Physical aging effect on the boson peak and heterogeneous nanostructure of a silicate glass

The effect of physical aging on a silicate glass has been investigated by low-frequency Raman scattering. It was observed that the low-frequency side of the excess of Raman scattering, or boson peak, due to harmonic vibration modes decreases in intensity with the thermal annealing (aging) at a temperature lower than the glass transition temperature (T_g), after quenching (rejuvenation) from a temperature higher than T_g. Moreover, it was found that the lowering of the very low-frequency scattering mainly due to anharmonic modes becomes more pronounced when the aging temperature is decreased. These observations are interpreted in the frame of the energy landscape, and by considering the model of the glass heterogeneous cohesion at the nanometric scale.     

Influence of pressure on fast dynamics in polyisobutylene

Influence of pressure on fast dynamics and elastic properties in polyisobutylene is studied using Raman, Brillouin and neutron scattering spectroscopy. Analysis of the results shows that the boson peak frequency increases with pressure stronger than the longitudinal sound velocity measured by Brillouin scattering. Moreover, the boson peak intensity decreases under pressure stronger in Raman scattering than in neutron scattering suggesting a decrease in the light-to-vibrations coupling coefficient C(ν). The strong decrease of the microscopic peak intensity under pressure in Raman spectra supports this suggestion. We argue that variations in C(ν) might be related to amplitude of structural fluctuations. We speculate that change in disorder and/or overall density under pressure is the main cause for the observed variations.     

Temperature variation of effective geometry parameter, αg,refractive indices and the determination of orientational orders in three room temperature nematic liquid crystal mixtures

Orientational order parameter, S and effective geometry, α_g are calculated from the refractive index in three nematic room temperature liquid crystal mixtures, blOO3, blOO6, and blO38. The method due to Kuczynski et al. used to determine S from birefringence. The effective geometry parameter is α_g = n_o/n_e. (n_o and n_e are the ordinary and extraordinary refractive indices of the liquid crystal material.) The behavior of α_g as a function of temperature and order parameter has also been reported. The S value calculated from α_g is identically equal to the value obtained from Kuczynski et al method, i.e., from birefringence, δn. Finally, dn_e/dT and dn_o/dT are obtained and the crossover temperatures, T_CO the characteristic temperature, which is in fact the temperature of minimum in the ordinary index of refraction in three mixtures for different wavelengths are calculated from dn_o/dT.     

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.     

Replica field theory for anharmonic sound attenuation in glasses

A saddle-point treatment of interacting phonons in a disordered environment is developed. In contrast to crystalline solids, anharmonic attenuation of density fluctuations becomes important in the hydrodynamic regime, due to a broken momentum conservation. The variance of the shear modulus Δ² turns out to be the strength of the disorder enhanced phonon-phonon interaction. In the low-frequency regime (below the boson peak frequency) we obtain an Akhiezer-like sound attenuation law Γ ∝ Τω². Together with the usual Rayleigh scattering mechanism this yields a crossover of the Brillouin linewidth from a ω² to a ω⁴ regime. The crossover frequency ω_c is fully determined by the boson peak frequency and the temperature. For network glasses like SiO₂ at room temperature this crossover is predicted to be situated one order of magnitude below the boson peak frequency.     

Non-Debye excess heat capacity and boson peak of binary lithium borate glasses

The non-Debye excess heat capacities of binary lithium borate glasses with different Li₂O compositions of x = 8, 14 and 22 (mol%) are investigated to understand origin of the boson peak. The low-temperature heat capacities are measured between 2 and 50 K by a relaxation calorimeter. The experimental non-Debye heat capacities with x = 14 is successfully reproduced using the excess vibrational density of states measured by inelastic neutron scattering. This finding indicates that the non-Debye heat capacities of lithium borate glasses originate from the excess vibrational density of states measureable by inelastic neutron scattering. Moreover, it is demonstrated that all of the excess heat capacity spectra lie on a single master curve by the scaling using boson peak temperature and intensity.     

On the spontaneous electro-optic effect in triglycine sulphate crystals with L-threonine admixture

The temperature and spectral dependences of the coefficients R _i2* of the quadratic electro-optic effect (EOE) in triglycine sulphate crystals with 5% L-threonine admixture have been calculated using the temperature and spectral dependences of the refractive indices n _i . It is established that the coefficients R _i2* of the admixture-containing triglycine sulphate crystals exhibit weak spectral and temperature dependences. In the polar direction, R _i2* are substantially different from the coefficients of admixture-free triglycine sulphate crystals. This indicates the occurrence of a bias field, the predominant orientation of the dipole moment of the polar defect, and a decrease in the spontaneous polarization in admixture-containing crystals.     

