Acoustic damping in Li2O–2B2O3 glass observed by inelastic X-ray and optical Brillouin scattering

The dynamic structure factor of lithium-diborate glass has been measured at several values of the momentum transfer Q using high resolution inelastic X-ray scattering. Much attention has been devoted to the low-Q-range, below the observed Ioffe–Regel crossover q_IR ≃ 2.1 nm⁻¹. We find that below q_IR, the linewidth of longitudinal acoustic waves increases with a high power of either Q, or of the frequency Ω, up to the crossover frequency Ω_IR ≃ 9 meV that nearly coincides with the center of the boson peak. This new finding strongly supports the view that resonance and hybridization of acoustic waves with a distribution of rather local low frequency modes forming the boson peak is responsible for the end of acoustic branches in strong covalent glasses. Further, we present high resolution Brillouin light-scattering data obtained at much lower frequencies on the same sample. These clearly rule out a simple Ω²-dependence of the acoustic damping over the entire frequency range.     

Low-frequency Raman scattering and small-angle X-ray scattering of glasses inclined to phase decomposition

Low-frequency (<1000 cm⁻¹) Raman scattering of lithium aluminosilicate (12Li₂O : 15Al₂O₃ : 73SiO₂ with 4 mol% TiO2) glasses with addition of titanium dioxide has been studied. With a heat treatment at temperatures 660°C, 700°C, 720°C and 820°C and for various times and sequences of temperature, our samples decompose into nanometer sized dispersed aluminotitanate particles. In Raman spectra of these glasses an evolution of a boson peak was observed. The width of the relatively broad boson band decreases as does the frequency of the band. From small-angle X-ray scattering data we conclude that the boson peak is connected with elastic vibrations of amorphous or crystalline regions of inhomogeneity with a dimension of ∼1.7 nm in initial glasses or larger depending on the heat treatment sequences.     

The effects of ZnCl2 and ErCl3 on the vibrational spectra and structure of tellurite glasses

A series of zinc tellurite glasses, containing up to 40 mol% ZnCl₂ and doped with 1–10 mol% ErCl₃, was prepared by melting and casting and their structure was analyzed by polarized Raman and variable incidence infrared reflection spectroscopies. Kramers–Kronig analysis of the infrared reflectivity led to the identification of the vibrational mode components. The Raman spectra are dominated by an intense, depolarized boson peak at ∼45 cm⁻¹ and a high frequency, polarized peak at ∼767 cm⁻¹. The introduction of ZnCl₂ and ErCl₃ modifiers led to a blue shift of the high frequency peak, while the intensity of the boson peak was found to increase continuously with the Er³⁺ content. It is shown that the erbium and zinc compounds both break TeOTe bonds, introducing non-bridging chlorine species, connected mostly to the zinc atoms.     

Structural studies of lead aluminate glasses

Lead aluminate melts were quenched rapidly with a roller quencher and bulk glasses were formed over a composition range from 72.5 to 80.0 mol% PbO. Pulsed neutron diffraction, ²⁷Al MAS NMR and Raman spectroscopy were used to study the structure of a series of these glasses. The results show that in the glasses the aluminium is four coordinated by oxygen across the compositional range, with a bond length of about 1.76 Å. The Pb–O peak in the neutron correlation function is asymmetric, and it can be modelled in terms of two bond lengths of ∼2.25 Å and ∼2.47 Å, with the majority of the coordination at the shorter distance. There is evidence that most or all of the lead ions are on asymmetric sites, coordinated by three oxygens in a trigonal pyramid arrangement. Both the neutron diffraction and Raman results indicate that the Pb–O bond lengths become shorter with increasing lead content.     

The boson peak in melt-formed and damage-formed glasses: A defect signature?

We describe the preparation and characterization of a glassy form of the moderately good glassformer PbGeO₃, by mechanical damage, and compare its properties with those of the normal melt-quenched glass and the crystal. The damage-formed glass exhibits a DSC thermogram strikingly similar to that of a hyperquenched glass, implying that it forms high on the energy landscape. The final glass transition endotherm occurs within 4 K (0.006T_g) of that of the melt-quenched glass, but crystallization occurs at a lower temperature, as if pre-nucleated. In particular, we have studied the low frequency vibrational dynamics of the alternatively prepared amorphous states in the boson peak region, and find the damage-formed glass boson peak to be almost identical in shape to, but more intense than, that of the normal melt-formed glass, as previously found for hyperquenched glasses. In view of the quite different preparation procedures, this similarity would seem to eliminate equilibrium liquid clusters as a source of the boson peak vibrations, but leaves plausible a connection to force constant fluctuations or to specific vitreous state defects.     

