Structure and dynamics of liquid MgO under high pressure

Microstructure, dynamics, and diffusion mechanism in liquid MgO have been studied by molecular dynamics simulation. Models consisting of 2000 atoms were constructed under a wide range of pressure and at a temperature of 3800 K. The local structure is analyzed through the coordination number distribution and topology statistics of coordination units (basic structural units) MgO _x (x=2, 3, 4, 5, 6, 7). As regards the structural dynamics, the nearest-neighbor atomic exchange among coordination units, spatially heterogeneous dynamics, clustering, and structural stability (lifetime of basic structural units) are investigated in detail. Investigation of structural dynamics allows us to gain insight into various important atomic (molecular) properties and to clarify the diffusion mechanism in liquid MgO under high pressure.     

Large-scale potential fluctuations caused by SiO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> compositional inhomogeneity

The short-range order in amorphous SiO _x (0 ≤ x ≤ 2) films has been studied by high-resolution X-ray photoelectron spectroscopy. Both the random bonding and random mixture models do not describe experimental photoelectron spectra of SiO _x (x ≤ 2). An intermediate model of the SiO _x structure has been proposed. The measured photoelectron spectra of the SiO _x (x ≤ 2) valence band indicate the presence of the silicon phase and silicon oxide.     

First-principles molecular dynamics simulations of the structure of germanium dioxide under pressures

We have carried out first-principles molecular dynamics simulations of glass and liquid germanium dioxide (GeO₂) over a wide range of pressure. Our results show that in the glass GeO₂ system nearly all Ge–O coordination environments are fourfold at low compression, whereas at high compression five- and sixfold coordination types coexist. In the liquid GeO₂ system although most Ge–O coordination environments are fourfold, some threefold coordination types exist at low compression. Pentahedral units also exist in the liquid state while less than that in the glass state. At high compression, pentahedral units disappear and GeO₆ octahedron is dominant in the liquid state going with some sevenfold coordination.     

Microstructural analysis of liquid and amorphous SiO2 nanoparticles

Microstructural properties of liquid and amorphous SiO₂ nanoparticles have been investigated via molecular dynamics (MD) simulations with the interatomic potentials that have weak Coulomb interaction and Morse-type short-range interaction under non-periodic boundary conditions. Structural properties of spherical nanoparticles with different sizes of 2, 4 and 6 nm obtained at 3500 K have been studied through partial radial distribution functions (PRDFs), coordination number and bond-angle distributions, and compared with those observed in the bulk. The core and surface structures of liquid SiO₂ nanoparticles have been studied in detail. We found significant size effects on structure of nanoparticles. Calculations also show that if the size is larger than 4 nm, liquid SiO₂ nanoparticles at the temperature of 3500 K have a lightly distorted tetrahedral network structure with the mean coordination number Z_Si-O≈4.0 and Z_O-Si≈2.0 like those observed in the bulk. Moreover, temperature dependence of structural defects and SiO_x stoichiometry in nanoparticles on cooling from the melt has been found and presented.     

Photoluminescence and growth mechanism of amorphous silica nanowires by vapor phase transport

Amorphous silica [SiO_x (1<x<2)] nanowires were fabricated on silicon substrate in an acidic environment by heating the mixture of ZnCl₂, and VO₂ powders at 1100 °C. The length of SiO_x nanowires ranges from micrometers to centimeters, with uniform diameters of 10–500 nm depending on substrate temperature. Room-temperature photoluminescence spectra of the SiO_x nanowires showed two strong luminescence peaks in the red and green region, respectively. The photoluminescence was suggested to originate from nonbridging oxygen hole center (red band), and hydrogen-related species in the structure of SiO_x (green band). The study on chemical reactions and growth of the SiO_x nanowires revealed the formation process of silica nanowires in acidic environment was closely related to the vapor–solid–liquid mechanism.     

