5)系列荧光粉, 利用较大离子半径的Sr和Ba元素替代Eu掺杂Ca_2.955-xM_xSi₂O₇ 中的Ca元素,研究Sr和Ba替代对样品结构和发光特性的影响. X射线衍射测试结果表明,少量Sr和Ba替代不会改变基质的晶体结构, 样品仍然为单斜晶系.未替代前, Ca_2.955Si₂O₇: 0.045Eu²⁺ 样品的发射峰在574 nm左右,随着Sr含量的增加,样品的发射峰发生蓝移; 而Ba含量在x= 0.1-0.4时不会引起发射峰位置的移动, 但x= 0.5样品的发射峰发生蓝移.同等含量的Sr和Ba部分替代样品中的Ca元素, Ba替代样品的光谱强度较强.     

Atomic structure of Sr0.75Ba0.25Nb2O6 single crystal and composition-structure-property relationin (Sr,Ba)Nb2O6 solid solutions

The structure of the Sr_0.75Ba_0.25Nb₂O₆ single crystal has been investigated by x-ray diffraction. The occupancies of the Ba and Sr sites in two structural channels are determined. It is found that these sites are split in the large (pentagonal) channel. A qualitative correlation is revealed between the smearing of the phase transition and the displacement of the Sr atom from the m symmetry plane in the pentagonal channel at different [Sr]/[Ba] ratios. The degree of acentricity of the NbO₆ octahedra is analyzed as a function of the [Sr]/[Ba] ratio.     

Luminescence properties and energy transfer in Eu2+, Mn2+ codoped Na(Sr,Ba)PO4 phosphor

Eu²⁺ and Mn²⁺ singly doped and codoped Na(Sr,Ba)PO₄ phosphors were synthesized, and their luminescent properties were investigated. A broad blue emission and a broad orange emission band were observed in Na(Sr,Ba)PO₄:Eu²⁺, Mn²⁺ phosphor. The resonant-type energy transfer from Eu²⁺ to Mn²⁺ was demonstrated, and the energy transfer efficiency was also calculated according to their emission spectra. Based on the principle of energy transfer, the emission intensity ration of Eu²⁺ and Mn²⁺ could be appropriately tuned by adjusting the contents of activators. Due to the strong absorption in the 250–400 nm range, Na(Sr,Ba)PO₄:Eu²⁺, Mn²⁺ phosphor could be used as a potential candidate for near-UV white light-emitting diodes (LEDs).     

The oxidation of Ca,Sr, and Ba studied by appearance potential spectroscopy

The reaction of oxygen with evaporated films of Ca, Sr and Ba has been studied with soft X-ray appearance potential spectroscopy (SXAPS). Both O 1s and Ca 2p_{$\text{1}\text{2}\text{,}\text{3}\text{2}$}, Sr 2p_{$\text{3}\text{2}$} and Ba 3d_{$\text{5}\text{2}$} spectra were recorded. For Ca and Sr two distinct reactivity regimes were distinguishable: (i) a low exposure regime characterized by a single broad peak in the O 1s spectra and unchanged metal spectra, (ii) a high exposure regime or oxide regime characterized by multipeaked O1s spectra and appreciable changes in the metal spectra. The exposure at which the nucleation of oxide starts is determined. The bonding in the metal-oxygen complex is discussed.     

Sr/Si界面沉积SrTiO3初始生长阶段的扫描遂道显微术研究

本文工作利用脉冲激光沉积术(PLD)和超高真空扫描隧道显微术(UHV-STM),研究了在Sr/Si(001)-(2×1)衬底表面上真空室温沉积几个单层SrTiO₃薄膜的初始生长过程.经660 ℃退火处理后,Sr/Si衬底表面上形成了纳米岛状结构.经分析,这些纳米小岛为C49-TiSi₂和 C54-TiSi₂.实验结果表明,在没有氧气的情况下退火,Sr/Si界面无法有效阻止SrTiO₃薄膜与Si衬底之间的相互作用. 关键词： 脉冲激光沉积术(PLD) 扫描隧道显微镜(STM) 3')" href="#">SrTiO₃ 2')" href="#">C54-TiSi₂     

