Showing papers in "Philosophical Magazine Part B in 1995"
TL;DR: In this paper, two approaches have been adopted in attempting to determine the strength of the one-phonon infrared absorption by the B nitrogen aggregate in diamond, which is complicated because diamonds in which the B aggregate is the only point defect also contain extended defects that may or may not involve nitrogen.
Abstract: Two approaches have been adopted in attempting to determine the strength of the one-phonon infrared absorption by the B nitrogen aggregate in diamond. On the one hand, we have heat treated purely type IaA specimens with known infrared absorption strengths (and thus also known nitrogen concentrations) to bring about partial aggregation of the A centres to B centres, monitoring changes in the strengths of the A, and resultant B, one-phonon components by decomposition of the ensuing infrared spectra. Secondly, we have made direct chemical assays of nitrogen in diamonds that show dominant B absorption features. This approach is complicated because diamonds in which the B aggregate is the only point defect also contain extended defects that may or may not involve nitrogen. These defects may or may not, in turn, contribute to absorption in the one-phonon region. Nevertheless, the nitrogen concentrations have been measured for two distinct groups of diamonds that both uncritically might at first glance ...
214 citations
TL;DR: The structure of a microporous titanosilicate, ETS-10, has been solved using a combination of electron microscopy, X-ray and electron diffraction, solid-state NMR and distance-least squares analysis as discussed by the authors.
Abstract: The structure of a microporous titanosilicate, ETS-10, has been solved using a combination of electron microscopy, X-ray and electron diffraction, solid-state NMR and distance-least squares analysis. We describe in detail how these tools have been applied to the structural solution. ETS-10 is a highly disordered, yet crystalline, wide-pore microporous framework material containing octahedrally coordinated titanium and tetrahedrally coordinated silicon. The disordered material can be described with respect to two ordered polymorphs: polymorph A, space group P41 or P43 with a = b = 14·8 A and α = β = γ = 90° polymorph B, space group C2/c with a = b = 21·00 A, c= 14·51 A and α = γ = 90°, β= 111·12°. Both polymorphs have'a three-dimensional 12-ring pore system and polymorph A has a spiral channel.
190 citations
TL;DR: In this article, the structure of copper films grown on (0001) sapphire by molecular beam epitaxy at 200°C and 600°C was studied by in situ reflection high-energy electron diffraction (RHEED) and transmission electron microscopy (TEM).
Abstract: The structure of copper films grown on (0001) sapphire by molecular beam epitaxy at 200°C and 600°C was studied by in situ reflection high-energy electron diffraction (RHEED) and transmission electron microscopy (TEM). Both RHEED and TEM confirm that growth is epitaxial at 200°C, but {111}Cu textured at 600°C. However, in both instances growth occurs with the copper (111) plane parallel to the interface. In the case of the 200°C films the close-packed directions of copper lie parallel to the close-packed directions of sapphire in the interfacial plane; these films contain low angle grain boundaries with the interface normal to the axis of rotation. High resolution and atomic resolution electron microscopy of the interface viewed in cross-section suggest that the atomic structure of the copper/sapphire interface is incoherent. The textured growth at 600°C and near single crystal growth at 200°C are explained in terms of entropy contributions to the free energy of the Cu/Al2O3 interface.
100 citations
TL;DR: In this paper, it is proposed that the compression in such fronts causes local metallization when either the density, or the bending of covalent bonds, reaches a critical level which can be estimated from simple models.
Abstract: Detonation fronts propagate too fast for the chemical reactions that drive them to be thermally activated. It is proposed that the compression in such fronts causes local metallization when either the density, or the bending of covalent bonds, reaches a critical level which can be estimated from simple models. In the case of the density, this can be estimated from the Herzfeld-Mott model; and in the case of bond-bending, from parabolic variation of the LUMO and HOMO molecular energy levels. Adiabatic expansion of the delocalized electrons provides the high pressure needed to accelerate the ions in the front. Specific examples that are discussed are lead azide, ammonium ions, and nitrate ions. In addition it is argued that this approach is consistent with: observed critical compressions; detonation velocities; the resonance energies of chemical bonds, polarizabilities and the stabilities of energetic materials. Metallization explains why hyper-fast reactions can occur athermally, and propagate sup...
