Ba-IV-type incommensurate crystal structure in group-V metals

The long-unknown crystal structure of Bi-III has been solved. It comprises a body-centered-tetragonal (bct) "host" and a bct "guest" component made up of chains that lie in channels in the host; the guest is incommensurate with the host along the tetragonal c axis. Diffraction data for Sb-II reveal that it too can be fitted with the same composite structure. The structures of these two high-pressure phases of Bi and Sb are similar to those reported recently in the alkaline-earth metals Ba and Sr.     

Fluctuating lattice constants of indium under high pressure

Recent high-pressure investigations of elemental In have yielded controversial results. We show that the observed high-pressure face-centered orthorhombic (fco) structure can be explained as an intermediate state between two body-centered tetragonal (bct) structures with different c/a ratios (c/a < square root [2] and c/a > square root [2], respectively). In a pressure range from about 50 to 200 GPa these two bct structures correspond to local minima of the total energy with respect to orthorhombic distortion of the ground-state bct In structure. The fco saddle point represents a tiny barrier and even at low temperatures rapid structural fluctuations should occur. Such a situation has not been identified in any other elemental metal.     

Effect of barium doping on the physical properties of zinc oxide nanoparticles elaborated via sonochemical synthesis method

The aim of this work is to study the effect of barium (Ba) doping on the optical, morphological and structural properties of ZnO nanoparticles. Undoped and Ba-doped ZnO have been successfully synthesized via sonochemical method using zinc nitrate, hexamethylenetetramine (HMT) and barium chloride as starting materials. The structural characterization by XRD and FTIR shows that ZnO nanoparticles are polycrystalline with a standard hexagonal ZnO wurtzite crystal structure. Decrease in lattice parameters from diffraction data shows the presence of Ba²⁺ in the ZnO crystal lattice. The morphology of the ZnO nanoparticles has been determined by scanning electron microscopy (SEM). Incorporation of Ba was confirmed from the elemental analysis using EDX. Optical analysis depicted that all samples exhibit an average optical transparency over 80%, in the visible range. Room-temperature photoluminescence (PL) spectra detected a strong ultraviolet emission at 330 nm and two weak emission bands were observed near 417 and 560 nm. Raman spectroscopy analysis of Ba-doped samples reveals the successful doping of Ba ions in the host ZnO.     

High-pressure synthesis, amorphization, and decomposition of silane

By compressing elemental silicon and hydrogen in a diamond anvil cell, we have synthesized polymeric silicon tetrahydride (SiH(4)) at 124 GPa and 300 K. In situ synchrotron x-ray diffraction reveals that the compound forms the insulating I4(1)/a structure previously proposed from ab initio calculations for the high-pressure phase of silane. From a series of high-pressure experiments at room and low temperature on silane itself, we find that its tetrahedral molecules break up, while silane undergoes pressure-induced amorphization at pressures above 60 GPa, recrystallizing at 90 GPa into the polymeric crystal structures.     

Patterning nonanethiol protected gold films by barium atoms

Self assembled monolayers (SAM) formed from nonanethiols on thin gold films were exposed to a beam of ground state and metastable neutral barium atoms through a nickel mask. The interaction of the Ba atoms with the nonanethiol layer, followed by an etching process, creates well defined structures on the gold film, with features below 100 nm. We compared the interaction of ground state Ba atoms and SAM molecules with respect to metastable Ba atoms, finding that by using metastable atoms the Ba dose per SAM molecule is reduced. The results indicate that nanofabrication in the nanometer range with barium atoms is feasible. PACS 07.77.Gx; 42.82.Cr; 81.16.Ta     

Superconducting Phase Diagram of the Yttrium,Barium, and YBa-core in YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7–δ</Subscript> by an Ising Model

We investigate the temperature dependent of magnetizations, hysteresis properties and present the superconducting phase diagrams of the YBa₂Cu₃O_7–δ (YBCO) by an Ising model within the effective-field theory. We obtain the magnetizations of the YBCO and its components (Cu1, Cu2, O1, O2, O3, O4, Ba, Y, CuO-shell, YBa-core, and total YBCO) versus the reduced temperature and external magnetic field. We find that the copper atoms have two different magnetizations (Cu1 and Cu2) and oxygen atoms have four different magnetizations (O1, O2, O3, O4) but yttrium and barium has one magnetization. The differences in the magnetizations of the copper and oxygen atoms result from the lattice location of them on the YBCO lattice. The magnetization of the Cu1 atoms is the lowest and the yttrium is highest than those of the others; hence magnetizations of the YBCO increase from the corner to inner of the orthorhombic YBCO lattice. We also find that the YBa-core, yttrium and barium atoms of the YBCO exhibit superconductivity behaviors. Therefore, Y–Ba core, yttrium and barium atoms have the superconducting phase diagram and it has Meissner, vortex, and normal states. It is also found that the upper critical coercive field (H_c2) and the critical vortex temperature (T_v) of the Ba are high than those of the Y and YBa-core.     

