Degree Of Medium-range Order Research Articles

Short to medium range order and atomic dynamic property within the surface of nanoscale metallic glasses revealed by molecular dynamics simulations

The surface microstructure and its atomic dynamic property of a metallic glass nano pillar were investigated by using molecular dynamics simulations. The short-range order motifs in the surface contain a large number of unstable bonded pairs and clusters, leading surface to approach the structural property of liquid phase. A proportion of <0, 0, 12, 0> clusters in the surface constitute chain-like super clusters, but its overall degree of medium-range order is lower than that in the core region. The atomic mobility in the surface at different scales are enhanced significantly. Generally, the lower the degree of order for the cluster, the stronger its migration ability. The surface region could be divided into three consecutive sublayers, which are solid-like layer, semi-solid layer and liquid-like layer. The content of liquid-like clusters increases sequentially in these three sublayers. A schematic illustration of the relationship between the hierarchical structure and the possible relaxation modes was proposed.

Impact of Yb3+/Er3+ ions on crystallization of phosphosilicate glass melts

Transparent glass ceramics were prepared from the phosphosilicate system by melt-quenching devitrification (MQD) method, i.e., nanocrystals spontaneously form during cooling of the melts. Introduction of 2.5 wt.% Yb2O3 and 0.5 wt.% Er2O3 into the glass melt induced the change of type and concentration of crystals. The comparison of rheological and thermodynamic properties of both undoped and Yb3+/Er3+ doped melts showed that addition of Yb3+/Er3+ oxides caused increase of liquid fragility, and degree of medium-range order. In addition, the thermodynamic barriers for nucleation ΔG* as a function of reduced temperature T/Tm were calculated with an assumption of wetting angle θ=90°, Yb3+/Er3+ doped melt tended to firstly nucleate as compared to undoped melt at small undercooling.

Structural study of amorphous CoFeB thin films exhibiting in-plane uniaxial magnetic anisotropy

The structure of amorphous ferromagnetic ${\text{Co}}_{40}{\text{Fe}}_{40}{\text{B}}_{20}$ thin films, in particular the origin of induced magnetic in-plane anisotropy, is investigated by reduced density function (RDF) analysis of electron-diffraction patterns. In this research, the RDF methodology is developed in order to measure the direction-dependent bond lengths and coordination numbers. The directional variations in these parameters are predicted to be the likely origins of induced anisotropy. With nearest-neighbor distances measured to an accuracy of $0.02\text{ }\text{\AA{}}$, no variations in the characteristic bond lengths with direction are observed. By studying the coordination numbers, it is shown that any directional ordering effect must be less than 5% and 1% for transition-metal--metalloid and transition metal-transition metal neighbors, respectively, in CoFeB films subjected to a 400 Oe magnetic field during deposition. The overall structure of ${\text{Co}}_{40}{\text{Fe}}_{40}{\text{B}}_{20}$ thin films is further investigated by reverse Monte Carlo simulations, which enable an estimation of the coordination number of the transition-metal components in the CoFeB alloy. These simulations find a range of local coordination polyhedra present in the material. Despite an average coordination number of approximately 8 for the transition-metal component, a degree of medium-range order, and a stoichiometry close to ${\text{Co}}_{50}{\text{Fe}}_{50}$, large local deviations from this octahedral value mean that the structure is not based on bcc-type basic units.

Effect of Ag addition on local structure of Cu–Zr glassy alloy

The effect of Ag substituting Cu on the structural features of the Cu55Zr45, Cu45Zr45Ag10, and Cu35Zr45Ag20glassy alloys was studied using the real-space pair distribution and radial distribution functions. The experimental x-ray diffraction data obtained in a synchrotron beam were used to derive pair and radial distribution functions through Fourier transformation processing. These results suggest that a certain degree of medium-range order in this alloy is maintained up to about 2.5 nm distance. It is suggested that the addition of Ag causes formation of a more homogeneous local atomic structure compared with that of a binary Cu–Zr alloy, which could be considered as a reason for the improved glass-forming ability of this alloy.

Real-space structural studies of Cu–Zr–Ti glassy alloy

Abstract Structural features of a glassy Cu 60 Zr 30 Ti 10 alloy are studied by using the real-space pair distribution and radial distribution functions. The experimental X-ray diffraction data obtained in a synchrotron beam were used to derive radial distribution function through Fourier transformation processing. Based on these and the results of transmission electron microscopy observation it is suggested that a certain degree of medium-range order in this alloy maintains up to about 2 nm distance. It is also shown that the interatomic distances closely correspond to those of oC68 Cu 10 Zr 7 compound which allows obtaining high relative density of the glassy alloy. The formation of icosahedral local order is also presumed.