94 citations
TL;DR: In this paper, the authors discuss the long-time out-of-equilibrium dynamics of mean-field spin-glass models and present a systematic solution for the long time complexity of the model.
Abstract: We discuss the systematic solution for the long-time out-of-equilibrium dynamics of mean-field spin-glass models. We describe the physical assumptions and work out a simple example.
90 citations
TL;DR: In this article, the authors used the Gaussian disorder model of charge transport in situations where dipolar contributions to the disorder are important, and showed that the central portion of the DOS is well approximated by a Gaussian distribution in many cases, the low and high-energy tails generally deviate from this distribution.
Abstract: One contribution to the distribution of site energies (DOS) in a random medium with localized charge-transport sites is the electrostatic potential of randomly placed and randomly oriented dipolar species nearby. This contribution has recently been modelled using a lattice on which a certain fraction (f) of the points is occupied by dipoles with random orientations. Simulations indicated that the DOS is close to a Gaussian distribution as long as f is not too small. This fact was taken to justify the application of the Gaussian disorder model (GDM) of charge transport in situations where dipolar contributions to the disorder are important. Analytical results on this model are presented here, including an expression for the r.m.s. width of the DOS that differs considerably from the previous result. While the central portion of the DOS is well approximated by a Gaussian distribution in many cases, the low- and high-energy tails generally deviate from this distribution. A simple principle is introdu...
78 citations
TL;DR: The oertex model, which consists of a set of equations for the motion of vertices obtained from the full curvature-driven equation of motion of cell boundaries (faces), has the potentiality for efficient improvements of indefinite degree by introducing arbi- trary numbers of virtual vertices.
Abstract: The time evolution of three-dimensional cellular patterns is discussed on the basis of the oertex model which consists of a set of equations for the motion of vertices obtained from the full curvature-driven equation of motion of cell boundaries (faces) by a reduction in the degrees of freedom. Some difficulty in the earlier vertex models, originating from the fact that faces are non-coplanar, is now side-stepped by introducing virtual vertices as supplementary degrees of freedom. Equations of motion are then readily derived assuming that the whole process is purely dissipative. By solving these equations numerically, scaling properties concerning cellular pattern growth are obtained. In addition, geometrical features such as the three-dimensional versions of the Aboav-Weaire law and the Lewis law are verified with fair accuracy. We emphasize that our vertex model has the potentiality for efficient improvements of indefinite degree by introducing arbi- trary numbers of virtual vertices.
69 citations
TL;DR: In this paper, the conductivity spectra of three supercooled glass-forming molten salts, LiCl·7H2O, Ca(NO3)2·4H 2O and 3KNO3·2Ca(NO 3)2, were taken at various temperatures above T G. The data cover a wide frequency range from a few MHz to the far infrared.
Abstract: The conductivity spectra of three supercooled glass-forming molten salts, LiCl·7H2O, Ca(NO3)2·4H2O and 3KNO3·2Ca(NO3)2, have been taken at various temperatures above T G. The data cover a wide frequency range from a few MHz to the far infrared. The d.c. conductivities exhibit Vogel-Fulcher-Tammann temperature dependences, but with increasing frequency a change to Arrhenius is observed. The crossover occurs along with the onset of the power-law frequency dependence of the conductivity. Increasing the frequency further up to the far infrared, we observe a second crossover, now into a v 2 frequency dependence due to the excitation of vibrational motion. The vibrational contributions can be subtracted out of the spectra. This has been carefully done for 3KNO3·2Ca(NO3)2. The resulting non-vibrational conductivity attains a thermally activated ‘high-frequency’ plateau at about 1 THz, as in a solid electrolyte. On a short time scale, the dynamics of glass-forming fragile melts is, indeed, found to be ve...