Effect of strontium substitution on superconductivity in YBa 2 Cu 4 O 8

Superconducting powders with the composition Y(Ba 1 mx Sr x ) 2 Cu 4 O 8 ( x = 0.00-0.50) have been prepared by an aqueous sol-gel method. The effects of strontium substitution on the properties of compounds were studied by resistivity measurements, X-ray powder diffraction, thermogravimetric analysis, electron microscopy and elemental analysis. The data indicate that single-phase Y(Ba 1 mx Sr x ) 2 Cu 4 O 8 superconducting samples were obtained. They also show that doping with strontium has a strong effect on the superconducting properties of the YBa 2 Cu 4 O 8 phase. The critical temperature is enhanced from 78 K (for a non-substituted sample) to 88 K (for Y(Ba 1 mx Sr x ) 2 Cu 4 O 8 ). Elemental analysis data clearly indicate that small amounts of strontium enter the copper sites in the Y(Ba 1 mx Sr x ) 2 Cu 4 O 8 compounds. A point-defect chemistry approach, which explains the enhancement of T c by substituting strontium for barium, is presented.     

涂镧石墨管-电热原子吸收法测定环境样品中微量钡

建立了硝酸镁作基体改进剂 ,镧涂层石墨管原子吸收法测定钡的方法 ,有效地避免了碳化物的生成 ,提高了测定灵敏度 ,测定土壤标样和环境水样中钡的含量。方法的检出限为 2 1× 10 - 1 2 g ,对于 15ng·mL- 1钡的测定相对标准偏差为 5 4%。     

Substitutions in the Nd/Ba cation subsystem in thin films of the NdBa2Cu3Oy high-temperature superconductor

Thin films of the Nd_1+x Ba_2?x Cu₃O_y high-T _c superconductor (NBCO) with different neodymium/barium ratios have been obtained by laser ablative cosputtering of targets with different elemental compositions. The films with excess neodymium (x>0) had a low surface particle density and were rough, but their critical temperature decreased with increasing x. Conversely, barium-rich films (x<0) exhibited independence of the superconducting properties of the film composition, with an appreciable number of particles observed on the surface. Substitution of Ba for Nd in NBCO thin films is apparently impeded, so that excess barium precipitates in the form of (Ba,Cu)O_z particles. The structure and superconducting properties of NBCO reveal a strong dependence on the conditions of film saturation by oxygen.     

Synthesis, characterization and microwave absorption properties of titania-coated barium ferrite composites

Anatase titania-coated barium ferrite composites were prepared by a heterogeneous precipitation method in the presence of barium ferrite particles. The obtained samples were characterized by ξ-pH, TEM, EDX and XRD. The complex permittivity and permeability were studied in the frequency range of 2-12 GHz. The structure and microwave response properties are investigated. The results show that the coverage of titania has a great influence on microwave response of barium ferrite. The formation of an anatase titania nano-layer on the surface of a barium ferrite particle changes the character of the frequency dispersion of the complex permittivity. Comparing the anatase titania-coated barium ferrite composites with the uncoated barium ferrite, the complex permittivity of the anatase titania-coated barium ferrite composites is higher than that of uncoated barium ferrite. The complex permeability of composites was found to decrease with an increase in frequency as well as with the molar ratio of Ti:Ba. The enhancement of the complex permittivity may be due to dipolar polarization and interfacial polarization. The maximum reflection loss was obtained at the Ti:Ba ratio of 1:10, and the peak of the maximum reflection loss shifts to a lower frequency value with increasing titania fraction. By changing the thickness of titania coverage, the frequency dependence of the complex permittivity could be adjusted, which provides us an opportunity for the synthesis of tailored particles.     

高压下金属Ba的结构稳定性以及热动力学的第一原理研究

采用第一原理方法计算了高压下金属Ba的三个高压相 Ba-I, Ba-Ⅱ和Ba-V的稳定性及热动力学性质.结果表明, Ba的三个高压相在0 K时在其压力范围内都是动力学和力学稳定的;但随压力增加, Ba-I 和Ba-Ⅱ 的声子谱频率出现异常"软化",而Ba-V则出现"硬化".虽然 Ba-Ⅱ 和 Ba-V 同为六方密堆(hcp)结构,计算表明它们在高压下表现出了不同的弹性各向异性.计算同时发现 Ba-Ⅱ 在更高的压力下仍满足力学稳定条件,但声子谱有虚频存在, 表明动力学失稳是Ba-Ⅱ在压力下向Ba-I!V相转变的原因. 计算和比较了同为六方密堆(hcp)结构的Ba-Ⅱ和Ba-V在高压下的声速、 德拜温度、体模量、剪切模量等力学和热学性质, 展现了金属Ba在压力下的稳定机制和热动力学性质.     