Composition dependence of the refractive index and its photo-induced variation in the binary glass systems: Ge1−xSex and As1−xSex

The composition dependence of the refractive index and its photo-induced variation have been calculated for the Ge_1?xSe_x and As_1?xSe_x binary glass systems, using the random network model. It is found that the refractive index has a minimum in Ge_1?xSe_x at x = 0.8, while it shows a monotomic increase with increasing As content in As_1?xSe_x glasses. The refractive index of the Ge_1?xSe_x system decreases with illumination and the variation Δn of GeSe₂ is ?0.0316, while the refractive index of the As_1?xSe_x system increases with illumination and the variation of As₂Se₃ is 0.01. These results are in agreement with the experimental results reported by several workers.     

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.     

The defect diffusion model and the properties of glasses and liquids

Several previously unexplained relaxation phenomena are interpreted quantitatively in terms of the defect diffusion model (DDM). These include widely different pre-exponentials, exponents and consequently fragility. In addition, the DDM is used to give a qualitative explanation of many properties that glasses and liquids exhibit. This includes interpretations of the glass transition (T_g), the liquid–liquid transition (T_B), the crystalline melting temperature (T_m), and the different values of the characteristic temperature, T_C, that some materials exhibit for different types of physical measurements. Next, qualitative explanations of the origin of secondary relaxations such as the excess wing and the β relaxation are given. In addition, the boson peak is discussed in terms of the DDM. Finally, it is pointed out that in the DDM, universality is embodied in the defects i.e. free volume.     

Synthesis and physical properties of glasses in the Sb2O3-PbCl2-MoO3 system

Lead chloroantimonite glasses form stable binary glasses that may accommodate numerous oxides or halides as a third component. Molybdenum trioxide is a glass progenitor leading to molybdate glasses. Ternary glasses have been synthesized and studied in the Sb₂O₃-PbCl₂-MoO₃ system. Compositional limits of glass formation are reported and two series of glass samples have been prepared corresponding to the general formulas: (90 − x)Sb₂O₃-xPbCl₂-10MoO₃ and (90 − x)Sb₂O₃-xMoO₃-10PbCl₂. Glass transition temperature is close to 290 °C at high Sb₂O₃ content and decreases as antimony oxide is substituted by MoO₃ or PbCl₂. Position, width and intensity of crystallization peak suggest that devitrification rate is small in some composition ranges. The evolution of density, thermal expansion, refractive index and microhardness has been studied as a function of composition parameter x. Deviations from linearity are observed. They suggest structural changes in the case of the MoO₃/Sb₂O₃ substitution while it appears that molar volume increases linearly versus lead content in the other series of glasses. Refractive index is close to 2.04. Optical transmission ranges from 550 nm in the visible spectrum to 5.5 μm in the infrared. It is limited by extrinsic absorption bands arising from hydroxyls and silicon impurities. Young's, bulk and shear moduli have been measured for the two series of samples.     

Semiconducting TiO2SiO2 glasses (II): Optical properties

The refractive index, optical absorption coefficient α and the thermomodulated absorption dα/dT have been measured on 70% TiO₂?30% SiO₂ glasses with up to 8% Ti³⁺. The direct absorption data show intense ligand field absorption at photon energies hν = 1.9 and 3.0 eV, arising from Ti³⁺ in a distorted octahedral environment. In the bandgap region at 3.5 eV α obey αhν ～ (hν ? E_g)²; it is qualitatively different from the bandgap absorption in crystalline TiO₂. The da/dT spectra show peaks in the bandgap region and at 1.1 eV in the near IR. This last peak is attributed to absorption by small polarons, and its line shape is compared with theoretical predictions.     

Effect of cooling rate on enthalpy and volume relaxation of polystyrene

This paper presents the results of measurements of physical aging on polystyrene with a narrow molecular weight distribution (M_w/M_n = 1.03). The evolution of the aging process was followed by recording the relaxed enthalpy and the accompanying decrease in volume, using differential scanning calorimetry and mercury-in-glass dilatometry, respectively. The measurements were carried out after cooling the sample at constant rate to the aging temperature. The cooling rate varied between 0.0037 and 1 °C/min. The aging data were fitted with the KWW and the TNM functions. The differences in the time scales of enthalpy, h, and volume, v, relaxation were relatively small. From the h(v) graphs the derivative dh/dv was found to amount to about 1.8 GPa, a value significantly exceeding the predictions of the thermodynamic model containing an additional internal variable.     

Isobaric heating of polystyrene and evaluation of the Narayanaswamy parameter x from volume recovery data

Structure parameter x, defining the relative contributions of temperature and structure to the relaxation times, which is applied in Tool-Narayanaswamy-Moynihan (TNM) equation, was evaluated from volume heating isobars for polystyrene (PS) applying the peak shift method. The peak shifted with changing structural state of PS prior heating, which was reached by different period of aging time at 89 °C < T_g up to 2544 h. In this novelized procedure for volume data, originally tested only for enthalpic measurements, high sensitivity of peak temperature (position of inflection point of the heating isobar) was shown, together with high linearity of data. This allows to directly and sensitively calculate the shift and consequently structure parameter x. The value found in the research, x = 0.40 ± 0.02, is in good agreement with published results obtained by the peak-shift method from enthalpy data of PS.     

