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ABSTRACT: We present a detailed investigation of the vibrational dynamics of glassy sulfur (g-S). The large frequency range spanned in this study has allowed us to carefully scrutinize the elastic properties of g-S and to analyze their relation to various features of both the glassy and the liquid state. In particular, the acoustic properties of g-S present a quasi-harmonic behavior in the THz frequency range, while at lower frequency, in the GHz range, they are affected by a strong anharmonic contribution. Moreover, the high frequency (THz) dynamics of g-S does not present signatures of the elastic anomalies recently observed in a number of glasses. Despite this apparent contradiction, we show that this finding is not in disagreement with the previous ones. Finally, by considering the correct long wavelength limit of the density fluctuations in the glassy state, we estimate the continuum limit of the nonergodicity factor and we investigate recently proposed relations between the fast dynamics of glasses and the slow dynamics of the corresponding viscous melts.
The Journal of Physical Chemistry B 12/2011; 115(48):14052-63. · 3.70 Impact Factor
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ABSTRACT: The polymer chain conformation under confinement and the morphology of the nanohybrid materials are investigated in hydrophilic polymer/layered silicate nanocomposites. A series of poly(ethylene oxide)/sodium montmorillonite hybrids were synthesized utilizing melt intercalation with compositions covering the complete range from pure polymer to pure clay. Intercalated nanocomposites with mono- and bilayers of PEO chains are obtained in all cases. The intercalated chains as well as the ones adsorbed on the outer surface of the clay particles remain purely amorphous; their conformation, however, exhibits different characteristics from those of the amorphous neat polymer melt. It is only for compositions where a large amount of excess polymer exists outside the completely full galleries that the polymer crystallinity is recovered.
11/2011;
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ABSTRACT: Nanoindentation is used to study the mechanical properties at the nanoscale of the ternary Agx(As0.33S0.67)100−x glassy system for 0 ≤ x ≤ 25. Direct evidence for phase separation of a Ag-poor and a Ag-rich phase in the composition range 2 ≤ x ≤ 20 is provided. The volume of the Ag-rich phase increases with x and percolates at x ≈ 7. The mechanical properties of the Ag-rich phase are comparable to those of the x = 25 glass, while those of the Ag-poor phase are closer to (however, quite different from) the ones of the base glass AsS2. The structure of both phases at short and medium range order is investigated by micro-Raman spectroscopy and is correlated to their mechanical properties.
Applied Physics Letters 10/2011; 99(17):171911-171911-3. · 3.84 Impact Factor
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ABSTRACT: For the case example of GeTe, we demonstrate that the use of techniques that probe the average structure may lead to misleading conclusions regarding the nature of phase change. GeTe is a narrow band-gap semiconductor and a ferroelectric with the simplest conceivable structure that—according to previous studies—undergoes a ferroelectric-to-paraelectric displacive phase transition at ∼705 K. In this work, we provide direct experimental evidence that, contrary to the existing paradigm, the local distortion remains essentially unchanged with temperature and argue that the previous conclusion about the displacive nature of the ferroelectric-to-paraelectric transition was due to misinterpretation of Bragg diffraction, a technique that is only sensitive to the average structure and does not “see” random local distortions. The reported results have far-reaching implications for other materials exhibiting displacive phase transitions where conclusions have been reached based on results obtained using averaging techniques.
Phys. Rev. B. 10/2010; 82(15).
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ABSTRACT: We present here a study of the acoustic dynamics of polymeric glassy sulfur. The data have been collected in the GHz frequency range by means of Brillouin light scattering for different scattering geometries. The choice of the experimental setup allows us to obtain information on the refractive index, n, that comes out to be close to that corresponding to the high temperature polymeric liquid phase. The longitudinal acoustic excitations have been investigated as a function of temperature from deep into the glassy state up to the glass transition temperature. The temperature dependence of the sound velocity is compared to the one measured in the liquid phase. We find that the sound velocity of the glass can be linearly extrapolated from that of the polymeric liquid measured in the THz frequency range with inelastic X-ray scattering.