X-ray scattering for the atomic structure of a barium-induced Si(111)-3 $\times$ 2 surface

We have investigated the atomic arrangement of a barium (Ba)-induced Si(111)-3×2 surface by using surface-sensitive synchrotron X-ray scattering. In-plane fractional-order reflections obtained from three rotational domains of 3×1 symmetry have been used to examine several competing structural models including the honeycomb-chain-channel (HCC) and the double-π-bonded chain (DπC) models. By fitting two different data sets, both the goodness-of-the fits χ² and the residuum R-factor strongly favor the HCC over the DπC model. We conclude that a Ba-paired HCC structure with a \(\frac{1}{3}\) monolayer of Ba coverage best describes the Ba-induced 3×2 surface with a Ba-Ba interatomic spacing d?=?1.93 Å.     

单一基质双光色Ba10-x(PO4)4(SiO4)2:xEu2+荧光粉的两步法制备与光谱调控

采用两步法成功合成了单一基质双光色Ba_10-x（PO₄）₄（SiO₄）₂：xEu²⁺荧光粉,研究了稀土离子占据不同的晶格格位对荧光粉光谱特性的影响。结果表明：两步法合成的荧光粉发射光谱由414 nm的蓝光波带和504 nm绿光波带两种光色组成,而传统的高温固相法制备的荧光粉只有504 nm处的绿光发射。荧光粉发光性能与Eu²⁺离子在磷灰石晶体结构中占据的晶格位置关系十分密切。两步法荧光粉双光色的形成主要是由于在第一步氧化气氛合成过程中Eu³⁺离子取代了基质结构中的Ba_Ⅰ和Ba_Ⅱ两个格位的Ba²⁺离子;在第二步还原过程结束后,Eu²⁺离子仍然占据着两种格位,从而形成了两种具有不同配位环境的发光中心。此外,双发射峰的相对强度能够通过Eu²⁺离子对Ba_Ⅰ格位的取代率而调节,进而实现光谱的调变。     

“Alternative” auger analysis reveals important properties of M-type and scandate cathodes

It is shown that in the evaluation of Auger spectra, as measured on cathode surfaces, the elemental sensitivities have to be corrected for differences in elemental number density. The substrate and the surface covering material have to be treated differently in the analysis. The result for normal M-type cathodes is that — besides the Ba---O cover — there is an excess oxygen concentration on the surface of about two times the Ba---O concentration. This in spite of the fact that the oxygen to barium peak-to-peak height ratio in the measured spectra is about equal to 2. For a degraded M-cathode the excess oxygen level is found to be much higher than for the normal cathodes. A new type of scandate cathode is described and discussed. Its analysis shows more Ba and, relatively, much less excess oxygen than for the M-type cathodes. This may account for the substantially higher emission.     

Hf-C体系的高压结构预测及电子性质第一性原理模拟

本论文中, 采用晶体结构预测软件USPEX结合第一性原理方法全面地搜索了Hf-C体系在高压下的晶体结构, 预测得到了两种新的化合物及HfC在高压下的相变路径. 压力低于100 GPa 时, 除了常压下的结构HfC, Hf₃C₂, Hf₆C₅, 并没有得到新的热力学稳定结构. 在200 GPa时, 预测得到了一种新化合物——Hf₂C, 空间群为I4/m; 且HfC的结构发生了相变, 空间群由Fm3m变为C2/m. 在300 GPa时, 预测得到了另一种新化合物——HfC₂, 空间群为Immm. 而在400 GPa时, HfC的结构再次发生相变, 空间群为Pnma. 通过能量计算, 得到了Hf-C体系的组分-压力相图: 在压力分别低于15.5 GPa和37.7 GPa时, Hf₃C₂和Hf₆C₅是稳定的; 压力分别大于102.5 GPa和215.5 GPa时, Hf₂C和HfC₂变成稳定化合物; HfC的相变路径为Fm3m→C2/m→Pnma, 相变压力分别为185.5 GPa 和322 GPa. 经结构优化后, 得到了这四种高压新结构的晶体学数据, 如晶格常数、原子位置等, 并分析了其结构特点. 对于Hf-C 体系中的高压热力学稳定结构, 分别计算了其弹性性质和声子谱曲线, 证明是力学稳定和晶格动力学稳定的. 采用第一性原理软件VASP模拟高压结构的能带结构、态密度、电子局域函数和Bader 电荷分析, 发现HfC(C2/m, Pnma结构), Hf₂C 和HfC₂ 中Hf-C 键具有强共价性、弱金属性和离子性, 且C-C 间存在共价作用.     