64 citations
TL;DR: In this article, the authors have calculated elastic constants and yield stress of disordered foams as functions of the gas fraction Π@, greatly improving and extending the previous results of Bolton and Weaire.
Abstract: The computer simulation of two-dimensional foams allows a comprehensive description of the mechanical properties of these systems. In particular, we have calculated elastic constants and yield stress of disordered foams as functions of the gas fraction Π@, greatly improving and extending the previous results of Bolton and Weaire. A comparison with random elastic networks is appropriate in the limit of wet froth, close to the rigidity-loss transition at Πc ≊ 0.84. As the transition is approached, small increments of strain trigger avalanches of topological changes.
56 citations
TL;DR: In this paper, amorphous hydrogenated carbon films (a-C: H) were deposited in a capacitively coupled r.f.c. discharge reactor and the precursor gas and the amount of doping and inert gas added during deposition were changed in a systematic manner.
Abstract: Amorphous hydrogenated carbon films (a-C: H) were deposited in a capacitively coupled r.f. discharge reactor. The precursor gas and the amount of doping and inert gas added during deposition were changed in a systematic manner. The d.c. conductivity was measured in the temperature range 80 to 500 K. The conductivity behaviour of the investigated samples, as well as to our knowledge that of all samples investigated by other authors, can be described in a uniform way by a characteristic temperature Θ (≊ 1500 K). At this temperature the extrapolated conductivity of all samples should be the same. This behaviour will be discussed in the context of different conductivity models as, for example the variable-range hopping, transport in band-tail states and hopping between graphite-like clusters, a statistical shift of the Fermi level, and the multiphonon tunnelling model. It is shown that only the latter model can, under certain conditions, satisfactorily explain the common behaviour of the samples inve...
52 citations
TL;DR: In this article, the authors present an explanation for thermal quenching in undoped and weakly doped hydrogenated amorphous silicon (a-Si:H) with computer simulations employing the theory of Simmons and Taylor.
Abstract: Thermal quenching (TQ) of the photoconductivity [sgrave]p is the decrease in [sgrave]p with increasing temperature. We present an explanation for TQ of [sgrave]p usually observed above 100 K in undoped and weakly doped hydrogenated amorphous silicon (a-Si:H). With computer simulations employing the theory of Simmons and Taylor, we show that TQ is caused by the natural density of gap states of a-Si: H. The onset of thermal quenching occurs at the temperature TTQ where the trapped hole density in the valence band tail has decreased to twice the density ND of dangling bonds. We elucidate the experimental observation that TTQ shifts to lower temperatures as the Fermi level shifts toward the valence band or as ND is increased and explain the reported superlinear dependence of the inverse photoconductivity [sgrave]p −1 on ND . We test and discuss the validity of the Simmons-Taylor theory by comparing the simulated and experimental temperature dependences of the Rose exponent γ, which relates the photoc...
TL;DR: In this article, the dispersion of the index of refraction of silver-doped As33S67 thin films in the visible and near-infrared regions was analyzed on the basis of a single effectiveoscillator model proposed by Wemple and DiDomenico.
Abstract: We report data on the dispersion of the index of refraction of silver-doped As33S67 thin films in the visible and near-infrared regions. The measured data is analysed on the basis of a single-effective-oscillator model proposed by Wemple and DiDomenico. The silver content dependence of the oscillator energy Eo seems to vary in proportion to optical gap Eg opt of the sample, whereas the dispersion energy or oscillator strength Ed increases with increasing silver content. This latter fact indicates that the effective coordination of the cation increases as silver is incorporated into the chalcogenide host matrix.
TL;DR: In this paper, the effect of temperature and field cyclings on the out-of-phase susceptibility of an insulating spin glass was investigated and an analysis of the field effect on the escape times of the states was proposed.
