Existence Of Medium-range Order Research Articles

Identification of medium range order defects and their critical effect on spallation of Cu64Zr36 metallic glass

The absence of a well-defined structural defect is a longstanding problem, acting as the obstacle for understanding of mechanical behavior in metallic glasses. Here, through a compelling integration of experimental and computational technologies containing Synchrotron X-ray diffraction, extended X-ray absorption fine structure spectroscopy (EXAFS) as well as atomistic simulations, we present more direct evidences for the existence of medium range order (MRO) defects based on the discontinuity of stiff icosahedral network. As revealed by the numerical simulation of spallation process for two Cu64Zr36 metallic glasses with different cooling history, the MRO defects can be used as the efficient predictor of void nucleation, as evidenced by the strong correlation between high MRO defects location and subsequent void sites. This observation implies that the initial distribution as well as further generation of MRO defects play a critical role for the mechanical failure process of metallic glasses. It can be also extended to experimental conditions since the atomic packing of metallic glasses in molecular dynamic (MD) simulations is in great agreement with that obtained from reality as characterized by results of X-ray and EXAFS.

Medium-range order in amorphous ices revealed by persistent homology

Despite the amorphous nature of glassy water, x-ray or neutron scattering experiments reveal sharp peaks in the structure factor, indicating the existence of medium-range order (MRO) in the system. However the real space origin of the peaks has yet to be disclosed. Herein, we use a combined approach of molecular dynamics simulations and persistent homology (PH) to investigate two types of glassy water, low-density amorphous (LDA) and high-density amorphous (HDA) ices. We present prominent MRO ring structures in each type of the ices, distinguished by their size and shape as well as the number of their components: MRO rings in HDA are observed smaller, less planar and more membered, compared to those in LDA. The PH-extracted MRO rings successfully reproduce the quantitative features, including the position and width, of the first sharp diffraction peaks in the structure factor, hence suitably serving as the origin of experimental MRO signatures in the amorphous ices. Our study supports that PH is an effective tool to identify hidden MRO in amorphous configurations.

Bergman-type medium range order in amorphous Zr77Rh23 alloy studied by ab initio molecular dynamics simulations

In recent years, some arguments about the existence of medium-range order (MRO) in the Zr-Rh system have been put forward. However, research on the structural features of the Zr-Rh binary alloy at the atomic level is still lacking. This study uses ab initio molecular dynamics simulations to systematically study the local structures of Zr77Rh23 from the liquid to the glassy states. Pair correlation function (PCF), coordination number (CN), Honeycutt–Anderson(HA) index, bond-angle distribution functions, and the Voronoi tessellation method are used to reveal a clear icosahedral-like configuration in the amorphous Zr77Rh23 alloy. It is noteworthy that the splitting in the second peak of the partial PDF implies the existence of a medium range order (MRO) in the Zr77Rh23 system. We obtain the local order in three-dimensional atomic density distributions by using a new atomistic cluster alignment (ACA) method. Interestingly, a Bergman-type MRO is observed in the glassy Zr77Rh23. Furthermore, the spatial distribution and connections of icosahedral-like clusters are shown to further demonstrate the MRO network. Our findings shed light on the nature of atomic local structures of amorphous Zr77Rh23 alloy and have important implications to understanding the formation of various MROs in metallic glasses.

Local structures and optical properties of As-Se-Te(S) chalcogenide glasses

The partial, total and average coordination number of atoms and, their neighbor distributions have been determined by the neutron diffraction method and application of the Reverse Monte Carlo modeling for experimental data in As40Se60, As40Se30S30 and As40Se30Te30 chalcogenide glasses. The total coordination number of the arsenic and chalcogen atoms is 3 and 2 for all compounds. The average coordination number is 2.4, i.e. the 8-N rule is performed and main structural element is AsSe3 type pyramids, where depending on the chemical composition Se atom is replaced by S or Te atoms. The proportion of homopolar bonds is significant in As40Se60 and As40Se30S30 compositions, but sharply decreases in As40Se30Te30. In all compositions the neighbor distances between arsenic and selenium atoms are the same within error: 2.35 Å (SeSe); 2.45 Å (AsAs) and 2.40 Å (AsSe). The existence of medium range order (MRO) is proved by the analysis of the total and partial pair structural factors and correlation functions in above mentioned compositions. The geometric dimensions of the MRO area, i.e. the quasi-periodicity of density fluctuations in the MRO area and the size of nano-voids have determined within Elliott's void-based model. The change of the optical parameters by variation of the chemical composition has been established from the analysis of the optical transmission spectra.