Origin of boson peak in glasses: Role of Ioffe–Regel crossover and atomic soft modes

A theoretical ‘complete soft-mode-dynamics’ model of the origin and properties of the boson peak accompanied by a high-frequency sound, observed in glasses of a certain type, is described. The origin is determined by interaction of non-acoustic vibrations with acoustic phonons and a Ioffe–Regel crossover for their inelastic scattering. The non-acoustic excitations are associated with vibrations of atomic soft-mode ‘defects’. Two types of boson peak can be predicted in agreement with experiments.     

The effect of cooling rate on the glass transition of an amorphous polymer

Cooling down from the equilibrium state at different rates reveals the dynamic behavior of glass forming materials. In particular, the dependence of the glass transition region on the cooling rate, q is commonly agreed to contain information regarding the activation energy of the relaxation time, τ. In this work experimental and theoretical aspects of such a relationship have been highlighted. Experimentally, the glass transition zone of amorphous polystyrene films has been investigated over two decades of cooling rate (0.5–50 K/min) by using refractive index measurements. The shift of the glass transition temperature and the broadening of the transition zone at increased cooling rate have been characterized. Theoretically, the cooling experiments have been simulated within the integral formulation of the Kovacs–Aklonis–Hutchinson–Ramos (KAHR) model using the Vogel temperature dependence for the relaxation time. The Frenkel–Kobeko–Reiner equation, τq = constant, provided the needed relationship between the experiments and the theory, enabling the evaluation of the relevant parameter of the kinetic model, i.e. the Vogel activation energy and the zero configurational entropy temperature, from the shift of the glass transition temperature with cooling rate.     

Refractive index behavior in phosphate glass from the view point of B3+ and Al3+ coordination

Higher refractive index and higher Abbe value were obtained simultaneously at B₂O₃/Al₂O₃ = 1 in the system of 65P₂O₅–xAl₂O₃–(20 ? x)B₂O₃–10CaO–5Li₂O, although the refractive index generally shows a positive correlation with the dispersion. We investigated the molar volume and the molar refraction by Lorentz–Lorenz’s formula and also studied the coordination number of ¹¹B and ²⁷Al by MAS NMR. By the replacement of Al₂O₃ with B₂O₃, the molar refraction constantly decreased, but the molar volume was minimized at B₂O₃/Al₂O₃ = 1. The refractive index behavior of the glasses was mainly determined by the molar volume in the system. The coordination number of B³⁺ was only IV and it did not change if the composition was changed. On the other hand the coordinations IV, V and VI were observed for Al³⁺. The ratio of Al³⁺(VI) was maximized at B₂O₃/Al₂O₃ = 1. It is considered that the higher coordination of Al³⁺ brings the improvement of the packing and it leads to high refractive index.     

Physical aging effects in selenide glasses accelerated by highly energetic γ-irradiation

Effect of ⁶⁰Co γ-irradiation on As–Se glasses stored for 20 years and subjected to saturated natural physical aging is studied using differential scanning calorimetry. It is shown that γ-irradiation activates further physical aging, which leads to an increase in glass transition temperature and endothermic peak area near glass transition region for As_xSe_100−x samples with x < 30. The observed changes are associated with additional structural relaxation of radiation-modified glass network.     

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.     

Distribution coefficient of boron in Si crystal ingots grown in cusp-magnetic Czochralski process

Silicon single crystals are grown by the Czochralski method with various growing conditions. Effective segregation coefficient of boron is found to depend on the magnetic field in cusp-magnetic Cz method. Effects of zero-Gauss plane (ZGP), ZGP shape and magnetic intensity (MI) on the dopant concentration and its distribution in the crystal are experimentally investigated. The shape of ZGP is not only flat but also parabolic due to the magnetic ratio (MR), which is the ratio of the lower to upper electric-current densities in the configurations of the cusp-magnetic field. Equilibrium distribution coefficient of boron calculated by BPS model is 0.698. With the crystal rotation (w) of 16 rpm and the crucible rotation of ?0.5 rpm, the effective distribution coefficient (k_e) is 0.728 in zero magnetic intensity and increases up to 0.8093 in the parabolic ZGP shape. Although the magnetic strength near the crystal–melt interface decreases with increasing MR, it increases in the bulk melt, and hence k_e increases. Flow stability in the bulk melt influences k_e. At the magnetic field and growing conditions, k_e increases with increasing initial charge size of the silicon melt. There is no significant influence of ZGP on the radial distribution of the boron concentration. Simulation results of melt flow in the presence of a parabolic ZGP are outlined, and the segregation results in the experiments are compared with published experimental data.