High pressure effect on structural transition of Fe cluster during rapid quenching processes

Molecular dynamics simulations of 21,952 atoms have been performed to study local structure evolution and glass formation in iron melts rapidly cooled under high pressures.In the quenching processes,structural transition details have been analyzed by using the radial distribution function g(r) and bond pair analysis technique.It is shown that high pressure strengthens the order degree of the glass system and favors the glass formation.That means with the increase of pressures,a more compacted local structure with more ideal icosahedra can be obtained.The calculations of free volume of the system indicate that the decrease of the number of the defected ideal icosahedra under high pressure contributes to the glass formation of Fe.     

Pressure effect on the structural transition of liquid Au

We present a molecular dynamics simulation within the framework of empirical tight-binding potential on the liquid structure of Au under different pressures during the rapid cooling process. The pair correlation function (PCF) and the pair analysis (PA) technique are used to reveal the structural characteristics of liquid Au under normal and high pressures. The split of the second peak of g(r) is associated with the glass transition at the cooling rate of 2.06×10¹³ K/s under different pressures. The nearest distance is shortened under high pressures. High-pressure is in favor of FCC-type and BCC-type atomic clusters. The number of icosahedral structures increases in the high temperature region as pressure increase, while high-pressure is not preferable for icosahedra at low temperature. With the increase of the pressure, the possibility that the system forms crystalline structure is enhanced. The influence of a higher pressure on the microstructure of liquid metal is much clearer than that of a lower pressure.     

Structural organization of icosahedral coordination polyhedra in a molecular dynamics model of the Ni60Ag40 metallic glass

Features of the formation of a percolation cluster structure composed of interpenetrating and contacting icosahedra during the glass transition in an amorphous Ni₆₀Ag₄₀ alloy have been studied using the molecular-dynamics method and statistical geometric analysis. Data on the morphology of clusters composed entirely of the interpenetrating icosahedra and on the character of their conjugation in the percolation cluster are presented.     

Plasma channel formation in nitrogen upon electron beam injection

A theoretical study is performed of plasma channel formation and current neutralization upon injection of a high current electron beam into nitrogen under high pressure (p 70 torr). The complex ion formation and recombination mechanism is considered together with its effect on the parameters of the plasma formed and the dynamics of beam current neutralization. Analytical expressions are obtained for the densities of simple and complex ions in the limiting cases and numerical calculations are carried out by computer for a more general case. It is shown that in nitrogen at a pressure of 70 torr when complex ions are considered the plasma channel conductivity decreases by approximately 35–40%.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 59–65, December, 1990.     

The structure of MgO-SiO2 glasses at elevated pressure

The magnesium silicate system is an important geophysical analogue and neutron diffraction data from glasses formed in this system may also provide an initial framework for understanding the structure-dependent properties of related liquids that are important during planetary formation. Neutron diffraction data collected in situ for a single composition (38 mol% SiO(2)) magnesium silicate glass sample shows local changes in structure as pressure is increased from ambient conditions to 8.6 GPa at ambient temperature. A method for obtaining the fully corrected, total structure factor, S(Q), has been developed that allows accurate structural characterization as this weakly scattering glass sample is compressed. The measured S(Q) data indicate changes in chemical ordering with pressure and the real-space transforms show an increase in Mg-O coordination number and a distortion of the local environment around magnesium ions. We have used reverse Monte Carlo methods to compare the high pressure and ambient pressure structures and also compare the high pressure form with a more silica-poor glass (Mg(2)SiO(4)) that represents the approach to a more dense, void-free and topologically ordered structure. The Mg-O coordination number increases with pressure and we also find that the degree of continuous connectivity of Si-O bonds increases via a collapse of interstices.     

<Emphasis Type="Italic">In situ</Emphasis> study of the mechanism of formation of pressure-densified Sio<Subscript>2</Subscript> glasses

The volume of glassy a-SiO₂ upon compression to 9 GPa was measured in situ at high temperatures up to 730 K and at both pressure buildup and release. It was established that the residual densification of a-SiO₂ glass after high-pressure treatment was due to the irreversible transformation accompanied by a small change in volume directly under pressure. The bulk modulus of the new amorphous modification was appreciably higher (80% more than its original value), giving rise to residual densification as high as 18% under normal conditions. It was shown that the transformation pressure shifted to a lower pressure of about 4 GPa with a rise in temperature. A conclusion was drawn about the existence of at least two pressure-induced phase transitions accompanied by structure rearrangement in a-SiO₂. A nonequilibrium phase diagram is suggested for glassy SiO₂. It accounts for all the presently available experimental data and is confirmed by the existing modeling data.     