Journal of Non-Crystalline Solids 10/2010; · 1.54 Impact Factor
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ABSTRACT: We report an in situ high pressure investigation of the structural change in vitreous As(2)S(3) up to 60 GPa using the diamond anvil cell and energy dispersive x-ray absorption spectroscopy. The main finding of the present study is a gradual elongation of the average As-S bond length, which takes place in the pressure range of 15-50 GPa. This change is interpreted as a signature of the coordination number increase around As atoms. The negative shift of the As K absorption edge position confirms the progressive metallization of the glass at high pressure. The observed changes are reversible after pressure release.
The Journal of chemical physics 12/2009; 131(22):224502. · 3.09 Impact Factor
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ABSTRACT: Vibrational spectroscopy is applied on samples obtained from the excavation area of the medieval Monastery (10th century) of Karaach-Teke in Bulgaria. The results of the corresponding study, reveal the type of materials used for the creation of the wall-paintings and give evidence of Byzantine influence, a fact that further supports the well known impact of Byzantium on the technology and thematic-aesthetic features of iconography in Bulgaria during this era. In addition, the complementarity of FTIR and micro-Raman spectroscopies in the identification of pigments is indicated.
Annali di Chimica 08/2007; 97(7):491-503. · 0.99 Impact Factor
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ABSTRACT: Developing a novel experimental technique, we applied photon correlation spectroscopy using infrared radiation in liquid sulfur around T(lambda), i.e., in the temperature range where an abrupt increase in viscosity by 4 orders of magnitude is observed upon heating within few degrees. This allowed us - overcoming photoinduced and absorption effects at visible wavelengths - to reveal a chain relaxation process with characteristic time in the millisecond range. These results do rehabilitate the validity of the Maxwell relation in sulfur from an apparent failure, allowing rationalizing of the mechanical and thermodynamic behavior of this system within a viscoelastic scenario.
Physical Review Letters 08/2007; 99(2):025701. · 7.37 Impact Factor
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ABSTRACT: In this work, individual single-wall carbon nanotubes (i-SWCNTs) and their parent bundled material (b-SWCNTs) are investigated by means of high pressure Raman spectroscopy. Pressure application results in the broadening and intensity redistribution of the radial breathing modes (RBMs) in both materials. These changes are more pronounced in the case of bundled SWCNTs as it is also evident by the abrupt increase of the G+ bandwidth at ∼5.5 GPa, in contrast to the smoother pressure response of the individual tubes. Our experimental findings reflect the size dependent cross-section deformation of carbon nanotubes which evolves differently in the two materials investigated. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
physica status solidi (b) 12/2006; 244(1):100 - 104. · 1.32 Impact Factor
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ABSTRACT: Pressure effects on the vibrational modes of As2O3 glass have been investigated with the aid of Raman scattering. Emphasis was put on both the intramolecular vibrational modes at high energies as well as the low-energy excitations (Boson peak) of the glass. The pressure dependence of the spectral features of Raman bands was found to be significantly different compared with the corresponding features of the structurally similar As2S3. Striking dissimilarities were also observed for the pressure dependence of the Boson peak in these two glasses.
High Pressure Research 12/2006; 26(4):401-406. · 0.78 Impact Factor
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ABSTRACT: A new mobile instrumentation that takes advantage of the simultaneous utilisation of two micro-analytical techniques (Raman and XRF) has already been effected within the PRAXIS European project (Competitive and Sustainable growth programme, PRAXIS: A portable Raman–X-Ray instrument. No G6RD-CT-2001-00602). The instrument permits structural characterisation of the pigments incorporated in a painting under study by evaluation of the Raman spectra together with elemental analysis of the same materials provided by XRF spectra. Although direct structural characterisation with great specificity is directly accomplished by Raman spectroscopy, the signal is often masked by strong fluorescence, a fact that imposes time consumption. On the other hand, the average signal offered by XRF over larger volumes enables unique information to be obtained from invisible under-layers. Moreover, the detection of trace elements by XRF, not influencing the Raman spectra, renders the characterisation of painting materials more comprehensive. Validation of the instrument's in situ and non-destructive capabilities was performed after its in vitro application on an experimental icon (painted with traditional Byzantine techniques). The data acquired by the two techniques from the same spots on the painting offer complementary results, which enable the identification of almost all pigments even in the case of over-painted art objects. Finally, the instrument's abilities on the identification of materials on specimen's cross sections are revealed. Copyright © 2006 John Wiley & Sons, Ltd.