Raman spectroscopy of nano-structured silicon to study the embedded crystallites

Raman spectra of a variety of polymorphous (pm-Si:H) and amorphous silicon (a-Si:H) samples deposited by plasma enhanced chemical vapor deposition (PECVD) at different pressures were recorded in the range of 150 cm-1 to 750 cm-1 using a 514 nm excitation source. A comparison of Raman spectra between a-Si:H and pm-Si:H samples reveals significant differences. The Transverse Optical (TO) peak in case of pm-Si:H is asymmetric and shifted to higher wave numbers. In the literature, discrepancies between predictions of various quantum confinement models and experimental spectra have typically been attributed to either strain or negligible fraction of nanocrystallites. We show that a quantum confinement model along with a Gaussian size distribution is able to accurately predict particle size of nanocrystallites embedded in the amorphous matrix. The crystallite size and size distribution obtained by fitting the TO peak is consistent with high-resolution transmission electron microscopy observations. In our case typical mean crystallite size obtained is about 3 nm with FWHM of the distribution varying in the range 0.2–1 nm. A comparison of ratio of heights in TA and TO peaks of a-Si:H and pm-Si:H material indicates that pm-Si:H as a material has higher medium range order (MRO). This ratio has been used to compare the degree of MRO in pm-Si:H samples prepared under different conditions. Thus, we demonstrate that Raman Spectroscopy along with our model can be used to obtain the crystallite size distribution and provide a measure of degree of medium range order.

Local structure reconstruction in hydrogenated amorphous silicon from angular correlation and synchrotron diffraction studies

Hydrogenated amorphous silicon (a-Si:H) is a widely used thin film semiconductor material which is still incompletely understood. It is generally assumed to form a continuous random network, with a high concentration of coordination defects (dangling bonds), which are hydrogen terminated. Neither the exact nature of these sites nor the degree of medium range order has been fully determined. In this paper, we present the first results for the local structure, from a combined study using angular correlation of positron annihilation radiation (ACAR) and synchrotron radiation diffraction. Reciprocal space information is obtained directly, for the mesoscale structure and the local defect structure, from the orientation dependent diffraction and 2D-ACAR patterns, respectively. Furthermore, inversion of both patterns yields a comparison of real space information through maps of the silicon–silicon pair correlation function and the electron–positron autocorrelation function B 2 γ ( r). From this information, it is possible to identify the dominant structural defect as a vacancy-size dangling bond cluster, around which the network strain is fully relaxed.

Fluctuation Microscopy in the STEM

Fluctuation microscopy is an electron microscopy technique sensitive to medium-range order (MRO) in disordered materials. It has been applied to study amorphous germanium and silicon, leading to the conclusion that these materials exhibit more MRO than the conventional continuous random network model for their structure.As originally proposed by Treacy and Gibson, fluctuation microscopy utilizes mesoscopicresolution (1.5 nm) hollow-cone dark field (HCDF) imaging in a TEM. The normalized variance of such images,is a measure of the magnitude of fluctuations in the diffracted intensity from mesoscopic volumes of the sample and is sensitive to MRO via the three- and four-body atom distribution functions. Studying V as a function of the diffraction vector magnitude k gives information about the degree of MRO and the internal structure of ordered regions. V as a function of the inverse resolution Q gives information about the characteristic MRO length scale.

An X-ray absorption fine structure study of amorphous precursors of a gallium silicate zeolite

Amorphous gallium silicates isolated in the early stages of the hydrothermal synthesis of the gallium silicate zeolite hydroxosodalite have been studied at both the Ga K and Si K absorption edges by extended X-ray absorption-fine structure (EXAFS) and X-ray absorption near edge structure (XANES) spectroscopies. A series of crystalline gallium oxide materials were also studied for comparison; this demonstrated that octahedral and tetrahedral gallium can be unequivocally distinguished using both EXAFS and XANES. The local structure of the amorphous phases is found to resemble the structure of the zeolite. Gallium is found in six-coordinate sites in the amorphous gallium oxide starting material used, but even after the shortest heating periods it converts to a solely four-coordinate environment. Both gallium and silicon K-edge EXAFS indicate some degree of medium-range order in the amorphous materials (i.e. beyond the first coordination shell). This is indicative of an extended network constructed from alternating Ga(OSi) 4 and Si(OGa) 4 units forming the structure of the amorphous materials, reminiscent of the structure of the zeolite.