The structure of densified As2O3 glasses

In situ high energy X-ray diffraction and Raman experiments have been carried out to probe the structure changes of vitreous As₂O₃ under pressure. The first sharp diffraction peak reduces in intensity up to 10 GPa, indicating a breakdown of intermediate range order with pressure. All features in the Raman spectra broaden with increasing pressure up to 11.6 GPa. The mode at 378 cm⁻¹ associated with As₄O₆ molecule-like vibrations increases in intensity up to 6.2 GPa and decreases at higher pressures. In addition, X-ray and neutron structure factors have been measured for normal density and permanently densified As₂O₃ glasses recovered from 10 and 23 GPa. The results show the local AsO₃ pyramids and 3-membered rings essentially remain intact after compression. The increase in density is mainly associated with an inward shift of the third nearest neighbor peak in the X-ray radial distribution function, which indicates an increased packing of 3-membered AsO₃ rings.     

Time resolved thermal lens measurements of the thermo-optical properties of Nd2O3-doped low silica calcium aluminosilicate glasses down to 4.3 K

In this work, the thermal lens spectrometry was applied to measure the thermo-optical properties of Nd₂O₃-doped low silica calcium aluminosilicate glasses as a function of temperature, between 4.3 and 300 K. The thermal relaxation calorimetry was used to determine the specific heat, c_p. The results showed a decrease of the thermal diffusivity of about one order of magnitude from 4.3 K up to 300 K, with a T^?1 dependence in the interval between 20 and 70 K and a T^?0.35 between 4.3 and 20 K. The fluorescence quantum efficiencies of the doped samples were calculated down to 50 K, showing a variation of the order of 12% and 25% for the samples with 0.6 and 1.04 mol% of Nd₂O₃, respectively. In addition, the temperature corresponding to the maximum in c_p/T³, the so-called boson peak, was observed at about 17 K for the undoped sample and at lower temperatures for the doped glasses. In conclusion, our results showed the ability of the time resolved thermal lens to determine the thermo-optical properties of glasses at temperatures lower than 300 K, bringing new possibilities for experiments in a wide range of optical materials.     

Continuous silica fiber reinforced silica composites densified by polymer-derived silicon nitride: Mechanical properties and microstructures

Continuous silica glass fiber reinforced silica composites were prepared via silica sol suspension infiltration/sintering method followed by densification treatment by preceramic polymer infiltration/pyrolysis process, and the mechanical properties and microstructures of the composites were investigated. Pyrolyzed at 873 K in ammonia atmosphere, the preceramic polymer polyhydridomethylsilazane resulted in a low-carbon near-stoichiometric ceramic with an empirical formula of Si_1.0N_1.38C_0.01O_0.04H_0.78. After the densification treatment, the density of the composites increased a little from 1.64 g/cm³ to 1.79 g/cm³, and the flexural strength increased greatly from 69.4 MPa to 95.9 MPa. However, the densified composites exhibited a tendency to brittle failure due to the relatively strong interfacial bonding between the glass fibers and polymer-derived silicon nitride, and also the degradation of glass fibers during post-treatment.     

In situ crystallization of lithium disilicate glass: Effect of pressure on crystal growth rate

Crystallization of a Li₂O · 2SiO₂ (LS₂) glass subjected to a uniform hydrostatic pressure of 4.5 and 6 GPa was investigated up to a temperature of 750 °C. The density of the compressed glass is ∼2% greater at 4.5 GPa than 1 atm and, depending on the processing temperature, up to 10% greater at 6 GPa. Crystal growth rates investigated as a function of temperature and pressure show that lithium disilicate crystal growth is an order of magnitude slower at 4.5 GPa than 1 atm resulting in a shift of +45 °C (±10 °C) in the growth rate curve at high pressure compared to 1 atm conditions. At 6 GPa lithium disilicate crystallization is suppressed entirely, while a new high pressure lithium metasilicate crystallizes at temperatures 95 °C (±10 °C) higher than those reported for lithium disilicate crystallization at 1 atm. The observed decrease in crystal growth rate with increasing pressure for the lithium disilicate glass up to 750 °C is attributed to an increase in viscosity with pressure associated with fundamental changes in glass structure accommodating densification.     

X-ray diffraction measurements for liquid Ge–Si alloys using synchrotron radiation

Energy dispersive X-ray diffraction measurements have been carried out for liquid Ge_1−xSi_x alloys (x = 0.0, 0.3, 0.5, 1.0) using synchrotron radiation at SPring-8. We measured the X-ray diffraction spectra of liquid Ge and Si up to a high temperature range, (liquid Ge from 1270 to1870 K and liquid Si from 1680 to 2020 K), liquid Ge_0.7Si_0.3 at 1620 K, and liquid Ge_0.5Si_0.5 at 1540, 1590, 1670 and 1720 K. The total structure factors of the liquid Ge–Si alloys have a characteristic shoulder on the high-wave-vector side of the first peak. We deduced a pair distribution function from the Fourier transform of the observed structure factor, which was weakly dependent on the temperature. The nearest-neighbor coordination number of liquid Ge–Si alloys is close to that of pure liquid Ge and Si. The first peak of the pair distribution function moved to a shorter distance with increasing Si concentration. These results may indicate that the atomic radii of the Si and Ge atoms in the pure liquid are preserved in the liquid alloys.     