Abstract: We present in this paper some new experimental results on the effect of temperature and field cyclings on the out-of-phase susceptibility of an insulating spin glass. The temperature-cycling experiments are consistent with a previously derived hierarchical scenario in which the probed metastable states continuously split into new substates as the temperature is lowered. The new results, however, suggest that the states of lowest free energy are not the same at different temperatures, supporting the idea of chaotic behaviour. Changing the magnetic field makes the system flow from one hierarchically organized set of states to another set of states with a different magnetization. An analysis of the field effect on the escape times of the states is proposed. It accounts for the observed behaviour of the a.c. susceptibility. It, moreover, predicts a scaling form for the time decay of the thermoremanent magnetization in terms of the probing field, which is nicely satisfied by the available experimental...
TL;DR: In this paper, a closed-form expression for the recombination function associated with a single type of recombination centre that can exist in three charge states is applied to the problem of bulk collection in a p-i-n solar cell of hydrogenated amorphous silicon.
Abstract: A closed-form expression for the recombination function associated with a single type of recombination centre that can exist in three charge states is applied to the problem of bulk collection in a p—i—n solar cell of hydrogenated amorphous silicon. It is shown that a linear approximation of the corresponding recombination function can be applied to determine bulk collection, if the p—i—n diode is under reverse bias voltage, a condition that allows the assumption of a constant electric field. The effects of two kinds of recombination function on the bulk collection are compared: first, the function found for a recombination centre that can exist in two charge states (i.e. the classical Shockley—Read—Hall expression) and second, the function found for a recombination centre that can exist in three charge states (as proposed by the present authors for dangling-bond recombination). The comparison demonstrates a significant difference between the two approaches with respect to the parameters that lim...
TL;DR: The double flash technique of F. K. Dolezalek and W. E. Spear was used to determine the recombination mechanism of drifting electrons and holes in stabilized a-Se as discussed by the authors.
Abstract: The double flash technique of F. K. Dolezalek and W. E. Spear (1975, J. Phys. Chem. Solids, 36, 819) for the measurement of recombination coefficients was used to determine the recombination mechanism of drifting electrons and holes in stabilized a-Se (Cl-doped a-Se: 0.3% As). The experiments show that the recombination process follows the Langevin mechanism.
TL;DR: In this paper, in situ transmission electron microscopy (TEM) and differential scanning calorimetry was used to study the crystallization behavior of amorphous Si(a-Si) in trilayer and multilayer systems.
Abstract: We report on the crystallization behaviour of amorphous Si(a-Si) in a-Si/Ag/a-Si trilayer and a-Si/Ag multilayer systems using in situ transmission electron microscopy (TEM) and differential scanning calorimetry. The crystallization temperature of a-Si in these Si/Ag layered systems is 385–415°C, about 200°C lower than that found in pure a-Si, whereas the heat of crystallization is 12 ± 1 kJ mol−1, in good agreement with published values. During the reaction, in situ cross-section TEM showed that Ag grains border the a-Si matrix and the crystallized Si(c-Si) grains and that the Ag grains migrate toward the a-Si region, leaving the c-Si phase behind. In situ atomic resolution TEM also revealed that the lattice points of the migrating Ag grains are stationary during this process and that the interface between the c-Si and Ag phases advances by a ledge mechanism. These observations indicated that the Si atoms diffuse through the Ag grains and precipitate onto the c-Si phase, whereas the migration of...
TL;DR: In this article, the electron emission and luminescence associated with the plastic deformation of ionic crystals have been investigated and it was concluded that these phenomena can be attributed to thermally activated intersections between edge dislocations lying on oblique slip planes.
Abstract: The electron emission and luminescence associated with the plastic deformation of ionic crystals have been studied. It is concluded that these phenomena can be attributed to thermally activated intersections between edge dislocations lying on oblique slip planes. Dislocation intersections of this kind are shown to result in colour centre generation; recombination of the centres gives rise to the deformation-induced electron emission and luminescence.
TL;DR: In this article, a numerical simulation method has been developed by which shear stress is applied to a two-dimensional atomic configuration of Lennard-Jones particles, which is based on a microconvergence relaxation procedure.