Luminescence properties of Eu2+ in M2MgSi2O7 (M=Ca, Sr, and Ba) phosphors

The photoluminescence properties of alkali-earth magnesium silicates (M₂MgSi₂O₇, M=Ca, Sr, and Ba) doped with Eu²⁺ were investigated. Solid solutions of Ba _x Sr_2−x Si₂O₇, Ca₂MgSi₂O₇, and Sr₂MgSi₂O₇ were prepared. Ba _x Sr_2−x Si₂O₇ retained a tetragonal crystal structure similar to the structure of the other compounds up to a stoichiometry of x=1.6, which enabled a systematic study of the common structure. Monoclinic Ba₂MgSi₂O₇ was prepared, and the luminescence properties were compared with those of other samples. The emission and excitation spectra of tetragonal M₂MgSi₂O₇ (M=Ca, Sr, and Ba) changed as a function of the covalency, site symmetry, and crystal field strength. The luminescence properties showed excellent agreement with theoretical predictions based on these factors. The Stokes shift differentiated the emission behaviors of the tetragonal and monoclinic structures.     

Tunable luminescent properties by adjusting the Sr/Ba ratio in Sr1.97−xBaxMgSi2O7:Eu0.01, Dy0.02 phosphors

The long afterglow phosphors Sr_1.97−xBa_xMgSi₂O₇:Eu²⁺_0.01, Dy³⁺_0.02 (x=0, 0.4, 0.8, 1.2, 1.6 and 1.97) were synthesized via high temperature solid-state reaction. The phase identification reveals that the crystal plane spacing becomes greater with the decrease in the Sr/Ba ratio. Phase transition occurs when x=1.97. A nonlinear relationship between the emission peak and the crystal plane spacing is obtained with the decrease of the Sr/Ba ratio. This ascribes to the splitting of the 5d level of the Eu²⁺ and the change of the crystal field strength. The duration of the afterglow becomes shorter with the decrease of the Sr/Ba ratio. It may ascribe to deeper trap depth, lower trap concentration and the embarrassment of the transfer of carriers.     

Photoemission investigation of the high temperature superconductors La2−x Sr x CuO4

Photoemission spectra of the La_2–x Sr _x CuO₄ system which recently has been demonstrated of being capable to produce high temperature (T _c >35 K) superconducting compounds are reported. The photoemission spectra of the basic system (La₂CuO₄) and of samples which contain up to 20% Sr are not significantly different. The density of states at the Fermi energy is small (smaller than that of metallic Cu) and the samples show a possible small shift of the Cu 2p lines towards the trivalent position with increasing tendency to superconductivity. From the Sr 3d spectra a considerable amount of oxygen defects coordinated to the Sr ions is estimated.     

Raman spectroscopy of BaTiO3/(Ba,Sr)TiO3 superlattices

A series of BaTiO₃/Ba_{1 ? x} Sr _x TiO₃ (BT/BST) superlattices were prepared by pulsed laser deposition on MgO substrates with a constant period of 80 Å (40 Å BT and 40 Å BST) and varying compositions of the BST layer so that the Ba/Sr concentration ratios were 0/100, 30/70, 40/60, 50/50, 60/40, 70/30, 100/0. The soft mode E(1TO) of the polarized Raman spectra transformed depending on the Ba/Sr ratio in the BST layer. As the Sr concentration in the BST layers increased from 0 to 100%, the E(1TO) soft mode half-width varied from 171 to 103 cm^?1 and its frequency increased from 31 to 109 cm^?1 due to the interaction between the epitaxial layers forming the superlattices.     

Relationships between X-ray photoelectron spectra,structure and superconductivity in LnBa2Cu3O7-x

X-ray photoelectron spectroscopy has been carried out on HoBa₂Cu₃O_7-x (on abraded, fractured and ion beam reduced surfaces). These measurements show that there are only minor differences with similarly obtained results from YBa₂Cu₃O_7-x. The spectral features of the Ln species (Y and Ho), the O 1s, and Ba 3d envelopes are considered in some detail. The low energy contribution to the O 1s envelope has been assigned in part to the effects of Ba–O bonding, either in the LnBa₂Cu₃O_7-x material, or in BaO in minor phase regions, and in part to the effects of the electronic linkage of the Cu planes in the superconducting compound. The anomalous low binding energy contributions to the Ba 3d envelope are also specific to the electronic structure of these materials. It is likely that Ba (and Sr and Ca for the Bi-Sr-Ca-C-O system) plays an important electronic, as well as structural role, in promoting high temperature superconductivity.     