High-frequency excitations in glassy crystals

We present broadband dielectric loss spectra of the orientationally disordered (‘plastic’) crystal ortho-carborane, extending well into the infrared region and compare the results to the low-molecular weight glass former glycerol. Focusing on the high-frequency dynamics beyond 1 GHz, we find a loss minimum showing evidence for excess intensity, quite similar to the findings in canonical glass formers. In the THz region a strongly asymmetric peak shows up, which exhibits marked differences to the corresponding peaks found in glycerol and other glass-forming liquids. We discuss the relation of this feature to the boson peak detected, e.g., in light and neutron scattering experiments.     

Pressure dependence of the Boson peak in glassy As2S3 studied by Raman scattering

A detailed pressure dependence study of the low energy excitations of glassy As₂S₃ is reported over a wide pressure range, up to 10 GPa. The spectral features of Boson peak are analyzed as a function of pressure. Pressure effects on the Boson peak are manifested as an appreciable shift of its frequency to higher values, a suppression of its intensity, as well as a noticeable change of its asymmetry leading to a more symmetric shape at high pressures. The pressure-induced Boson peak frequency shift agrees very well with the predictions of the soft potential model over the whole pressure range studied. As regards the pressure dependence of the Boson peak intensity, the situation is more complicated. It is proposed that in order to reach proper conclusions the corresponding dependence of the Debye density of states must also be considered. Comparing the low energy modes of the crystalline counterpart of As₂S₃, as well as the experimental data concerning the pressure dependencies of the Boson peak frequency and intensity, a structural or glass-to-glass transition seems to occur at the pressure ∼4 GPa related to a change of local dimensionality of the glass structure. Finally, the pressure-induced shape changes of the Boson peak can be traced back to the very details of the excess (over the Debye contribution) vibrational density of states.     

Effect of uni-axial loading on the nanostructure of silica aerogels

Aerogels are unique materials offering a combination of remarkable properties that make them useful in a wide range of applications. However, aerogel materials can be difficult to work with because they are fragile. The intent of the work presented here was to study the relationship between axial loading and pore structure in aerogel material. Silica aerogel samples with a bulk density of 0.1 g/mL were compressed by uni-axial force loads from 1 to 5 kN which resulted in stress levels up to 23 MPa. The resulting change in the pore distribution was observed using nitrogen desorption analysis and scanning electron microscopy. Uncompressed aerogel samples exhibit peak pore volume at diameters of about 20 nm. As the aerogels are subjected to increased loading, the location of the peak volume moves to smaller diameters with a reduced volume of pores occurring above this diameter. The peak diameter, the average pore diameter and pore volume all decrease and scale with increasing maximum stress while the surface area of the aerogel samples remains unaffected at about 520 m²/g. When combined with data from the literature, the relation between maximum pore diameter and applied stress suggests a failure mechanism dominated by bending induced fracture.     

Liquid structure of Rb–Hg alloys studied by neutron diffraction

The structures of liquid Rb–Hg alloys were studied as a function of composition by neutron diffraction. In the intermediate Rb concentration range, the obtained structure factors show a small prepeak, which may be an evidence of the formation of Hg polyanion units in liquids. The Reverse Monte Carlo (RMC) analysis was applied to separate the total radial distribution function into the corresponding partial radial distribution functions. Up to 10 at.% Rb, no obvious changes are found for the first peak position of the partial radial distribution functions of the Hg–Hg pair and that of the Hg–Rb pair. The first peak position between the Hg–Rb pairs increases above 20 at.% Rb. In addition to the first peak, a subpeak between Hg–Hg pairs can be seen in the large distance. At 60 at.% Rb, the nearest neighbor distance between Hg atoms shows the closest value in the concentration range studied. These results indicate that with the progress of charge transfer the solvation structure in the dilute Rb concentration range changes into the structure containing polyanions composed of Hg species.     

Structural and dynamical properties of Fe78Si9B13 alloy during rapid quenching by first principles molecular dynamic simulation

Structural evolution of the Fe₇₈Si₉B₁₃ alloy during rapid quenching was investigated by ab initio molecular dynamics simulation. The second peak splitting has been perceived even at 1473 K in the partial pair correlation functions though not in the total pair correlation function. The (0, 3, 6, 0) polyhedra are abundant in the liquid state while the distorted (0, 3, 6, 0) polyhedra are the featured local structure around B atoms in the amorphous state. The diffusion coefficients of the three elements are evaluated to understand the dynamics of quenching. From 1173 to 873 K the three coefficients are coherent. We think this temperature range corresponds to the supercooled liquid region, and 873 K serves as the glass transition temperature.     