A study of thermodynamic properties of dilute Fe-Ru alloys by <Superscript>57</Superscript>Fe Mössbauer spectroscopy

The room temperature Mössbauer spectra of ⁵⁷Fe were measured for Fe_1?x Ru _x solid solutions with x in the range 0.01 ≤ x ≤ 0.08. The obtained data were analysed in terms of short-range order parameter (SRO) and the binding energy E _b between two ruthenium atoms in the studied materials using the extended Hrynkiewicz-Królas idea. The extrapolated value of E _b for x = 0 was used to compute the enthalpy of solution H _FeRu of Ru in Fe matrix. The result was compared with corresponding values given in the literature which were derived from experimental calorimetric data as well as with the value resulting from the cellular atomic model of alloys by Miedema. It was found that all the H _FeRu values are negative or Ru atoms interact repulsively. At the same time, the Mössbauer data were used to determine values of the short-range order parameter α ₁. For the as-obtained samples in which atoms are frozen-in high temperature state, close to the melting point, the negative α ₁ values were found. The findings indicates ordering tendencies in such specimens. On the other hand, in the case of the annealed samples where the observed distributions of atoms should be frozen-in state corresponding to the temperature 700 K, the Fe_1?x Ru _x alloys with x ≥ 0.05 exhibit clustering tendencies (a predominance of Fe-Fe and Ru-Ru bonds), which manifest themselves by positive values of the calculated SRO parameter. The clustering process leads to a local increase in ruthenium concentration and nucleation of a new ruthenium-rich phase with the hcp structure.     

Structural and compositional heterogeneities in liquid aluminosilicate: insight from a grain structure model

Network structure as well as structural and compositional heterogeneities in aluminosilicate (Al₂O₃-2SiO₂) under compression is investigated by analysis and visualization of simulation data. Structural and compositional heterogeneities are clarified through analysis of topology structure and size distribution of TO _x -clusters (T = Si, Al; x = 3, 4, 5, 6) as well as OT _y -clusters (y = 2, 3, 4). The TO _x -cluster can be considered as TO _x -grains. It appears that the structure of aluminosilicate is the mixture of TO _x -grains with a different short-range order structure and this is the origin of structural heterogeneity. Regarding their composition, the OSi _y - and OAl _y -clusters can be considered as silica- and alumina-grains respectively, and the structure of aluminosilicate can thus be considered to be formed from silica- and alumina-grains. This results in compositional heterogeneity. Moreover, the degree of polymerization and polyamorphism as well as dynamic heterogeneity is also discussed in detail.     

Local configurations and atomic intermixing in as-quenched and annealed Fe1−xCrx and Fe1−xMox ribbons

Local atomic configuration, phase composition and atomic intermixing in Fe-rich Fe_1?xCr_x and Fe_1?xMo_x ribbons (x = 0.05, 0.10, 0.15), of potential interest for high-temperature applications and nuclear devices, are investigated in this study in relation to specific processing and annealing routes. The Fe-based thin ribbons have been prepared by induction melting, followed by melt spinning and further annealed in He at temperatures up to 1250 °C. The complex structural, compositional and atomic configuration characterisation has been performed by means of X-ray diffraction (XRD), transmission Mössbauer spectroscopy and differential scanning calorimetry (TG-DSC). The XRD analysis indicates the formation of the desired solid solutions with body-centred cubic (bcc) structure in the as-quenched state. The Mössbauer spectroscopy results have been analysed in terms of the two-shell model. The distribution of Cr/Mo atoms in the first two coordination spheres is not homogeneous, especially after annealing, as supported by the short-range order parameters. In addition, high-temperature annealing treatments give rise to oxidation of Fe (to haematite, maghemite and magnetite) at the surface of the ribbons. Fe_1?xCr_x alloys are structurally more stable than the Mo counterpart under annealing at 700 °C. Annealing at 1250 °C in He enhances drastically the Cr clustering around Fe nuclei.     