判定金属玻璃微观结构中的二十面体类团簇

基于Voronoi几何分形法, 分析了理想二十面体团簇和ZrCu二元金属玻璃中各种团簇的结构特点, 提出了一种判定金属玻璃原子结构中二十面体类团簇的方法. 并选取三个ZrCu 非晶成分作为研究对象, 基于Voronoi团簇, 利用该方法提取了各种构型团簇, 证实其中四种构型团簇的基本几何结构与理想二十面体相似, 并具有同样近似于理想二十面体的高致密度、高规则度和高五次对称性, 因此可称之为二十面体类团簇. 此类二十面体类团簇可作为金属玻璃的主要结构单元, 普遍存在于非晶结构中; 二十面体类团簇及其连接能包含几乎所有的原子, 从而形成非晶结构. 研究结果提供了一种新的团簇判定方法, 有助于从微观结构层面分析合金中的非晶形成机理.     

Structure formation on titanium during oxidation induced by cumulative pulsed Nd:YAG laser irradiation

We investigated local oxidation and surface structure development of Ti targets under multi-pulse, high-repetition-rate Nd:YAG ( =1.064 m, 300 ns, =30 kHz) laser irradiation in air at atmospheric pressure. The experiments were performed at laser intensity levels below the single-laser-pulse melting threshold of Ti. The morphology of the irradiated areas was studied by scanning electron microscopy and profilometry. The variation of the oxide compositions and the crystalline state with increasing laser pulse number was analysed by X-ray diffractometry. Besides the known phenomena related to laser irradiation and oxidation of metal surfaces (micro-crack or pore formation), we evidenced new morphological features such as droplet-like structures inside the surface micro-cracks and micro-columns, and with increasing laser pulse number the formation of a dome-shaped structure over the whole irradiated zone. The occurrence of melting under multi-pulse irradiation was associated with the rise in the surface temperature enhanced by the oxidation, and the progressive evolution of the surface structures was associated with the consecutive melting–solidification processes as well as with the different stages of oxidation. PACS 61.80.Ba; 68.37.Hk; 81.65.Mq     

Thermoelectric properties of the trigonal and orthorhombic modifications of zinc telluride

Thermoelectric measurements are performed to study the phase transformations occurring in ZnTe under high pressure. It is shown that the thermoelectric power S of the cinnabar trigonal phase corresponds to a semiconductor with a hole-type conduction. In the Cmcm orthorhombic phase, the value of S≈+10 μV/K and the sign of the thermoelectric power testify to the metallic hole-type conduction, as in the high-pressure phases of other Group II chalcogenides (HgSe, HgTe, CdTe) with similar crystal lattices. In the transition region between the trigonal and orthorhombic phases, the pressure dependence of the thermoelectric power is found to exhibit an anomaly (a sharp dip), which leads to a change in the sign of S under decreasing pressure. This feature may presumably be related to the formation of the intermediate phase with the NaCl structure, which has an electron-type conduction in other zinc and cadmium chalcogenides.     

Molecular dynamics computation of clusters in liquid Fe–Al alloy

By means of constant pressure molecular dynamics (MD) simulation technique, a series of simulations of the Fe₅₀Al₅₀ alloy have been carried out. The atoms interact via semi-empirical n-body noncentral potential. The pair correlation functions and the pair analysis technique is applied to reveal the cluster evolution in the process of quick solidification. By using the bond orientation order parameters, we have measured both local and extended orientation symmetries for computer-generated models of dense liquid and glass. A lot of polyhedra in liquid system, for example, icosahedra, are also obtained. In order to test the reliance of the computation results, corresponding X-ray diffraction experiments have been performed on the material.     