Reconciliation of the microcrystalline and the continuous random network model for amorphous semiconductors

AbstractWe show by molecular dynamics simulations, that the radial distribution function of an amorphous material does not change significantly by introducing a considerable volume fraction of nanocrystals. The nanocrystals, embedded in a continuous random network, ensure a certain degree of medium range order in the amorphous material. Our simulations, which are on germanium, show that microcrystals smaller than 2 nm can comprise at least 20 % of the volume without significantly changing the radial distribution function from that of pure continuous random network. By increasing the size of the crystals, without altering the crystal to amorphous volume ratio, the radial distribution changes. The molecular dynamics simulations show that the nanocrystals are unchanged at low temperature. At higher temperature the mobility and critical size of the grains increase, transforming the sub-critical crystalline grains into the surrounding continuous random network matrix.

Comparative Fluctuation Microscopy Study of Medium-Range Order in Hydrogenated Amorphous Silicon Deposited by Various Methods

ABSTRACTWe have characterized by fluctuation electron microscopy the medium-range order of hydrogenated amorphous silicon thin films deposited by a variety of methods. Films were deposited by reactive magnetron sputtering, hot-wire chemical vapor deposition, and plasma enhanced chemical vapor deposition with and without H2 dilution of the SiH4 precursor gas. All of the films show the signature of the paracrystalline structure typical of amorphous Si. There are small variations in the degree of medium-range order with deposition method and H content. The PECVD film grown with high H2 dilution contains Si crystals ∼5 nm in diameter at a density of ∼109 cm−2. The amorphous matrix surrounding these crystals shows no difference in mediumrange order from the standard PECVD film. This supports explanations of the resistance of the H-dilution material to light-induced degradation that depend only on the presence of crystalline grains without modifications of the amorphous matrix.

Investigation of the medium range order of polyhedra forming the walls of MCM-41 — An X-ray diffraction study

Pure silica MCM-41, Al-MCM-41 (Si/Al=30, 20, 10) and Ti-MCM-41 (Si/Ti=40) have been synthesized and characterized by transmission electron microscopy (TEM) and X-ray powder diffraction (XRD). The calcined materials have been investigated by wide-angle X-ray scattering (WAXS), in order to elucidate the structural arrangement of the non-crystalline walls with regard to the medium-range order (MRO). The experimental radial distribution functions (RDF) of all samples under investigation indicate a considerable degree of MRO within the examined length scale of 8 Å. Even though, the addition of aluminium sulfate as source for aluminium, or titanium-isopropylate as source for titanium to the synthesis gel is known to affect the regularity of the pores, the MRO is not significantly affected. Modelling studies have been performed to describe the average atomic arrangement of the inorganic material of pure silica MCM-41. Best agreement between experimental and calculated data with regard to the MRO has been obtained using a structural arrangement of tetrahedra, which is also found in the structure of α-cristobalite.

Effect of glass short-range order on crystallization onset in Al–Y–Ni glasses

Studies of Al85YxNi15−x glasses have established a correlation between crystallization mode (growth versus nucleation and growth) and glass composition. Glasses with approximate compositions x=5,7,8,10 were studied to determine the nature of glass structure variation which gives rise to different crystallization modes. x=5 glasses contain quenched-in Al crystals which act as nucleating centers, leading to α-Al first stage crystallization product. x=10 is a homogeneous glass which undergoes a glass transition prior to crystallization; first crystalline product is an unstable fcc structure which breaks down via Al segregation. x=7,8 glasses share intermediate characteristics: x=7 contains quenched-in Al crystals and follows the initial x=5 pattern of primary Al crystallization; however, it undergoes a glass transition prior to crystallization. x=8 crystallizes by nucleation and growth yet has crystallization products as x=7. Isothermal annealing prior to the first crystallization stage of x=8 yields α-Al and the unstable fcc structure. Results imply that Al–Y–Ni glasses obey the cluster model (Al–Y versus Al–Ni clusters), where cluster–cluster correlation establishes the degree of medium-range order and presence or absence of Al crystals.

Local Structure of Simplified Waste Glass: Complementarity of XAS and MD Calculations