Abstract: To analyse the plasticity of quasicrystals a numerical simulation method has been developed by which shear stress is applied to a two-dimensional atomic configuration of Lennard-Jones particles. The method is based on a microconvergence relaxation procedure. We present results of simulations on a simple binary tiling which is obtained by decoration of the Tubingen triangle tiling. Slip through dislocations is observed. In contrast with periodic crystals the motion of dislocations through a quasicrystal does not leave the configuration undisturbed but creates a wall of phasons.
TL;DR: In this paper, the room temperature elastic properties of hexagonal C40 NbSi2 single crystal have been determined using resonant ultrasound spectroscopy (RUS), and five independent elastic stiffness constants cij were obtained: c11 = 380·2, c33 = 468·0, c44 = 145·3, c12 = 75·9 and c13 = 88·3 GPa.
Abstract: Room-temperature elastic properties of hexagonal C40 NbSi2 single crystal have been determined using resonant ultrasound spectroscopy (RUS). All five independent elastic stiffness constants cij were obtained: c11 = 380·2, c33 = 468·0, c44 = 145·3, c12 = 75·9 and c13 = 88·3 GPa. The shear modulus c66 in the (0001) plane, 152·2 GPa, is low relative to those of MoSi2 and WSi2. The isotropic elastic constants for polycrystalline material were also calculated. The results show that the bulk modulus K. shear modulus G and Young's modulus E are much higher than those of the constituent elements. The room temperature Poisson's ratio of the compound is 0·18, which is much smaller than those of the constituent elements and most observed materials. The elastic properties of C40 NbSi2 and C11b MoSi2 and WSi2 are compared and the possible influence on the mechanical behaviour is discussed.
TL;DR: Theoretical models relative to Raman scattering in disordered solids are discussed and compared with the available experimental evidence as mentioned in this paper, and experimental data relative to the Boson peak integrated intensity and computer simulations on three-dimensional site percolators, allow us to infer that the relevant mechanism that produces the peak is disorder-induced light scattering from acoustic modes of a disordered structure.
Abstract: Theoretical models relative to Raman scattering in disordered solids are discussed and compared with the available experimental evidence. In particular, experimental data relative to the Boson peak integrated intensity and computer simulations on three-dimensional site percolators, allow us to infer that the relevant mechanism that produces the peak is disorder-induced light scattering from acoustic modes of a disordered structure.
TL;DR: In this paper, the formation of barrier-type anodic films on sputter-deposited amorphous Al-30W (at.%) alloy has been studied for the first time to elucidate ionic transport mechanisms across the film and associated interfaces.
Abstract: The formation of barrier-type anodic films on sputter-deposited amorphous Al-30W (at.%) alloy has been studied for the first time to elucidate ionic transport mechanisms across the film and associated interfaces. The film, grown with high Faradaic efficiency at 50 Am−2 to 150 V in aqueous 0.01 m ammonium pentaborate electrolyte at 293 K, comprises two main layers of relatively uniform composition and with flat, parallel and sharp interfaces: An outer layer, representing about 15% of the film thickness and composed of tungsten-free anodic alumina, and an inner layer containing both aluminium and tungsten in an amorphous structure based on groupings of Al2O3 and WO3. The two layers form as a consequence of the slower migration of tungsten species in the film compared with that of A13 + ions. The inner layer has, as expected on compositional grounds, a lower ionic resistivity than the outer layer; yet, contrary to predictions, film formation proceeds in a uniform manner, i.e. without current channel...
TL;DR: In this paper, a metal-insulator transition at 230 K was discovered and the enthalpy and entropy of the transition were 3·50 kJ mol−1 and 15·6 J K−1 mol− 1 respectively.