Formation of embedded patterns in glasses using femtosecond irradiation

Channel formation in glasses is demonstrated and discussed. Photosensitive glass (Foturan) was microstructured using femtosecond irradiation at 800 nm and 400 nm wavelengths, with subsequent thermal annealing, and HF etching of a lithium-silicate phase formed by exposure-annealing. It was found that there is a significant difference in the volume of lithium-silicate when the exposure was made with and without optical breakdown. Channels with a minimum cross section of approximately 10 m were achieved.In silicate glasses, the optically induced dielectric breakdown was used for the recording of channel patterns. The highest wet etching rate in a HF based solution was observed when the irradiance corresponded to the 2.5–3.0 thresholds of dielectric breakdown and the adjacent pulses were overlapping by more than 50% of the diameter of the focussed beam. Under these conditions, the formation of a void inside the glass was confirmed by optical observation of single shot damage under a microscope. The mechanism of selective wet etching in silicate glasses is discussed in terms of the stress corrosion effect, which explains crack propagation in glasses via the reaction of stretched Si–O–Si bonds at the tip of a crack with water, resulting in SiOH formation. It is shown that intra-connection of voxels was the key factor to achieve etching of high aspect ratio patterns in silica glass. PACS 42.70.Ce; 81.05.Kf; 82.50.Pt; 87.80.Mj     

Geometric structure of Bergman clusters related to bulk amorphous alloys and quasicrystals

The fcc Zr₂Ni- and MgCu₂-type phases, being primary crystalline products of certain amorphous alloys with high glass-forming ability, should provide local structural information about the related amorphous alloys. The present paper gives a detailed analysis of the common structural features of these two fcc phases. They are both built from a similar building block consisting of tetrahedrally packed icosahedra. They also have shelled local structures similar to the first two shells of the Bergman-type clusters found in many icosahedral quasicrystals. Therefore the Bergman-type clusters probably play an important role in amorphous phase formation. Because of topological close packing inside dual-intergrown icosahedra, characteristic of Bergman clusters, the atomic radius ratios of the first- and second-shell atoms with respect to the central atom are nearly equal and fall in the range 1.1–1.2.     

Study of structural, optical and electrical properties of ZnO and SnO2 thin films

The investigation of structure, optical and electrical properties of tin and zinc oxide films on glass substrates by using magnetron sputtering are carried out. X-ray data show the formation of textured tin oxides film during deposition and its transformation to SnO₂ polycrystalline film at low temperature (200 C) if the concentration of oxygen in the chamber is high (O₂ — 100%, Ar — 0%). Optimal conditions of SnO₂ polycrystalline film deposition (pressure of Ar–O₂ mixture in chamber — 2.7 Pa, concentration of O₂ — 10%) are determined. Low resistivity of as-deposited ZnO film and increasing ZnO crystallite sizes and phase volume at temperatures higher than the melting point of Zn (419.5 C) are explained by formation of conductive Zn and ZnO particle chains and their destruction, respectively.     

Properties of yttrium-iron-metalloid glasses

We report the results of measurements on metallic glasses of the form Y₆₆(Fe_1–x M _x )₃₄. For M = B we find that there is an extended range of glass formation (0x0.40) and that properties such as atomic density, electrical resistivity, microhardness, and thermal stability are functions ofx. For M = C, Si, or Ge we find a restricted range of glass formation (0 x0.10) and no significant changes in the properties of the glasses. Using Fe⁵⁷ Mössbauer effect spectroscopy we find that the M = B case is again unique in that the structure of the glass is sensitive to B content. We relate the differences in glass formation for the M = B and M = C, Si, Ge glasses to the existence of Y rich compounds in the C, Si, and Ge cases and the lack of such compounds in the M = B case.     

Crystallite misorientation analysis in semiconductor wafers and ELO samples by rocking curve imaging

Rocking curve imaging is based on measuring a series of Bragg-reflection digital topographs by monochromatic parallel-beam synchrotron radiation in order to quantify local crystal lattice rotations within a large surface area with high angular and high spatial resolution. In this paper we apply the method to map local lattice tilts in two distinct semiconductor sample types with lattice misorientations up to 0.5° and with spatial resolution from 30  down to 1 . We analyse the measured surface-tilt data volumes for samples with almost smoothly varying specific misoriented defect formation in GaAs wafers and for an inherent subsurface grain structure of epitaxial lateral overgrowth wings in GaN. Back-projected tilt maps and histograms provide both local and global characteristics of the microcrystallinity.     