Journal of Raman Spectroscopy 09/2006; 37(10):1026 - 1034. · 3.09 Impact Factor
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ABSTRACT: The extent of polymerization of liquid sulfur up to 731 K has been obtained using Raman scattering. The data reveal the absence of a maximum in the polymerization curve. The obtained results are discussed in the spirit of computer simulations that suggest the existence of a maximum which depends on the bond breaking energy.
The Journal of Chemical Physics 05/2006; 124(14):146101. · 3.33 Impact Factor
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ABSTRACT: 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.
Journal of Non-Crystalline Solids. 03/2006;
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ABSTRACT: We report on an inelastic (Raman) light scattering study of the local structure of amorphous GeTe films. A detailed analysis of the temperature-reduced Raman spectra has shown that appreciable structural changes occur as a function of temperature. These changes involve modifications of atomic arrangements such as to facilitate the rapid amorphous-to-crystal transformation, which is the major advantage of phase-change materials used in optical data storage media. A particular structural model, supported by polarization analysis, is proposed being compatible with the experimental data as regards both the structure of a-GeTe and the crystallization transition. The remarkable difference between the Raman spectrum of the crystal and the glass can thus naturally be accounted for. Comment: Published in: J. Phys. Condens. Matter. 18, 965-979 (2006)
02/2006;
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ABSTRACT: Implications of reduction procedures applied to the low energy part of the vibrational density of states in glasses and supercooled liquids are considered by advancing a detailed comparison between the excess - over the Debye limit - vibrational density of states g(w) and the frequency-reduced representation g(w)/w^2 usually referred to as the Boson peak. Analyzing representative experimental data from inelastic neutron and Raman scattering we show that reduction procedures distort to a great extent the otherwise symmetric excess density of states. The frequency of the maximum and the intensity of the excess experience dramatic changes; the former is reduced while the latter increases. The frequency and the intensity of the Boson peak are also sensitive to the distribution of the excess. In the light of the critical appraisal between the two forms of the density of states (i.e. the excess and the frequency-reduced one) we discuss changes of the Boson peak spectral features that are induced under the presence of external stimuli such as temperature (quenching rate, annealing), pressure, and irradiation. The majority of the Boson peak changes induced by the presence of those stimuli can be reasonably traced back to simple and expected modifications of the excess density of states and can be quite satisfactorily accounted for the Euclidean random matrix theory. Parallels to the heat capacity Boson peak are also briefly discussed. Comment: To appear in J. Non-Cryst. Solids (Proceedings of the 5th IDMRCS, Lille, July 2005)
02/2006;
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ABSTRACT: By means of x-ray absorption fine structure and Raman scattering spectroscopies we demonstrate that the structure of amorphous GeTe is likely to be a mixture of 4(Ge):2(Te) and 3(Ge):3(Te)-coordinated structural units. Upon crystallization, a rhombohedral (distorted rocksalt) structure is established with about 10% of vacancies occurring on Ge sites. The vacancies are believed to play an important role in determining the ratio of 3(Ge):3(Te) and 4(Ge):2(Te) structural units.