ABSTRACTX-ray Absorption Spectroscopy (XAS) together with x-ray diffraction methods were used in inactive analogues of the French nuclear waste glass to analyze Pd-Te precipitates and the peculiarity of the sites occupied by some structural probe elements such as Mo, Zn, Zr and Si in the glassy part of the material. The influence of the precipitates on the structure of the glassy matrix around these structural probes was investigated by comparing noble metal bearing compositions together with noble metal free glasses.In noble metal bearing glasses, Pd is associated to Te as (Pd90Te10) precipitates. Pd K-edge EXAFS shows the presence of Te in the Pd coordination shell and is in accordance with a non-metallic character of the Pd-Te bond. Mo K-edge EXAFS shows that Mo occurs as molybdate groups non connected to the glassy matrix, which may be related to the occasional separation of Mo-rich phases in some compositions. Zr is located in an octahedron with d(Zr-O)=2.08Å 0.01Å whereas Zn is 4-fold coordinated with d(Zn-0)=-1.95 ű 0.01Å. Beyond this well defined first coordination shell, XAS detects some degree of medium range order which gives insight on the bonding of the site to the polymeric borosilicate network. The interatomic Zn-Si and Zr-Si EXAFS-determined distances agree with a ZnO4and Si04tetrahedra sharing corners geometry and Zr-Si octahedra sharing corner with Si04tetrahedra. A third shell of neighbors around Zr was evidenced in the noble metal bearing glasses and in a four oxide glass, indicating that Zr is an element useful for detecting subtle changes in the local structure of complex borosilicate glasses. Si K XANES shows modifications in the connection between Si04tetrahedra when noble metals are present. To obtain accurate and precise structural interpretations, a direct comparison between EXAFS data and Molecular Dynamic (M.D.) calculations on simplified nuclear glass comprising six oxides has been performed. MD calculations show that the distribution of the Zr-O distances is harmonic in the k-range accessible with EXAFS in glasses.

Structure of Sodium Gallotitanate Glass in Relation to Glass Formation

The structure of 40NaO0.5⋅20GaO1.5⋅40TiO2 glass has been investigated by X-ray radial distribution analysis. Nearly all the Ga3+ and Ti4+ ions are four coordinated, constructing highly cross-linked three-dimensional glass networks, and that the number of non-bridging oxygens is very small. The degree of medium-range order in this glass is much lower than in the Na2O⋅2TiO2 glass studied in a previous paper, possibly due to the almost complete absence of TiO6 octahedra in the former glass. The higher glass-forming ability of the Na2O-Ga2O3-TiO2 ternary system compared with the Na2O-TiO2 binary system is ascribed mainly to the higher fraction of four-coordinated Ti4+ ions in the former system. It is suggested that in oxide systems glass-forming ability increases with increasing fraction of four-coordinated Ti4+ ions.

Reflectivity spectra analysis of the Sb 40S 60 and Sb 28S 72 non-crystalline thin films

The measurements of the reflectivity and absorption coefficient in 0.5–5.9 eV energy range for Sb 40S 60 and Sb 28S 72 non-crystalline thin films have been carried out. A careful analysis of the absorption coefficient indicated the non-crystalline character of the studied samples, similarly to the diagnosis obtained from X-ray analysis. The reflectivity spectra, however, display some fine structure that suggests the existence of some degree of medium range order in the samples, in agreement with previous X-ray measurements. These results support the general theoretical discussion concerning the cluster size limit sufficient to reproduce the crystalline-like properties of electronic band structure.

Electronic and atomic structure of amorphous carbon.

The electronic structure of amorphous carbon and hydrogenated amorphous carbon (a-C:H) has been investigated through calculations on a number of model structures containing different configurations of ${\mathrm{sp}}^{2}$ and ${\mathrm{sp}}^{3}$ sites. We find that the most stable arrangement of ${\mathrm{sp}}^{2}$ sites is in compact clusters of fused sixfold rings, i.e., graphitic layers. The width of the optical gap is found to vary inversely with the ${\mathrm{sp}}^{2}$ cluster size, and the \ensuremath{\sim}0.5-eV optical gap of evaporated amorphous carbon is found to be consistent with a model of disordered graphitic layers of about 15 A\r{} in diameter, bounded by ${\mathrm{sp}}^{3}$ sites. It is argued that a-C forms such finite clusters in order to relieve strain. It is then shown that the 1.5--2.5-eV optical gap of a-C:H is unusually small and requires that both its valence and conduction band consist of \ensuremath{\pi} states on ${\mathrm{sp}}^{2}$ sites and that these sites must also be significantly clustered, such as in graphitic clusters containing four or more rings. In other words, the optical gap of both a-C and a-C:H depends on their degree of medium-range order, rather than just on their short-range order as is the case in most amorphous semiconductors. We have also studied the nature of states away from the gap in order to interpret the photoemission data and the carbon 1s core-level absorption spectra. The nature of defects and midgap states is discussed, and it is predicted that the defect density decreases with increasing band gap. Finally it is argued that the doping of a-C:H by group-III and -V elements proceeds via a substitution mechanism, as in a-Si:H, in spite of the coordination disorder present in a-C:H. Doping is also expected to be accompanied by an increase in gap states, as in a-Si:H.