Study of the <Emphasis Type="Italic">A</Emphasis> ion effect on the chemical bond of Ti and O ions in <Emphasis Type="Italic">A</Emphasis>TiO<Subscript>3</Subscript> (<Emphasis Type="Italic">A</Emphasis> = Sr,Ba, Ca,Pb) crystals with perovskite structure

The local Ti d densities of states in the valence band have been calculated in clusters of ATiO₃ (A = Ca, Sr, Ba, Pb) crystals with perovskite structure by the full multiple scattering method. The d density of states in PbTiO₃ crystal is found to significantly differ from that in other crystals. This fact indicates a significant effect of lead ions on the chemical bond between Ti and O ions in this crystal.     

Electronic structure of novel multiple-band superconductor SrPt<Subscript>2</Subscript>As<Subscript>2</Subscript>

The electronic structure of the recently discovered superconductor SrPt₂As₂ with T _c = 5.2 K has been calculated in the local-density approximation. Despite its chemical composition and crystal structure are somehow similar to FeAs-based high-temperature superconductors, the electronic structure of SrPt₂As₂ is very much different. The crystal structure is orthorhombic (or tetragonal if idealized) and has layered nature with alternating PtAs₄ and AsPt₄ tetrahedra slabs sandwiched with Sr ions. The Fermi level is crossed by Pt-5d states with rather strong admixture of As-4p states. Fermi surface of SrPt₂As₂ is essentially three-dimensional, with complicated sheets corresponding to multiple bands. We compare SrPt₂As₂ with 1111 and 122 representatives of FeAs-class of superconductors, as well as with isovalent (Ba,Sr)Ni₂As₂ superconductors. Brief discussion of superconductivity in SrPt₂As₂ is also presented.     

Strontium diffusion in Sr3−3x La2x □x(VO4)2 solid solutions

Layer-by-layer radiometric analysis in the temperature range 800–950 °C was used to measure the diffusion coefficients of ⁹⁰Sr, ⁴⁵Ca, ¹³³Ba radionuclides in Sr_3−3x La_2x □_x(VO₄)₂ (x=0,0.1,0.15,0.2,0.25) solid solutions. It was established that D _Ca ^* >D _Sr ^* >D _Ba ^* . An increase in the concentrations of vacancies (□) in the strontium sublattice leads to increased coefficients of diffusion of rare-earth metal cations. The activation energies for radionuclide diffusion were determined. It was shown that M²⁺ cations migrate via Sr(2) positions. Fiz. Tverd. Tela (St. Petersburg) 40, 223–226 (February 1998)     

Preparation of porous (Ba,Sr)TiO3 by adding corn-starch

A new method of preparing porous (Ba,Sr)TiO₃ ceramics has been introduced, using an ordinary ceramics processing technique. The effect of corn-starch on the positive temperature coefficient of resistivity characteristics and microstructure of the porous (Ba,Sr)TiO₃ ceramics has been investigated. When the corn-starch addition was 1-20 wt%, the PTCR jump was over 10⁶ and 1-2 orders higher than that of samples without corn-starch. Also, it was found that the (Ba,Sr)TiO₃ ceramics had porous microstructure by the addition of corn-starch. The porosity of the ceramics with 20 wt% corn-starch was 44%. The electrical properties of the (Ba,Sr)TiO₃ ceramics have been discussed, based on the microstructure, resistivity of grain boundaries, donor concentration of grains and the electrical potential barrier of grain boundaries.     

Dielectric and leakage current properties of Li doped (Ba,Sr)TiO3 thick film interdigital capacitors on the alumina substrates

Li doped (Ba,Sr)TiO₃ thick films were fabricated by employing the screen printing method on the alumina (Al₂O₃) substrates. Interdigital capacitor patterns with seven fingers of 200 μm gap, 250 μm length were designed and screen printed on the alumina substrates. Ba_0.5Sr_0.5TiO₃ materials, paraelectric state at the room temperature, have been chosen for the microwave devices due to high dielectric permittivity and low loss tangent, however, the sintering temperature of (Ba,Sr)TiO₃ is over 1350 °C. In order to lower the sintering temperature, Li (3 wt%) was added to the (Ba,Sr)TiO₃ materials. Li doped (Ba,Sr)TiO₃ thick films screen printed on the alumina (Al₂O₃) substrates were sintered at 900 °C for 1.5 h. The structural feature was analyzed with X-ray diffraction method. Temperature dependent dielectric properties were characterized from 303 to 403 K at 1 MHz. Within the ±100 V of bias voltage, current-voltage characteristics of Li doped (Ba,Sr)TiO₃ films were investigated from 303 to 403 K. Through the current-voltage characteristics, the resistivity of Li doped (Ba,Sr)TiO₃ films were calculated.In this paper, the significant negative temperature coefficient of resistance (NTCR) of Li doped (Ba,Sr)TiO₃ films will be presented through the activation energy fitting. Measured activation energy is approximately 0.366 eV.     