Short-to-medium-range order in Mg65Cu25Y10 metallic glass

The atomic configurations of liquid and glassy Mg₆₅Cu₂₅Y₁₀ alloy have been simulated in the temperature range of 300 K to 2000 K via ab initio molecular dynamics. The variations of pair correlation function (PCF), structure factor (SF), coordination number (CN) and bond pairs with the temperature for this alloy are characterized. It has been shown that the atoms are near densely packed and icosahedral type of short-range order (SRO) is predominant in the glass state. Icosahedral medium range order (MRO) can be formed by vertex or intercross connection of icosahedral SROs. In this work, an icosahedral MRO which is composed of 55 atoms has been found. It has been also clarified that Mg and Cu occupy the centre or vertex, and Y atoms only occupy the vertex of the icosahedron in this glassy alloy. It is believed that these findings have implication for understanding the glass forming mechanism of magnesium based metallic glasses.     

Preparation and photoluminescence properties of silica-coated CuO nanowires

We have fabricated cupric oxide (CuO)-core/silica (SiO _x )-shell nanowires by using a two-step process: thermal oxidation and sputtering. The structure and photoluminescence (PL) properties of the core/shell nanowires has been investigated by using scanning electron microscopy, transmission electron microscopy, X-ray diffraction and PL analysis techniques. The CuO cores and the SiO _x shells of the as-synthesized nanowires have crystalline monoclinic CuO and amorphous SiO _x structures, respectively. The PL emission intensity of the CuO-core/SiO _x -shell nanowires has been increased but the emission peak position has not been nearly shifted by annealing in a nitrogen atmosphere, whereas the emission peak position has been shifted a lot from 510 to around 650 nm as well as the emission intensity has been increased by annealing in an oxygen atmosphere. In addition, the origin of the PL enhancement in the CuO-core/SiO _x -shell nanowires by annealing and the growth mechanism of the CuO nanowires have been discussed.     

层状钙钛矿La1.3Sr1.7Mn2-xCuxO7的磁性及电特性

通过固相反应烧结法成功制备了层状钙钛矿La_1.3Sr_1.7Mn_2-xCu_xO₇多晶，主要研究了其磁电特性.结果表明，样品为Sr₃Ti₂O₇型钙钛矿结构.随着温度的降低，其磁性经历了一个很复杂的转变过程.当x=0时，在T^*=231K出现二维短程铁磁有序，在<     

Structural state of β-solid solution in rapidly quenched alloys of Al50Cu50− x Fex (x = 17, 13, 6) and ordered η1-AlCu phase formation

Conventional transmission and scanning electron microscopy were employed to investigate the structural state and phase transformations of the β(CsCl)-solid solution in rapidly quenched alloys of Al₅₀Cu_{50? x} Fe _x with respect to the Cu/Fe ratio. We studied the alloys from the central (x?=?17 and 13) and border areas (x?=?6) of the β-solid solution homogeneity region. The structural state of the β-solid solution was characterized by premartensitic structural instability of the bcc lattice for x?=?17 and 13, and the presence of combined short-range order accompanied by the intense diffuse scattering for x?=?6. This short-range order can be described by the ordering of atoms and vacancies in the planes (111)β and ω-like atomic displacements (longitudinally polarized waves of displacement in the direction of the [111]β). The structural state of the β-solid solution with the combined short-range order was regarded as a pretransition state for the revealed transformation with homogeneous precipitation of the nanodispersed phase. The precipitation phase was attributed to an orthorhombic Al(Cu, Fe) η₁-phase, belonging to the family of ordered β-based phases with orientation relationships of [100]_η1 ||[110]_β, [010]_η1 ||[110]_β, [001]_η1 ||[001]_β. We concluded that the atomic structure of the η1-phase is characterized by ordering, accompanied by ω-like atomic displacements of the adjacent layers in 3d metals (Cu, Fe) and aluminum.     