Journal of Physics Condensed Matter 10/2004; 16(44):S5103. · 2.55 Impact Factor
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ABSTRACT: The central idea of the preceding Comment [J. Chem. Phys. xxx, xxx (2004)] is tackled from different viewpoints and arguments are presented showing its invalidity. First, we show that the presence of non-S8 rings is negligible under the conditions in which our experiment was performed. Then we prove that, even if we consider the non-S8 ring’s presence in the concentrations indicated in the Comment, their contribution to the isotropic Raman intensity is negligible and hence the accuracy of polymer content determination does not change as the authors of the Comment supposed. Finally, we briefly examine the ensuing question of the extent to which quench-and-dissolution methods can give accurate and reproducible results concerning the polymer content in liquid sulfur, demonstrating their inadequacy for reproducible and accurate data. Since reliable ex situ experimental data concerning sulfur’s polymerization do not exist, we vindicate the essence of methodologies that enable the quantification of sulfur’s polymerization from in situ experiments. © 2004 American Institute of Physics.
The Journal of Chemical Physics 09/2004; 121(13):6575-6577. · 3.33 Impact Factor
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ABSTRACT: The pressure response of double-wall carbon nanotubes has been investigated by means of Raman spectroscopy up to 10 GPa. The intensity of the radial breathing modes of the outer tubes decreases rapidly but remain observable up to 9 GPa, exhibiting a behavior similar (but less pronounced) to that of single-wall carbon nanotubes, which undergo a shape distortion at higher pressures. In addition, the tangential band of the external tubes broadens and decreases in amplitude. The corresponding Raman features of the internal tubes appear to be considerably less sensitive to pressure. All findings lead to the conclusion that the outer tubes act as a protection shield for the inner tubes whereas the latter increase the structural stability of the outer tubes upon pressure application. Comment: PDF with 15 pages, 3 figures, 1 table; submitted to Physical Review B
09/2004;
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ABSTRACT: We report on a detailed, temperature-dependent, off-resonant Raman scattering study of glassy and supercooled selenium. Raman spectra in the frequency regime of the first-order scattering (5-450 cm(-1)) have been recorded over a wide temperature range, i.e., 143-353 K. To facilitate the analysis, the spectra have intuitively been divided in three spectral regions. The analysis of the high frequency region (bond-stretching vibrational modes) yielded information on the rings-chains equilibrium. In particular, the polymer content was found to amount to more than 85% around the glass transition temperature, exhibiting a weak temperature dependence, which extrapolates nicely to the high-temperature dissolution data. The intermediate frequency range (representative of the medium-range structural order) was treated together with the low frequency regime (where low-energy excitations, i.e., the quasielastic line and the Boson peak are the dominant contributions) owing to their strong overlap. The study of the bond-bending regime revealed information which made it possible to clarify the role of ringlike and chainlike fragments incorporated in polymeric molecules. The temperature evolution of the Boson peak and the frequency dependence of the Raman coupling coefficient Comega were also determined. An attempt to decompose the partial contribution of the pure Boson peak to Comega revealed valuable information concerning the limiting (omega-->0) behavior of the coupling coefficient.
The Journal of Chemical Physics 09/2004; 121(10):4747-58. · 3.33 Impact Factor
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ABSTRACT: The molecular orientation of uniaxially drawn commercial poly(vinyl chloride), PVC, specimens has been investigated by polarized Fourier transform infrared (FTIR) in the specular reflectance mode and by resonance Raman scattering in various combinations of sample orientation, scattering configuration and polarization geometry. The orientation was examined as a function of the specimen draw ratio. The Kramers−Kronig transformation has been used to obtain infrared absorption spectra from the FTIR reflectance data; the orientation was determined by the dichroic ratio of the C−Cl stretching and CH2 rocking modes of PVC. The resonance Raman spectra of polyene segments that result from partial thermal degradation/dehydrochlorination of PVC during processing have been utilized in order to determine the parameters of the orientation distribution function. The all-trans conjugated polyene sequences formed during degradation are considered as rodlike segments in the polymer backbone and are used as indicators of orientation within the amorphous-like polymer phase. As far as the trends are concerned, data analysis showed good agreement between FTIR and Raman results regarding the induced molecular orientation in industrial PVC specimens.
07/2000;