Abstract: A new thiospinel CuIr2S4 has been synthesized. We have discovered a metal-insulator transition at 230 K. The physical properties of this compound have been investigated with special emphasis on heat capacity measurements in the range 13·8 K < T < 300K using an adiabatic calorimeter. The metal-insulator transition is accompanied by a structural phase transition from tetragonal symmetry (at the lower temperature) to cubic symmetry (at the higher temperature). The phase transition has been observed to be first order. A sharp heat capacity anomaly is exhibited at 230·8 K. The enthalpy and entropy of the transition are 3·50 kJ mol−1 and 15·6 J K−1 mol−1 respectively. A hysteresis effect at the phase transition was detected clearly in the heat capacity measurements. The metallic phase of CuIr2S4 shows Pauli paramagnetism which indicates a rather high density of states at the Fermi level, D(∊F) = 0·67 states/eV atom. The insulating phase of CuIr2S4 reveals semiconductive behaviour with an activation ene...
TL;DR: Amorphous silicon-carbon thin films have been deposited by a newly designed ultra-high vacuum plasma enhanced chemical vapour deposition system starting from SiH4+CH4 gas mixtures at different Y(CH4) = CH4/(SiH4 + CH4) ratios with deposition conditions optimized to grow high quality material, having energy gaps in the range 1·9−3·5 eV as discussed by the authors.
Abstract: Amorphous silicon-carbon thin films have been deposited by a newly designed ultra-high vacuum plasma enhanced chemical vapour deposition system starting from SiH4 + CH4 gas mixtures at different Y(CH4) = CH4/(SiH4 + CH4) ratios with deposition conditions optimized to grow high-quality material, having energy gaps in the range 1·9–3·5 eV. The effect of the addition of hydrogen to the plasma with Z(H2)=H2/(SiH4 + CH4 + H2) ranging from 0 to 0·95 has been investigated. A detailed analysis of elemental composition, obtained by Rutherford backscattering and elastic recoil detection analysis, and of structural properties, obtained by infrared and Raman spectroscopies, has been carried out. The results have been correlated to the plasma conditions, the disorder and the optoelectronic properties of the a-Si1−x C x :H films.
TL;DR: In this article, in situ transmission electron microscopy (TEM) and differential scanning calorimetry was used to investigate the crystallization behavior of amorphous Ge (a-Ge) in trilayer and multilayer systems.
Abstract: The crystallization behaviour of amorphous Ge (a-Ge) in a-Ge/Ag/a-Ge trilayer and a-Ge/Ag multilayer systems has been investigated using in situ transmission electron microscopy (TEM) and differential scanning calorimetry. The crystallization temperature of a-Ge in these systems is 240–270°C, about 250°C lower than that found in pure a-Ge. During the reaction, in situ cross-section TEM showed that Ag grains separate the a-Ge matrix and the crystallized Ge (c-Ge) grains, and that the Ag grains migrate towards the a-Ge region, leaving the c-Ge phase behind. Our in situ atomic resolution TEM also revealed that the lattice points of the migrating Ag grains are stationary. These observations indicate that the Ge atoms diffuse through the Ag grains for the growth of the c-Ge phase, whereas the migration of the Ag grains is caused by the reverse self-diffusion of the Ag atoms. The mechanism is essentially identical to that in other metal-mediated crystallization reactions (e.g. Al-Si, Ag-Si). The observ...
TL;DR: In this paper, a high-pressure X-ray diffraction study has been performed on a perfect icosahedral Al62Cu25·5Fe12·5 (i-(Al-Cu-Fe)) phase at room temperature up to 33·5 GPa using a diamond-anvil cell to determine the equation of state.
Abstract: A high-pressure X-ray diffraction study has been performed on a perfect icosahedral Al62Cu25·5Fe12·5 (i-(Al—Cu—Fe)) phase at room temperature up to 33·5 GPa using a diamond-anvil cell to determine the equation of state. X-ray diffraction was undertaken in energy-dispersive mode on a synchrotron beam line. The remarkable result is that i-(Al—Cu—Fe) remains icosahedral over this pressure range. The relative volume change V/V o as a function of pressure is least squares fitted to a Murnaghan-type equation of state so that the zero-pressure bulk modulus B 0 is 155 ± 10GPa, maintaining its pressure derivative B r 0 = 2. Two approximant phases with very similar chemical compositions but with different structures, a rhombohedral phase (Al62·B Cu26Fe11·2) and a pentagonal phase (Al64 Cu24Fe12), have been studied under the same conditions. No drastic modification of the diffraction spectra appears up to 33 GPa and both phases remain unchanged after release of the pressure. The relative volume change V/V 0...
