Mid-range Order Research Articles

Local Mid-Range Order in La2O3–Nb2O5–BsO3 and BaO–Nb2O5–P2O5 Glasses According to Full X-Ray Scattering Data

Local Mid-Range Order in La2O3–Nb2O5–BsO3 and BaO–Nb2O5–P2O5 Glasses According to Full X-Ray Scattering Data

Unraveling the bond structure, porosity, and mechanical properties amorphous ZIF-4 and its topological equivalents: Large scale ab initio calculations

Four large-scale amorphous zeolite imidazolate frameworks (a-ZIFs) have been constructed based on the continuous random network model. Structural properties correlation (SPC) between the mechanical properties and structural order (short-range, mid-range, or long-range orders) of ZIFs samples have been investigated by well-defined density functional theory calculations. The results demonstrated the effect of short-range and mid-range local order on the mechanical properties of the a-ZIFs, respectively. By combining different metal nodes and organic ligands, the porosity of a-ZIFs can be dramatically changed, leading to changes in Young’s modulus and shear modulus. The most important, SPC between structural order and mechanical properties of a-ZIFs is established, which will pave the way to designing high-strength and toughness ZIF-based glass.

Order Indices and Entanglement Production in Quantum Systems.

The review is devoted to two important quantities characterizing many-body systems, order indices and the measure of entanglement production. Order indices describe the type of order distinguishing statistical systems. Contrary to the order parameters characterizing systems in the thermodynamic limit and describing long-range order, the order indices are applicable to finite systems and classify all types of orders, including long-range, mid-range, and short-range orders. The measure of entanglement production quantifies the amount of entanglement produced in a many-partite system by a quantum operation. Despite that the notions of order indices and entanglement production seem to be quite different, there is an intimate relation between them, which is emphasized in the review.

Evidence of Paracrystalline Cation Order in the Ruddlesden-Popper Phase LaSr3NiRuO8 through Neutron Total Scattering Techniques.

Cation ordering in perovskite-derived phases can lead to a wealth of tunable physical properties. Ordering is typically driven by a large difference between the cation size and charge, but many Ruddlesden-Popper phases An+1BnO3n+1 appear to lack such B-site ordering, even when these differences are present. One such example is the "double" Ruddlesden-Popper n = 1 composition LaSr3NiRuO8. In this material, a lack of B-site ordering is observed through traditional crystallographic techniques, but antiferromagnetic ordering in the magnetism data suggests that B-site cation ordering is indeed present. Neutron total scattering, particularly analysis of the neutron pair distribution function, reveals that the structure is locally B-site-ordered below 6 Å but becomes slightly disordered in the midrange structure around 12 Å. This provides evidence for paracrystalline order in this material: cation ordering within a single perovskite sheet that lacks perfect registry within the three-dimensional stack of sheets. This work highlights the importance of employing a structural technique that can probe both the local and midrange order in addition to the crystallographic structure and provides a structural origin to the observed magnetic properties of LaSr3NiRuO8. Further, it is proposed that paracrystalline order is likely to be common among these layered-type oxides.

Mid-range order in trapped quasi-condensates of bosonic atoms

Finite Bose systems cannot display a genuine Bose–Einstein condensate with infinite long-range order. But, if the number of trapped atoms is sufficiently large, a kind of Bose–Einstein condensation does occur, with the properties of the arising quasi-condensate being very close to the genuine condensate. Although the quasi-condensate does not enjoy long-range order, it has mid-range order. This paper shows that the level of mid-range order in finite Bose systems can be characterized by order indices of density matrices.

Fine structure, mechanical characteristics and mid-range order in xZnO–yNa2O–(1-x-y)SiO2 glasses

The differentiation between the behavior of Zn2+ and Na+ cations in silicate network is shaded by the lack of characteristic infrared or Raman bands belonging to the modes involving the zinc- or sodium-oxygen bonds. It was shown in this study that the stepwise substitution of Na2O for ZnO in the xZnO–yNa2O–(1-x-y)SiO2 glasses with keeping approximately the same SiO2 content causes the high-frequency shift of the Raman-active boson peak (BP) that is the decrease of the correlation length in structural glass heterogeneities. Intercomparison of this trend with variations of the measured structural strength and microhardness of the glasses under consideration afforded ground to suggestion of a possible dual role of Zn2+ ion as either glass-modifier or glass former in specific compositions of zinc-alkali-silicate glasses.