Ru(0001) 表面BaO吸附层的原子结构和氮分子的吸附性质

采用密度泛函理论研究了Ru(0001) /BaO表面的原子层结构和氮分子的吸附性质. 研究结果表明, 在低覆盖度下氧化钡倾向于以相同的构型形成Ru(0001) 表面原子层. 在此构型中, 氧原子位于表面p(1× 1) 结构的hcp谷位, 而钡原子则位于同一p(1× 1) 结构的顶位附近. 钌氧键键长等于0.209 nm, 比EXAFS的实验值大0.018 nm. 在Ru(0001) /BaO表面氮分子倾向吸附于钡原子附近. 相应位置的氮分子吸附能位于0.70到0.87 eV之间, 大于氧原子附近的氮分子吸附能. 钡原子附近的钌原子对氮分子具有更强的活化性能. 相应位置的氮分子拉伸振动频率等于1946 cm^{- 1}, 比氧原子附近的最大分子振动频率小约130 cm^-1. Ru(0001) /BaO表面氮分子键强度介于清洁Ru(0001) 和Ru(0001) /Ba表面之间. Ru(0001)/BaO表面不同位置的氮分子吸附性质差异是由钡和氧原子化学性质不同造成的. 表面钡原子的作用能够减少吸附氮分子的σ^*轨道电子密度, 增加π^*轨道电子密度, 从而增强氮分子和钌原子间的轨道杂化作用, 弱化氮分子键.     

Barium isotope fractionation during experimental formation of the double carbonate BaMn[CO3](2) at ambient temperature

In this study, we present the first experimental results for stable barium (Ba) isotope ((137)Ba/(134)Ba) fractionation during low-temperature formation of the anhydrous double carbonate BaMn[CO(3)](2). This investigation is part of an ongoing work on Ba fractionation in the natural barium cycle. Precipitation at a temperature of 21±1°C leads to an enrichment of the lighter Ba isotope described by an enrichment factor of-0.11±0.06‰ in the double carbonate than in an aqueous barium-manganese(II) chloride/sodium bicarbonate solution, which is within the range of previous reports for synthetic pure BaCO (3) (witherite) formation.     

Superconductivity at 38 K in the iron arsenide (Ba1-xKx)Fe2As2

The ternary iron arsenide BaFe2As2 becomes superconducting by hole doping, which was achieved by partial substitution of the barium site with potassium. We have discovered bulk superconductivity at T{c}=38 K in (Ba1-xKx)Fe2As2 with x approximately 0.4. The parent compound BaFe2As2 crystallizes in the tetragonal ThCr2Si2-type structure, which consists of (FeAs);{delta-} iron arsenide layers separated by Ba2+ ions. BaFe2As2 is a poor metal and exhibits a spin density wave anomaly at 140 K. By substituting Ba2+ for K+ ions we have introduced holes in the (FeAs);{-} layers, which suppress the anomaly and induce superconductivity. The T{c} of 38 K in (Ba0.6K0.4)Fe2As2 is the highest in hole doped iron arsenide superconductors so far. Therefore, we were able to expand this class of superconductors by oxygen-free compounds with the ThCr2Si2-type structure.     

Atomistic modeling of thermodynamic equilibrium and polymorphism of iron

We develop two new modified embedded-atom method (MEAM) potentials for elemental iron, intended to reproduce the experimental phase stability with respect to both temperature and pressure. These simple interatomic potentials are fitted to a wide variety of material properties of bcc iron in close agreement with experiments. Numerous defect properties of bcc iron and bulk properties of the two close-packed structures calculated with these models are in reasonable agreement with the available first-principles calculations and experiments. Performance at finite temperatures of these models has also been examined using Monte Carlo simulations. We attempt to reproduce the experimental iron polymorphism at finite temperature by means of free energy computations, similar to the procedure previously pursued by Müller et al (2007 J. Phys.: Condens. Matter 19 326220), and re-examine the adequacy of the conclusion drawn in the study by addressing two critical aspects missing in their analysis: (i) the stability of the hcp structure relative to the bcc and fcc structures and (ii) the compatibility between the temperature and pressure dependences of the phase stability. Using two MEAM potentials, we are able to represent all of the observed structural phase transitions in iron. We discuss that the correct reproductions of the phase stability among three crystal structures of iron with respect to both temperature and pressure are incompatible with each other due to the lack of magnetic effects in this class of empirical interatomic potential models. The MEAM potentials developed in this study correctly predict, in the bcc structure, the self-interstitial in the (110) orientation to be the most stable configuration, and the screw dislocation to have a non-degenerate core structure, in contrast to many embedded-atom method potentials for bcc iron in the literature.