TL;DR: In this paper, the participation of atomic hydrogen in the light-induced metastability mechanism of hydrogenated amorphous silicon (a-Si:H) has been observed by low-temperature infrared transmission spec...
Abstract: Evidence for participation of atomic hydrogen in the light-induced metastability mechanism of hydrogenated amorphous silicon (a-Si:H) has been observed by low-temperature infrared transmission spec...
TL;DR: In this article, the relative probabilities of Au morphologies were compared with a probability model to understand the role of different parameters that affect the transformations, and the experimental data and probability model show good agreement and reveal several interesting trends for the first time.
Abstract: Morphological transformations of Au particles supported on SiO were observed by real-time high-resolution electron microscopy, and the data were analysed quantitatively to extract the relative probabilities of the different morphologies and compared with a probability model to understand the role of the different parameters that affect the transformations. The experimental data and probability model show good agreement and reveal several interesting trends for the first time. The single crystal population dominates over the icosahedral multiply twinned particles (Ic) and the decahedral multiply twinned particles (Dc) over the observed size regime 2–8nm. The Ic shows a maximum in relative probability of occurrence at smaller sizes, whereas the Dc probability increases with size, leading to a transition region centred at 6·5 nm particle diameter at which Dc and Ic show almost equal relative probabilities. The activation energies of the morphologies are also affected by the surface stress and anisot...
TL;DR: In this paper, the structure of the shells is compatible with the model of spherical fullerene molecules based on so-called Goldberg polyhedra, and the shells within one cluster are oriented differently.
Abstract: Concentric-shell carbon clusters are produced by electron irradiation of carbon soot in a transmission electron microscope and investigated with the highest resolution available to date. Simulations of the electron microscopy images based on different models are performed and compared with experimental images. It is found that the structure of the shells is compatible with the model of spherical fullerene molecules based on so-called Goldberg polyhedra. In general, the shells within one cluster are oriented differently.
TL;DR: In this paper, a large molecular dynamics model of amorphous silicon is used to show how the first sharp diffraction peak is a direct consequence of fluctuations in the pair distribution function which persist to above 30 A.
Abstract: A large molecular dynamics model of amorphous silicon is used to show how the first sharp diffraction peak is a direct consequence of fluctuations in the pair distribution function which persist to above 30 A. The fluctuations can be explained in terms of the sum of neighbour-specific pair distribution functions, which reveal enhanced intensity of alternate (even-n) nth-neighbour peaks, or analogously in terms of the chemical ordering of interatomic voids about each atom.
TL;DR: The Raman spectra of metaphosphate glasses, which contain very high concentrations of either Eu3+ or Gd3+ ions, have been measured over a wide range of temperatures (10 −300 K) as discussed by the authors, indicating that these glasses share the common structural feature of a skeleton built up from (PO4)2 tetrahedral units.
Abstract: The Raman spectra of metaphosphate glasses, which contain very high concentrations of either Eu3+ or Gd3+ ions, have been measured over a wide range of temperatures (10–300 K). The high-frequency part of the spectrum (above 200 cm−1) is similar to that found for other phosphate glasses, indicating that these glasses share the common structural feature of a skeleton built up from (PO4)2 tetrahedral units. The low-frequency region is determined by two spectral contributions which are characteristic of the disordered topology of vitreous materials: the light-scattering excess and the ‘boson peak’. The former contribution, clearly observed for frequencies below 10cm−1, becomes larger with increasing temperature, while the latter does not change appreciably as the temperature is increased to room temperature. Interpretation of the light scattering excess and the boson peak in terms of the predictions of the soft potential model can account for their frequency and temperature dependences.