Amorphous chalcogenides as random octahedrally bonded solids: I. Implications for the first sharp diffraction peak, photodarkening, and Boson peak.

We develop a computationally efficient algorithm for generating high-quality structures for amorphous materials exhibiting distorted octahedral coordination. The computationally costly step of equilibrating the simulated melt is relegated to a much more efficient procedure, viz., generation of a random close-packed structure, which is subsequently used to generate parent structures for octahedrally bonded amorphous solids. The sites of the so-obtained lattice are populated by atoms and vacancies according to the desired stoichiometry while allowing one to control the number of homo-nuclear and hetero-nuclear bonds and, hence, effects of the mixing entropy. The resulting parent structure is geometrically optimized using quantum-chemical force fields; by varying the extent of geometric optimization of the parent structure, one can partially control the degree of octahedrality in local coordination and the strength of secondary bonding. The present methodology is applied to the archetypal chalcogenide alloys AsxSe1-x. We find that local coordination in these alloys interpolates between octahedral and tetrahedral bonding but in a non-obvious way; it exhibits bonding motifs that are not characteristic of either extreme. We consistently recover the first sharp diffraction peak (FSDP) in our structures and argue that the corresponding mid-range order stems from the charge density wave formed by regions housing covalent and weak, secondary interactions. The number of secondary interactions is determined by a delicate interplay between octahedrality and tetrahedrality in the covalent bonding; many of these interactions are homonuclear. The present results are consistent with the experimentally observed dependence of the FSDP on arsenic content, pressure, and temperature and its correlation with photodarkening and the Boson peak. They also suggest that the position of the FSDP can be used to infer the effective particle size relevant for the configurational equilibration in covalently bonded glassy liquids, where the identification of the effective rigid molecular unit is ambiguous.

Plasticity, crack initiation and defect resistance in alkali-borosilicate glasses: From normal to anomalous behavior

We provide a comprehensive description of the defect tolerance of sodium-borosilicate glasses upon sharp contact loading. This is motivated by the key role which is taken by this particular glass system in a wide variety of applications, ranging from electronic substrates, display covers and substrates for biomedical imaging and sensing to, e.g., radioactive waste vitrification. The present report covers the mechanical properties of glasses in the Na2O–B2O3–SiO2 ternary over the broad range of compositions from pure SiO2 to binary sodium-borates, and crossing the regions of various commercially relevant specialty borosilicate glasses, such as the multi-component Duran-, Pyrex- and BK7-type compositions and typical soda-lime silicate glasses, which are also included in this study. In terms of structure, the considered glasses may be separated into two groups, that is, one series which contains only bridging oxygen atoms, and another series which is designed with an increasing number of non-bridging oxygen ions. Elastic moduli, Poisson ratio, hardness as well as creep and crack resistance were evaluated, as well as the contribution of densification to the overall amount of indentation deformation. Correlations between the mechanical properties and structural characteristics of near- and mid-range order are discussed, from which we obtain a mechanistic view at the molecular reactions which govern the overall deformation reaction and, ultimately, contact cracking.

Raman spectroscopic investigation of boron doped hydrogenated amorphous silicon thin films

We report on the effects of hydrogen dilution and boron doping on the structural properties of amorphous silicon thin films prepared by plasma enhanced chemical vapor deposition. Raman studies were performed on samples differing in both boron concentration and hydrogen dilution of the silane precursor prepared at two growth temperatures. Changes in the transverse optical and acoustic phonon modes were analyzed to determine the effect of boron and hydrogen on short- and mid-range order of the amorphous crystal structure. The results show that, with either an increase of hydrogen dilution or growth temperature, the short- and mid-range order improves. However, the effect of growth temperature on the ability of hydrogen to improve the short- and mid-range order decreases as the growth temperature is increased. This is attributed to dissociation of weakly bonded species at higher growth temperature, leading to less hydrogen being absorbed. In addition, for fixed hydrogen dilution, an increase in boron doping results in a decrease in the short-range order due to a rearrangement of the chemical bonding in the thin film. Finally, a direct correlation is seen between the electrical resistivity and the short-range order for samples of differing hydrogen dilution grown at the higher growth temperatures.