Density and structure of undercooled molten silicon using synchrotron radiation combined with an electromagnetic levitation technique

X-ray diffraction and density measurements have been simultaneously performed to investigate the atomic structure of molten silicon in the undercooled temperature region with the use of an electromagnetic levitation technique. The density was obtained from the levitated sample shape by a non-contact method based on an image analysis technique. From structural analysis of the undercooled molten silicon, the first nearest neighbor coordination number and interatomic distance were determined to be about 5 and 2.48 Å, respectively. There are no temperature dependence of both the first nearest neighbor coordination number and interatomic distance, but a small change observed on the side of the first peak with the structure factor in the temperature range of 1550–1900 K. From these results, we conclude that the short-range order based on tetrahedral bonds of the undercooled molten silicon did not change with the degree of undercooling, however, the medium-range order was changed by the degree of undercooling.     

NMR in soft materials: A study of DMPC/DHPC bicellar system

The DMPC/DHPC bicellar system at the molar ratio of 2.8:1 has been characterised by measurements of self-diffusion coefficient (using PFGSE and PFGSTE NMR sequences), differential scanning calorimetry (DSC) and small angle scattering of synchrotron radiation (SAXS). The DSC curve shows only one endothermic peak characterised by the peak temperature T_peak = 295.7 K and the onset temperature T_onset = 290.1 K. This peak can be assigned to the nematic to smectic phase transition. Below the phase transition temperature, NMR diffusion experiments indicate a two-exponential decay of the spin echo amplitude allowing two diffusion coefficients D₁ and D₂ to be extracted from the experimental data. The maximum size (D_max) of the bicelle determined from SAXS data using the pair distance distribution function p(r) is 11.2 nm and the bilayer thickness is 5 nm.     

Amorphous silicon deposited by xenon ion beam assisted deposition

In this work, we present some properties of amorphous silicon deposited by ion beam assisted deposition (IBAD). The films were prepared using a Kauffman ion gun using xenon (Xe) gas to sputter a silicon target. Another ion gun was adopted to simultaneously bombard the film with Xe during the growth of the film with xenon in the 0–300 eV energy range. Rutherford backscattering (RBS) was used to determine the concentration of the implanted Xe atoms and the density of the films. It was observed that the implantation of Xe do not affect much the stress of the films, which is compressive and about −0.6 GPa for all samples. The concentration of implantation Xe reach a maximum at energy of about 50 eV decreasing as the ion energy increases. The density of the films follows the concentration of Xe, suggesting that the densification of the film is not due to a compactation process supplied by the Xe bombardment of the films, but rather due to the incorporation of a heavy atom into the matrix.     

Fast relaxation processes in glasses as revealed by depolarized light scattering

Applying tandem-Fabry-Perot interferometry together with a double monochromator, depolarized light scattering spectra were measured in order to investigate the fast relaxation processes and vibrations in molecular, ionic and polymeric glasses in the 1–5000 GHz range covering temperatures from the glass transition temperature T_g down to some 10 K. In addition to the boson peak, the spectra reveal quasi-elastic contributions that we attribute to (i) a nearly constant loss in the frequency range below ≅10 GHz and (ii) a power-law contribution with positive exponent α at higher frequencies. In the majority of glasses the latter may be attributed to thermally activated dynamics in asymmetric double well potentials as previously found for the light scattering spectra in silica. Following the Gilroy–Phillips model the exponent α shows a master curve as a function of T/V₀ for the various glasses, where V₀ specifies the width of the exponential distribution of barriers g(V), i.e., g(V) ∝ exp(−V/V₀). The parameter V₀ is found to be ∼T_g/2 in most cases. The relative strength of the dynamics in asymmetric double well potentials and the nearly constant loss contribution is different in the glasses studied.     

Characterization of mixed phase silicon by Raman spectroscopy

We have examined the common methods for determination of the crystallinity of mixed phase silicon thin films from the TO–LO phonon band in Raman spectra. Spectra are decomposed into contributions of amorphous and crystalline phase and empirical formulas are used to obtain crystallinity either from the integral intensities (peak areas) or from magnitudes (peak maxima). Crystallinity values obtained from Raman spectra excited by Ar⁺ laser green line (514.5 nm) for a special sample with a profile of structure from amorphous to fully microcrystalline were compared with surface crystallinity obtained independently from atomic force microscopy (AFM). Analysis of the Raman collection depth in material composed of grains with absorption depth 1000 nm in an amorphous matrix (absorption depth 100 nm), was used to explain reasons for systematic difference between surface and Raman crystallinities. Recommendations are given for obtaining consistent results.