Covalent bond connectivity, medium range order and physical properties of GexIn8Se92−x glasses

The GexIn8Se92−x glasses (14 ≤ x ≤ 23.5) were extensively characterized by X-ray diffraction (XRD). The chemical composition is determined using energy dispersive spectrometers (EDS). The atomic structure of these glasses is investigated by using (XRD) technique. The first sharp diffraction peak (FSDP) in the structural factor indicates the existence of medium range order (MRO) in these glasses. The position of the FSDP and its full width at half maximum (FWHM), obtained after fitting the XRD of the glasses. The repetitive characteristic distance, R and structural correlation length, L, which characterizing the intermediate structure are deduced. The effect of composition and average coordination number <CN>, on the atomic volume, the density and compactness are studied. The connectivity of the system is discussed in terms of the total number of constraints which also influence the mean bond energy of the system. The intensity of the FSDP, the atomic volume and compactness of the structure, have anomalous change at <CN> = 2.67 indicating, a topological threshold occurs. The occurrence of this dependence leads to the Ge content has a marked effect on the MRO and physical properties parameters.

Pulsed neutron diffraction study on the short range structure of B 2O 3–Ag 2O glasses

The short range structure of (1− x)B 2O 3– xAg 2O ( x=0.05–0.30) glasses was investigated by time-of-flight neutron scattering experiments. Radial distribution analyses were applied to extract the structural information, but the structural parameters for each atomic pair were finally optimized in the Q-space. The bond distances of near neighbor B–O pairs were estimated at 0.138 and 0.145 nm, and the corresponding coordination numbers were 3 and 4, respectively. These were ascribed to the B–O correlations in distorted six-membered rings consisting of planer trigonal units and tetrahedral ones. The silver atoms serve to hold the networks together through coordination with oxygen atoms, thereby the nearly octahedral arrangements of oxygen atoms around a silver atom were found to exist in these glasses. The octahedral units were distorted to such an extent that the Ag–O distances were ranging from 0.25 to 0.32 nm. The existence of medium range order was also confirmed in the structure factor S( Q) over the composition range of this work.

Control of Medium Range Order in Amorphous Silicon via Ion and Neutral Bombardment

ABSTRACTWe have observed the existence of medium range order via fluctuation microscopy in amorphous silicon grown at 230°C. We hypothesize that this structure develops during the highly non-equilibrium growth process; high densities of ordered surface nuclei are produced which are subsequently buried and forced into an unfavorable energy state. These nm sized regions are distorted in the bulk due to strain, but remain topologically crystalline. In this work, we alter the growth energetics both at the surface and sub-surface during magnetron sputter film deposition with two kinds of particle bombardment, respectively: a controllable flux of low-energy (20eV) Ar+ ions, and higher energy (100eV) D vs. H neutrals. With this method, we demonstrate for the first time control over the intensity of this medium-range structural order at a constant substrate temperature as seen primarily with fluctuation electron microscopy, but also Raman scattering, spectroscopic ellipsometry, and SAXS. We suggest that these bombardments can increase adspecie surface mobility or drive local sub-surface restructuring (“kinetic annealing”), thus increasing or decreasing the size, density and/or strength of the ordered regions.

Structure anddynamics of a model glass: influence of long-range forces

We vary the amplitude of the long-range Coulomb forces within aclassical potential describing a model silica glass and study theconsequences on the structure and dynamics of the glass, viamolecular dynamics simulations. This model allows us to follow thevariation of specific features such as the first sharp diffractionpeak and the boson peak in a system going continuously from afragile (no Coulomb forces) to a strong (with Coulomb forces) glass.In particular we show that the characteristic features of a strongglass (existence of medium-range order, bell-shaped ring sizedistribution, sharp boson peak) appear as soon as tetrahedral unitsare formed.

Microinhomogeneities of glasses of the system PbO–SiO2

Small angle x-ray scattering (SAXS) and Rayleigh–Mandelstam–Brillouin (RMB) light scattering as well as ultrasonic sound velocities have been studied in glasses of the system PbO–SiO2 which has an unusually wide range of glass forming ability. The results of scattering are compared with calculations based on the concept of frozen-in equilibrium thermal fluctuations as the origin of static microinhomogeneities (MIH) in glasses. MIH of compositions seem to be the main source of scattering, and the calculations are found to be in qualitative agreement with both SAXS and RMB measurements. Glasses with PbO content above 40 mol.% are more homogeneous than ideal solutions of PbO and SiO2 whereas MIH in glasses with smaller PbO content are comparable with those expected for ideal solutions of PbO and SiO2. In the latter range SAXS measurements indicate the existence of medium-range order with correlation length of 5–7 Å.