Vibrational signatures of chemical- and density-induced structural changes in simulated amorphous silica

Molecular dynamics simulations show that vibrational modes in glassy SiO2 are affected differently by density-induced changes to the structural order compared with those induced by specific cationic substitutions (Al, P, Na). Using standard measures of local and midrange positional order, we find that P disrupts network order in terms of second nearest neighbours, but preserves local order. In contrast, Al decreases local order while maintaining network order. Increased density preserves structural correlations in the network although the length scale shrinks. The complex short-range and long-range structural trends in these modified glasses coincide with changes in distinct regions in the vibrational density of states (VDOS). This suggests that a greater role for VDOS as a tool to link simulation and experiment in amorphous materials.

Electrical Conductivity and Structural Order of p-Type Amorphous Silicon Thin Films

ABSTRACTThe dependence of dark conductivity and room temperature Raman spectra on boron and hydrogen incorporation in thin films of hydrogenated amorphous silicon (a-Si:H) prepared by plasma enhanced chemical vapor deposition was investigated. It was found that the dominant conductivity is Mott variable range hopping conduction. However, at lower temperatures, Efros-Shklosvkii hopping conduction is observed and contributes to the total conductivity. For structural characterization, transverse optical (TO) and transverse acoustic (TA) modes of the Raman spectra were studied to relate changes in short- and mid-range order to the effects of boron and hydrogen incorporation. With an increase of hydrogen incorporation and/or substrate temperature, both short and mid-range order improve, whereas the addition of boron results in the degradation of the short range order. The line width and frequency of the Raman TO Raman peak correlate with electrical measurements and suggest that this technique can be used for non-destructive characterization of a-Si:H.

Visualizing microscopic structure of simulated model basalt melt

We perform a detailed visualization-based analysis of atomic-position series data for model basalt melt obtained from first-principles (quantum mechanical) molecular dynamics simulations. To gain insight into the short- and mid-range order of the melt structure, we extract and visualize the details of radial distribution function (RDF) and coordination environment. The first peaks of all partial RDFs lie in the distance range of 1.6–4Å and the corresponding mean coordination numbers vary from less than 1 to more than 9. The coordination environments involving cations and anions differ substantially from each other, each consisting of a rich set of coordination states. These states vary both spatially and temporally: The per-atom coordination information extracted on the fly is rendered instantaneously as the spheres and polyhedra as well as along the corresponding trajectories using a color-coding scheme. The information is also visualized as clusters formed by atoms that are coordinated at different time intervals during the entire simulation. The Si–O coordination is comprised of almost all tetrahedra (4-fold) whereas the Al–O coordination includes both tetrahedra (4-fold) and pentahedra (5-fold). The animated visualization suggests that the melt structure can be viewed as a dynamic (partial) network of Al/Si–O coordination polyhedra connected via bridging oxygen in an inhomogeneous distribution of mobile magnesium and calcium atoms.