2D degenerate electron gas at Ba/<Emphasis Type="Italic">n</Emphasis>-AlGaN and Ba/<Emphasis Type="Italic">n</Emphasis>-GaN interfaces

Ba/n-GaN(0001) and Ba/n-AlGaN(0001) interfaces were investigated for the first time by means of ultraviolet photoelectron spectroscopy. The spectra of the photoemission from a valence band along with the spectra of the core levels of Ga 3d, Al 2p, and Ba 4d were studied. The formation of a 2D degenerate electron gas (an accumulated layer on the n-GaN and n-AlGaN surfaces during adsorption of Ba atoms) was revealed.     

The analysis of tunneling characteristics measured on Bi2Sr2CaCu2O8 surface

Cleaved in air a-b surface of Bi₂Sr₂CaCu₂O₈ (BSCCO-2212) was measured by means of STM and STS at 4.2 K in liquid hellium bath. From fitting experimental conductivity curves by Dynes' function two superconductivity parameters (gap value) and (smearing parameter) were obtained. The shape of gap structure superimposed on dI/dV characteristics depends on tip-sample distance, what is expressed by the increase of and decrease of with shortening of s. The phenomenon of becoming gap structure more distinct when approaching the tunneling tip to the surface is explained by us as the non-vacuum tunneling, where the surface contamination layer on non-metallic BiO top-surface layer strongly influences the tunneling process. Only for s short enough tunneling electrons penetrate to deeper situated CuO layers and reflect their superconducting behaviour. Non-vacuum STM images are therefore sensitive to the tip-sample distance adjustment. The dependence of gap parameters on lateral position of the tip above the sample can also occur. In such cases STS enables to state which elements of the image belong to the topography of the surface and which to its electron density of states.     

Electronic properties of new Ca(AlxSi1−x )2 and Sr(GaxSi1−x )2 superconductors in crystalline and nanotubular states

The band structure of new layered (AlB₂ type) Ca(Al_xSi_1−x )₂ and Sr(Ga_xSi_1−x )₂ superconductors is studied by the first-principle full-potential LMTO method. It has been shown that the superconducting properties of ternary silicides are due to the high density of (Ca,Sr)d states at the Fermi level, whereas the growth of T _C in going from Sr(Ga_xSi_1−x )₂ to Ca(Al_xSi_1−x )₂ is associated with the increase in phonon frequencies due to the decrease in atomic masses. Simulations are performed for the electronic properties of hypothetical (11,11) and (20,0) CaAlSi and SrGaSi nanotubes. In going from the crystalline to the nanotubular state, the silicide systems retain metal-like properties. The template and “film rolling” techniques can be used for obtaining silicide nanotubes.     

Morphology control of ATiO3 (A=Ba,Sr, Ca) by hydrothermal soft chemical process

Abstract

A hydrothermal soft chemical process for controlling the morphology of ATiO₃ (A = Ba, Sr, Ca) perowkites is proposed, and used for the synthesis of fibrous particles. In this process. a fibrous hydrous tetratitanate (H₂Ti₄O₉ · nH₂O) which has a layered structure was used as the precursor, and hydrothermally treated with Ba(OH)₂, Sr(OH)₂and Ca(OH)₂ solutions. The hydrous tetratitanate was transformed to the perowkite structure, but retained the fibrous morphology after the hydrothermal reactions.     

Anisotropic thermal expansion of AEFe2As2 (AE = Ba,Sr, Ca) single crystals

We report anisotropic thermal expansion of the parent, AEFe₂As₂ (AE = Ba, Sr, and Ca), compounds. Above the structural/antiferromagnetic phase transition anisotropy of the thermal expansion coefficients is observed, with the coefficient along the a-axis being significantly smaller than the coefficient for the c-axis. The high temperature (200 K ≤ T ≤ 300 K) coefficients themselves have similar values for the compounds studied. The sharp anomalies associated with the structural/antiferromagnetic phase transitions are clearly seen in the thermal expansion measurements. For all three pure compounds, the ‘average’ a-value increases and the c-lattice parameter decreases on warming through the transition, with the smallest change in the lattice parameters observed for SrFe₂As₂. The data are in general agreement with the literature data from X-ray and neutron diffraction experiments.