SiC外延层表面化学态的研究

用高分辨X射线光电子能谱仪(XPS)和傅里叶变换红外(FTIR)光谱仪研究了SiC外延层表面的组分结构. XPS宽扫描谱,红外掠反射吸收谱及红外镜面反射谱的解析结果说明SiC外延层表面是由Si—O—Si和Si—CH₂—Si聚合体构成的非晶SiC_xO_y:H. SiC外延层表面的化学态结构为Si(CH₂)₄,SiO(CH₂)₃,SiO₂(CH₃)₂,SiO₃(CH₃),Si—Si,游离H₂O,缔合OH,Si—OH,O和O₂. 根据化学态结构和元素电负性确定了化学态的各原子芯电子束缚能顺序,并与XPS窄扫描谱拟合结果相对比,建立了化学态与其束缚能的对应关系,进而用Si(CH₂)₄的实际C 1s束缚能值进行校正,确定了各化学态的束缚能. 结果发现,除了SiC_xO_y(x=1,2,3,4,x+y=4)的Si 2p束缚能彼此不同外,其C 1s和O 1s彼此也不相同,其中SiO₂(CH₃)₂和SiO₃(CH₃)的C 1s束缚能与CH_m和C—O中C 1s的相近,对此从化学态结构,元素电负性和邻位效应进行了解释. 关键词： SiC 化学态 XPS FTIR     

Chemical short-range-order effects on stability in δ-Pu–Ga alloys

We present results on the structural stability of Pu_{1? x} Ga _x alloys based on the local spin-density approximation and including non-local corrections to the exchange–correlation functional (generalized gradient approximation). First, we used both the linearized augmented plane-wave and the projector augmented wave (PAW) methods to calculate the energies of formation of five intermediate phases which occur in the Pu–Ga phase diagram. Then, PAW calculations were performed for various superstructures based on the fcc lattice to get the interaction parameters to describe the energetics of the δ-Pu–Ga solid solution. To discuss the effects of the chemical short-range order on the structural stability of the δ-Pu–Ga solid solution as a function of temperature, the cluster variation method configurational entropy has been introduced using the regular tetrahedron approximation. The solid part of the Pu-rich side of the phase diagram involving phase equilibria between the Pu phases, the δ-Pu–Ga solid solution and the Pu₃Ga compound is calculated. We show the importance of chemical short-range order on Ga solubility in δ-Pu.     

Structural studies of phase transitions in crystalline and liquid halides (ZnCl<Subscript>2</Subscript>, AlCl<Subscript>3</Subscript>) under pressure

The results of investigating the phase diagrams of ZnCl₂ and AlCl₃ halides, as well as the structure of the shortrange order of the corresponding melts under pressures up to 6.5 GPa, by the method of energy-dispersive x-ray diffraction are reported. When a ZnCl₂ crystal is compressed, a phase transition occurs from the γ phase (HgI₂ structure type) to the δ phase (distorted CdI₂ structure, WTe₂ type). The structural studies of the liquid state of ZnCl₂ and AlCl₃ indicate that the intermediate-range order decreases rapidly in the tetrahedral network of both melts as the pressure increases to 1.8 and 2.3 GPa for ZnCl₂ and AlCl₃, respectively. With further compression, the transitions in both melts occur with a change in the structure of the short-range order and with an increase in the coordination number. In this case, the transition in AlCl₃ occurs at ≈4 GPa and is a sharp first order transition, whereas the transition in ZnCl₂ occurs more smoothly in a pressure range of 2–4 GPa with a maximum intensity near 3 GPa. Thus, the AlCl₃ and ZnCl₂ compounds exemplify the existence of two phenomena—gradual decay of intermediate-range structural correlations and a sharper liquid-liquid coordination transition.     