Vacuum structure and string tension in Yang-Mills dimeron ensembles

We numerically simulate ensembles of SU(2) Yang-Mills dimeron solutions with a statistical weight determined by the classical action and perform a comprehensive analysis of their properties as a function of the bare coupling. In particular, we examine the extent to which these ensembles and their classical gauge interactions capture topological and confinement properties of the Yang-Mills vacuum. This also allows us to put the classic picture of meron-induced quark confinement, with the confinement-deconfinement transition triggered by dimeron dissociation, to stringent tests. In the first part of our analysis we study spacial, topological-charge and color correlations at the level of both the dimerons and their meron constituents. At small to moderate couplings, the dependence of the interactions between the dimerons on their relative color orientations is found to generate a strong attraction (repulsion) between nearest neighbors of opposite (equal) topological charge. Hence, the emerging short- to mid-range order in the gauge-field configurations screens topological charges. With increasing coupling this order weakens rapidly, however, in part because the dimerons gradually dissociate into their less localized meron constituents. Monitoring confinement properties by evaluating Wilson-loop expectation values, we find the growing disorder due to the long-range tails of these progressively liberated merons to generate a finite and (with the coupling) increasing string tension. The short-distance behavior of the static quark-antiquark potential, on the other hand, is dominated by small, ``instantonlike'' dimerons. String tension, action density and topological susceptibility of the dimeron ensembles in the physical coupling region turn out to be of the order of standard values. Hence, the above results demonstrate without reliance on weak-coupling or low-density approximations that the dissociating dimeron component in the Yang-Mills vacuum can indeed produce a meron-populated confining phase. The density of coexisting, hardly dissociated and thus instantonlike dimerons seems to remain large enough, on the other hand, to reproduce much of the additional phenomenology successfully accounted for by nonconfining instanton vacuum models. Hence, dimeron ensembles should provide an efficient basis for a more complete description of the Yang-Mills vacuum.

Study of polyethylene structure by means of low-frequency Raman spectroscopy: Longitudinal acoustic mode and boson peak

The opportunities given by the low-frequency Raman spectroscopy for studying the ordered structures in polymers are presented and illustrated by examples of application of two techniques. The longitudinal acoustic modes (LAM) excited in straight chain segments of macromolecules are characteristic in linear semicrystalline polymers, while the Boson peak (BP) manifests itself in amorphous (glassy) polymers. Both these features are due to the laser light scattering from the acoustic vibrations localized either in the ordered one-dimension structures (LAM) or in the mid-range order correlation sphere (BP).

Visualization-based analysis of structural and dynamical properties of simulated hydrous silicate melt

We have explored first-principles molecular dynamics simulation data for hydrous MgSiO3 liquid (with 10 wt% water) to gain insight into its structural and dynamical behavior as a function of pressure (0–150 GPa) and temperature (2,000–6,000 K). By visualizing/analyzing a number of parameters associated with short- and mid-range orders, we have shown that the melt structure changes substantially on compression. The speciation of the water component at low pressures is dominated by the isolated structures (with over 90% hydrogen participated) consisting of hydroxyls, water molecules, O–H–O bridging and four-atom (O–H–O–H and H–O–H–O) groups, where every oxygen atom may be a part of polyhedron or free (i.e., bound to only magnesium atom). Hydroxyls favor polyhedral sites over magnesium sites whereas molecular water is almost entirely bound to magnesium sites, and also interpolyhedral bridging (Si–O–H–O–Si) dominates other types of bridging. Water content is shown to enhance and suppress, respectively, the proportions of hydroxyls and molecular water. As compression increases, these isolated structures increasingly combine with each other to form extended structures involving a total of five or more O and H atoms and also containing threefold coordination species, which together consume over 80% hydrogen at the highest compression studied. Our results show that water lowers the mean coordination numbers of different types including all cation–anion environments. The hydrous melt tends to be more tetrahedrally coordinated but with the Si–Si network being more disrupted compared to the anhydrous melt. Protons increase the content of non-bridging oxygen and decrease the contents of bridging oxygen as well as oxygen triclusters (present at pressures above 10 GPa). The calculated self-diffusion coefficients of all atomic species are enhanced in the presence of water compared to those of the anhydrous melt. This is consistent with the prediction that water depolymerizes the melt structure at all pressures. Our analysis also suggests that proton diffusion involves two processes—the transfer of H atoms (requiring the rupture and formation of O–H bonds) and the motion of hydroxyls as hydrogen carriers (requiring the rupture and formation of Si–O and/or Mg–O bonds). Both the processes are operative at low compression whereas only the first process is operative at high compression.