Pressure-induced modifications of crystal and magnetic structure of oxygen deficient La<Subscript>0.7</Subscript>Sr<Subscript>0.3</Subscript>MnO<Subscript>3-d</Subscript> manganites

The crystal and magnetic structures of the oxygen deficient manganites La_0.7Sr_0.3MnO_3-d (d = 0.15, 0.20) have been studied by means of powder neutron diffraction over the 0–5.2 GPa pressure and 10–290 K temperature ranges. La_0.7Sr_0.3MnO_2.85 exhibits a coexistence of rhombohedral and tetragonal (I4/mcm) crystal structures and below T_g ~ 50 K a spin glass state is formed. La_0.7Sr_0.3MnO_2.80 exhibits a tetragonal (I4/mcm) crystal structure. Below T_g ~ 50 K a phase separated magnetic state is formed, involving coexistence of C-type AFM domains with spin glass domains. In both compounds the crystal structure and magnetic states remain stable upon compression. The factors leading to the formation of different magnetic states in La_0.7Sr_0.3MnO_3-d (d = 0.15, 0.20) and their specific high pressure behavior, contrasting with that of the stoichiometric A_0.5Ba_0.5MnO₃ (A = Nd, Sm) compounds showing pressure-induced suppression of the spin glass state and the appearance of the FM state, are analysed.     

The Cavity Method at Zero Temperature

In this note we explain the use of the cavity method directly at zero temperature, in the case of the spin glass on a lattice with a local tree like structure, which is the proper generalization of the usual Bethe lattice to frustrated problems. The computation is done explicitly in the formalism equivalent to one step replica symmetry breaking; we compute the energy of the global ground state, as well as the complexity of equilibrium states at a given energy. Full results are presented for a Bethe lattice with connectivity equal to three. The main assumptions underlying the one step cavity approach, namely the existence of many local ground states, are explicitely stated and discussed: some of the main obstacles towards a rigorous study of the problem with the cavity method are outlined.     

Interplay between structural and magnetic properties of Lu\mathsf{_{2}}Fe \mathsf{_{17}} at high pressure

A crystal structure of Lu₂Fe₁₇ was studied by means of neutron powder diffraction method in the pressure range up to 1.65 GPa and ambient temperature. Modification of the magnetic properties corresponding to the pressure-induced change of the crystal structure of Lu₂Fe₁₇ is analysed on the basis of local moment model in the mean field approximation. The experimentally observed suppression of FM state and stabilization of the non-collinear AFM state under high pressure in Lu₂Fe₁₇ can be explained qualitatively from model calculations. FM and non-collinear AFM states in Lu₂Fe₁₇ are found to be nearly degenerate at ambient pressure with the estimated difference between the total energy minima of mRyd and the application of high pressure removes this degeneracy in the favor of non-collinear AFM state.Received: 6 September 2004, Published online: 5 November 2004PACS: 61.50.Ks Crystallographic aspects of phase transformations; pressure effects - 75.25. + z Spin arrangements in magnetically ordered materials - 61.12.Ld Neutron diffraction     

Features of changes in the surface structure of K-208 glass under electron—proton irradiation

Changes in the surface structure of K-208 glass after single-time irradiation of its samples with 20-keV electrons and protons are studied using atomic-force microscopy. Irradiation is performed in a vacuum chamber under a pressure of 10^–4 Pa; the densities of the electron (? _e ) and proton (? _р ) fluxes are varied in the range of 10¹⁰–2.5 × 10¹¹ cm^?2 s^?1. Analysis of the samples irradiated in the case where the parameters ? _e and ? _р increased in a stepwise manner makes it possible to study the appearance, growth, and evolution of microscopic structures on their surfaces. The radiation-stimulated processes of defect annealing and the release and field diffusion of alkali metal ions are accompanied by crystallization of the irradiated glass layer, which gives grounds for the use of dislocation mechanisms for mass transfer in explaining the formation of microprotrusions on its surface. It is shown that the character of changes in the structure is determined by the values of the parameters ? _e and ? _р and the ratio between them. In particular, it is established that, in the case of electron— proton irradiation of the glass, electrostatic discharges begin to noticeably affect the formation of microprotrusions for ? _е > 3? _р .