Interaction of germanium with silicon dioxide

The interaction of germanium (Ge) adatoms with SiO₂ (silica) plays an important role in selective, heteroepitaxial growth of Ge(100) through windows created in silica on Si(100) and in the selective growth of Ge nanoparticles on hafnia, located at the bottom of pores etched through silica. Both processes rely on the inability of Ge to accumulate on silica. In hot wire chemical vapor deposition of Ge nanoparticles from GeH₄, etching of the silica has been invoked as one path to prevent accumulation of Ge on silica; whereas dense silica is not etched when Ge atoms are incident on the surface in molecular beam processes. Surface studies were conducted to determine the nature of oxidized Ge on SiO₂, to reconcile the etching claim with GeH₄, and to look for the additional etching product that must accompany GeO, namely SiO. Etching of silica is not found with GeH₄ or GeH_x fragments. A more complete examination of the Ge isotopes reveals instead the m/e 90 signal, previously attributed to GeO, originates from interactions between iron oxide impurities in the molybdenum holder, and hydrogen and GeH_x fragments. Coating the Mo with gold eliminates m/e 90 from Ge TPD spectra. The high temperature m/e 74 and m/e 2 peaks observed from 800 to 900 K are attributed to GeH_x decomposition to Ge and H followed by their desorption, while the appearance of GeO_x is attributed to possible reactions between GeH_x species with hydroxyl groups and/or oxidation of Ge clusters by background oxidants.     

Vibrational properties of crystalline and amorphous Ge1−xSixalloys

A five-atom cluster Bethe lattice method treating the short-range order with proper statistical effects has been proposed for understanding the crystalline and amorphous alloys. Numerical results for Ge_1?xSi_x alloys are in excellent agreement with the experimental data. In the system studied the network is random but the nearest-neighbour coordination should be treated exactly. The long-range coordination is seen to be concentration-dependent in the crystalline alloys but concentration-independent in the amorphous alloys.     

Spin-glass ordering in amorphous Tb-Cr alloys

This paper discusses amorphous TbxCr_100−x alloys (x=16, 28.5, 43, and 59 at.%) obtained by quenching from the vapor phase. It is found that the structure of the alloys is heterogeneous: the coexistence of two amorphous phases, characterized by different short-range order, is observed. The short-range order of these phases qualitatively corresponds to the structure of pure components (Cr or Tb). In the low-temperature region, the alloys transform to the spin-glass state. It is established that the transition temperatures to the spin-glass state depends on the composition linearly, increasing as the terbium concentration increases. It is shown that the magnetic properties of amorphous Tb-Cr alloys are determined by the random magnetic anisotropy associated with the Tb atoms. Fiz. Tverd. Tela (St. Petersburg) 41, 1236–1239 (July 1999)     

Electronic structure of phosphate glasses with a complex oxygen sublattice structure

Calculations of the electronic structure of phosphate glasses are performed in the MO LCAO x _α approximation discrete variation method. On the basis of an analysis of theoretical and experimental electronic spectra of the system BeO-P₂O₅ regularities are found in the formation of the valence band of alkaline-earth phosphate glasses with different types of anion sublattice. Data on the electronic structure are used to refine the models of short-range order; in particular, the possibility of oxygen in the threefold coordination state is confirmed. With the features of the spectrum of electronic states taken into account, localization of charge carriers, the nature of the optical transitions, and hole-transport phenomena are discussed. Fiz. Tverd. Tela (St. Petersburg) 39, 1366–1372 (August 1997)     

Patterned laser annealing of silicon oxide films

UV-absorbing silicon monoxide (SiO _x , x≈1) thin films on fused silica substrates are irradiated by an ArF excimer laser (wavelength 193 nm) in the sub-ablation threshold regime. Multi-pulse irradiation of films with ∼200-nm thickness at a fluence of about 100 mJ/cm² leads to a significant increase of the UV transmission, indicating the oxidation of SiO _x to SiO₂. The quality of the obtained films after this laser annealing process depends on the oxygen content of the environment. Irradiation in air at atmospheric pressure leads to the formation of sub-micron-sized oxide particles on top of the film. Structured illumination is applied either to form areas of the film with changed transmission and refractive index, or for the formation of regular particle patterns with sub-micron periods. These processes can be utilized for the fabrication of phase masks or for various types of surface functionalization.