Modification of the N bonding environment in GaN after high-dose Si implantation: An x-ray absorption study

The microstructure and electronic structure of epitaxially grown GaN, that has been subjected to high-dose Si implantation, is studied using x-ray absorption fine structure (XAFS) spectroscopy. More specifically, XAFS is used to probe the formation of N–Si bonds and to study the implantation induced distortions in the lattice. The analysis of the extended XAFS spectra reveals that implantation with 100keV Si ions with a fluence equal to 1×1018cm−2 renders the material amorphous and promotes the formation of Si–N bonds with a bond length equal to that corresponding in Si3N4. In addition to that, the N–Ga distances increase by ∼5% due to the lattice expansion caused by the incorporation of the Si ions and the formation of point and extended defects. The absence of long or midrange order is also verified by the near edge XAFS (NEXAFS) spectra where the characteristic peaks are smeared out. However, a characteristic sharp resonance line, that appears about 1eV above the absorption edge, indicates the existence of defect states which are strongly localized on the N atoms, most probably defect complexes involving N dangling bonds. In order to provide additional evidence on the nature of the implantation induced changes we resorted to simulations of the NEXAFS spectra using the FEFF8 code by applying chemical and lattice deformations in the immediate environment of the absorbing atom as well as to larger clusters.

Mid-infrared spectroscopic studies of alkali-activated fly ash structure

Infrared spectroscopic techniques were used in the present study to acquire a deeper understanding of mid-range order in the alkaline aluminosilicate (zeolite precursor) formed in the alkali activation of fly ash. Experiments were conducted with three type F fly ashes containing different proportions of vitreous and crystalline (primarily mullite and quartz) phases, activated with an 8 M solution of NaOH. All the mixes were alkali activated at 85 °C for the following reaction times: 2 h, 5 h, 8 h, 20 h and 7 days. The results indicate that the amount of alkaline silicoaluminate formed as the main reaction product, and its Si/Al ratio, induce a shift in the T–O stretching band appearing at 950–1100 cm −1. Moreover, the presence of zeolite-type crystalline phases generates substantial changes in the 800–500 cm −1 area of the spectra. Selective chemical attacks were used to facilitate identification of the spectra bands.

Fractal behavior of amorphous metal structure under the action of tensile stress

We report a series of time-separated topograms of the stressed surface of a foil made of metallic glass Fe70Cr15B15. The numerical analysis of the tortuosity of both normal profiles and horizontal sections of the surface relief demonstrates a fractal/self-affine geometry as beginning from the nanoscopic scale level. Unlike the foils made of convention metals whose surface profile do not exhibit any features of fractality, in the original metallic glass foil the scaling character of the profile depth distribution was revealed. This is a statistical representation of non-equilibrium nanoscopic structure resulting from the highly critical glass forming process in the amorphous alloy. The temporal variation of fractal dimension after applying the 500MPa tensile stress includes a rise to 1.46±0.06 at the initial stage of loading, dramatic drop to 1.12+0.03, and gradual increase to 1.22±0.02 just before the formation of regular shear bands, that is in ∼1.5h after the stress application. The mechanism of formation of the transient fractal structures is discussed in terms of mid-range order dynamics in disordered media.

On the reversibility of hydrogen effects on the properties of amorphous silicon carbide

We performed a theoretical investigation on the effects of hydrogenation and dehydrogenation in the structural properties of amorphous silicon carbide (a-SiC). The calculations were carried out combining interatomic potentials with free volume Monte Carlo simulations. Our prototype amorphous silicon carbide systems contained as much as 85 000 atoms, allowing for a good statistical sampling. We addressed the role of hydrogen on the local and mid-range order of the material. We found that carbon atoms segregate, forming small clusters, embedded in an extensive silicon network. Ring statistics showed that hydrogenation increases the concentration of micro-voids in the structure and reduces its mid-range order. On the other hand, dehydrogenation recovers most of the original structural order.

The role of the dynamic structural correlation in glasses in the initiation of brittle fracture

The previously reported data on the mid-range order and on the low-frequency vibrational interaction in glassy materials are interpreted in terms of dynamically defined correlation lengths between disordered structural units. The latter parameter is shown to be highly sensitive to the topological perturbations in the material which affect the cation–cation separation lengths, or, in general, to the local strains in the glass network. Virtual expansion of the correlation sphere involving ultimately strained chemical bonds imposes conditions on the formation of the fractal structure of primary nanocracks due to the scaling character of the inverse of the Grüneisen parameter.