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Density functional theory (DFT) and variational density functional perturbation theory (DFPT) simulations of amorphous poly-CO structures were performed to understand the stability of the polymerized structure at low pressures and to study the mechanism of destruction of the extended network at low pressures. Charge population analyses accompanied the search of the “weakest link” in the covalently bonded network. IR and Raman spectra of amorphous p-CO, calculated at 15 and 5.02 GPa, show significant contributions of CO molecules, carbonyl groups fragments decorating chains, and lactones of amorphous p-CO structures. DFT simulations of formation of amorphous polymeric structures were also done with the addition (as a result of replacement of CO molecules) of N or He atoms to the crystalline delta phase of CO. For the CO-N mixtures, the concentration of N was varied in the range from 6.25 % to 50% with different distribution patterns of N atoms in the unit cell. For all studied CO-N concentrations, isotropic compression led to CO polymerization beginning at a pressure of 11 GPa; the N was incorporated in the random network in low concentration. In CO-He mixtures He atoms appear to facilitate complete formation of the random structure which is almost completely polymerized at a pressure of 18 GPa. He atoms also help stabilize the structure at low pressures.
In this presentation, results of our recent investigations on the role of Ga on Al site in Zr69.5Al7.5-xGaxCu12Ni11 and Ce75Al25-xGax metallic glass compositions will be discussed. Ga like Al is normally expected to be in trivalent state. However, it may go in monovalent state depending on other alloying elements. The rapidly solidified melt spun ribbons of above two alloys gave rise to two important conclusions. The Zr69.5Al7.5-xGaxCu12Ni11 system displayed metallic glass formation in the range of x=0 to 7.5. In this process, we have come out with a new composition of glass without Al corresponding to x=7.5. In contrast to the above, for Ce-Al(Ga) system, we have observed phase separation in glass after dilute substitution of Ga. It seems that such a phase separation in this system cannot be understood in terms of summation of enthalpy of mixing of the various possible binaries in this system. The substitution of Ga in different valence states might have created chemical pressure leading to creation of two types of distinct major clusters. The phase separation may be due to this. This has also given rise to excursion of Ce 4f-states of the alloy. This and aforesaid ‘chemical pressure’ will be corroborated based on results of binary Ce-Al system under pressure by other investigators.
In this article, we review our recent structural studies on chalcogenide glass systems, GexSe1-x and AsxSe1-x, using anomalous X-ray scattering (AXS) in combination with reverse Monte Carlo (RMC) modeling. We show to what extent the present AXS + RMC works are effective to solve the long lasting topics in these chalcogenide glasses, such as the validity of the 8-N bonding rule, the relation to the rigidity percolation theory, the validity of chemical order, and the origin of prepeak.
In this paper we report the structure of voids in several thousand atom models of hydrogenated amorphous silicon. The models are produced by jointly employing experimental information from Smets and coworkers  and first principles simulations . We demonstrate the existence of a useful correlation between the presence of large irreducible rings and the voids in hydrogenated amorphous silicon networks. Molecular hydrogen is observed in the models, and discussed.
Fe K-edge X-ray absorption spectroscopy (XAS) was applied to study the structural response of iron phosphate glasses to atomic displacements arising from ion beam irradiation, as an analogue of α-recoil damage arising from actinide immobilization. Analysis of XAS spectra demonstrated reduction of Fe3+ to Fe2+ as a consequence of 2 MeV Kr+ and 2 MeV Au+ implantation to a fluence of 2 x 1016 ions / cm2 and 5 x 1015 ions / cm2, respectively.
Data on a viscous flow model based on network defects – broken bonds termed configurons – were analysed. An universal equation has been derived for the variable activation energy of viscous flow Q(T) of the generic Frenkel equation of viscosity η(T)=A∙exp(Q/RT) which is known to have two constant asymptotes – high QH at low temperatures and low QL at high temperatures. The defect model of flow used by e.g. Doremus, Mott, Nemilov, Sanditov states that higher the concentration of defects (e.g. configurons) the lower the viscosity. We have used the configuron percolation theory (CPT) which treats glass–liquid transition as a percolation-type phase transition. Additionally the CPT results in a continuous temperature relationship for viscosity valid for both glassy and liquid amorphous materials. We show that a particular result of CPT is the universal temperature relationship for the activation energy of viscous flow: Q(T)=QL+RT∙ln[1+exp(-Sd/R) exp((QH-QL)/RT)] which depends on asymptotic energies QL (for the liquid phase) and QH (for the glassy phase), and on entropy of configurons Sd. This equation has two asymptotes, namely Q(T<<Tg) = QH, and Q(T>>Tg) = QL. Moreover we demonstrate that the equation for Q(T) practically coincides in the transition range of temperatures with known Sanditov equation.
The 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.
We investigate the crystallization of amorphous Ni–P with near-eutectic composition, fabricated by electroless plating as a 10 µm thick continuous layer. Aiming to understand phase transformations that occur upon heating and, in particular, the microscopic mechanism of crystallization, we combine a variety of complimentary characterization techniques. DSC (differential scanning calorimetry) during isothermal heating reveals the crystallization kinetics. Conventional-, high-resolution-, and analytical TEM (transmission electron microscopy) and TEM-based electron diffraction provide high-spatial-resolution information on phase nucleation and spatial distribution of atom species, particularly the crystallography of the nucleating crystalline phases (Ni3P and Ni) and the spatial distribution of phosphorus in the partially and completely crystallized alloy. Our results indicate that crystallization proceeds by homogeneous nucleation of Ni3P grains. Internally, these exhibit a microstructure of radially oriented subgrains containing Ni nano-platelets in a specific crystallographic OR (orientation relationship) with Ni3P. However, the preferred Ni–Ni3P OR differs from those reported in the literature for similar material. Combining our observation on the structure and microstructure of partially and completely crystallized Ni–P with the observed crystallization kinetics provides a deeper understanding of the microscopic mechanism of crystallization.
We have used fluctuation electron microscopy (FEM) to measure the medium range order in the molecular packing of 40 nm thick indomethacin glass films. Vapor deposition of indomethacin can create glasses with extraordinary kinetic stability and high density. We find peaks in the FEM variance at diffraction vector magnitudes between 0.03 and 0.09 Å-1, corresponding to intermolecular packing distances of 1-3 nm. FEM experiments were performed with a 13 nm diameter electron probe, so these data are sensitive to medium-range order in intermolecular packing. The FEM variance from an indomethacin glass with normal stability cooled from the liquid is significantly smaller than the variance from the ultrastable glass, suggesting that ultrastable glass is more structurally heterogeneous at a 13 nm length scale. A dose of ∼7×105 e-/nm2 with a very low beam current of ∼ 2.5 pA at 200 kV was used to minimize electron beam damage to the sample, and the average electron diffraction from the sample is unchanged at total electron doses fourteen times larger than required for a FEM experiment. These preliminary results on medium-range order in molecular glasses suggest that we may be able to provide insight into the structural differences between the remarkable ultrastable thin films and ordinary glasses.
Understanding, predicting and eventually improving the resistance to fracture of silicate materials is of primary importance to design new glasses that would be tougher, while retaining their transparency. However, the atomic mechanism of the fracture in amorphous silicate materials is still a topic of debate. In particular, there is some controversy about the existence of ductility at the nano-scale during the crack propagation. Here, we present simulations of the fracture of three archetypical silicate glasses using molecular dynamics. We show that the methodology that is used provide realistic values of fracture energy and toughness. In addition, the simulations clearly suggest that silicate glasses can show different degrees of ductility, depending on their composition.