K. S. Andrikopoulos

Raman scattering study on structural and dynamical features of noncrystalline selenium

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.

Probing the sulfur polymerization transition in situ with Raman spectroscopy

In this paper we demonstrate the ability of inelastic (Raman) light scattering to probe polymerization transitions. We show that after proper treatment—that is, separating isotropic and anisotropic contributions and employing the reduced representation which removes the thermal population effect of vibrational energy levels—the Raman data can be used as an accurate quantitative indicator of monomer↔polymer transitions. In particular, we have applied this method to study the thermoreversible polymerization transition of liquid sulfur up to 300 °C. Raman spectra obtained from rapidly quenched samples over a broad low-temperature range, from −180 °C to ambient temperature, revealed the fact that the equilibrium between monomers and polymers in the solid amorphous state is precarious; thus quench-and-dissolution methods employed to determine the polymer content of the liquid are not accurate. Our data are compared with existing data obtained via quench-and-dissolution techniques showing considerable dissimila...

The glassy and supercooled state of elemental sulfur: Vibrational modes, structure metastability, and polymer content

We report a detailed investigation of vibrational modes, structure, and dynamics of elemental sulfur in the glassy and the supercooled state, using Raman scattering and ab initio calculations. Polarized Raman spectra are recorded--for sulfur quenched from 473 K--over a broad temperature range from 93 K to 273 K where the supercooled liquid crystallized. The temperature induced shifts of the majority of the vibrational modes are determined and compared with the corresponding ones of crystalline sulfur. Analysis of the reduced isotropic spectra showed that the structure of the quenched product is composed of eight member rings (S8) and polymeric chains (Sμ) with a relative fraction comparable to that of the parent liquid at 473 K. Low temperature spectra, where spectral line broadening due to thermal effects is limited, revealed that two different polymeric species are present in the glass with distinct vibrational frequencies. Their interpretation was assisted by ab initio calculations used to simulate the vibrational frequencies of polymeric chains S(8k) (k = 1, ..., 7). Theoretical results exhibit an increasing breathing mode frequency for sulfur chains up to k = 2, although it remains constant beyond the above value. The polymeric content is metastable; heating the glass above its glass transition temperature, T(g), destabilizes the chains and drives them back to the more thermodynamically stable rings. This bond interchange mechanism provides the structural origin of a secondary relaxation process in supercooled sulfur reported long ago, which has been also considered as a complication in the correct fragility estimation of this material. Finally, the Boson peak of the glass was found to exhibit strong temperature dependence even at temperatures below T(g).

“Rounding” of the sulfur living polymerization transition under spatial confinement

“Rounding” effects and other specific changes of liquid sulfur’s polymerization transition are studied using Raman scattering in the case of fluid’s spatial confinement in nanonoporous sol-gel glasses. Specifically, in this paper we demonstrate the smearing or “rounding” of the monomer↔polymer transition caused by geometrical confinement of the liquid, a phenomenon that is in marked contrast to ordinary (bulk) liquid sulfur, where the polymerization transition is quite sharp, bearing a close resemblance to a second-order-type transition. In noticeable agreement with recent theories of living polymerizations, the limited growth of polymeric species brings about a diminishing of the strength of the transition as evidenced in the temperature dependence of many physical properties. The careful determination of the extent of polymerization by analyzing Raman spectra made it possible to recalculate the magnitude of certain thermodynamic parameters changes across the polymerization transition. As a result, the t...

Nonergodicity factor, fragility, and elastic properties of polymeric glassy sulfur.

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.

Confinement effects on liquid–liquid transitions: pore size dependence of sulfur's living polymerization

Modifications in the living polymerization or λ-transition of liquid sulfur, caused by confinement in nanonoporous sol–gel-derived glasses, are investigated by Raman scattering. Raman spectra are analyzed to provide the extent of polymerization, through which the temperature-dependence of the respective heat capacity is calculated. The data reveal that the sharp, second-order-type transition which takes place in bulk sulfur experiences severe rounding (loss of sharpness of the λ-transition) and systematic shift (increase) of the characteristic transition temperature upon restricting the liquid to smaller pore sizes. The observed confinement-induced changes in the transition temperature are discussed in the framework of theoretical predictions of finite-size scaling phenomena revealing a dependence on the inverse first power of the mean pore size in accordance with theoretical and experimental data.

Some remarks on the low energy excitations in glasses: Interpretation of Boson peak data

The frequency-reduced representation, g(ω)/ω2, of the vibrational density of states g(ω), known as the Boson peak is frequently used to extract information on structural and dynamical properties of glasses and supercooled liquids. The g(ω)/ω2 representation is preferred over the true excess of the vibrational modes, defined as the difference between the total g(ω) and the Debye contribution, for practical reasons. Analysis of a large body of the available experimental data from inelastic neutron and Raman scattering reveals that reduction procedures distort to a great extent the otherwise symmetric excess density of states. The frequency and the intensity of the Boson peak are very sensitive to minor modifications of the distribution of the excess. The quantitative use of the g(ω)/ω2 function leads to some peculiar results that contradict our general perception about structure and dynamics of glass-forming materials. We stress that the physics of the low-energy excitations are hidden in the excess density of states, which should attract more attention both from the experimental as well as the theoretical point of view.

High pressure transition in amorphous As2S3 studied by EXAFS

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.

Response to “Comment on ‘Probing in situ the sulfur polymerization transition with Raman spectroscopy’ ” [J. Chem. Phys. 121, 6573 (2004)]

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 pol...

Elemental sulfur under high hydrostatic pressure. An up-to-date Raman study

We report a high pressure Raman study of orthorhombic elemental sulfur from ambient pressure to ∼ 25 GPa. Using a near infrared laser and low laser intensity on the scattering volume, we achieve off-resonant conditions up to larger pressures in comparison with previous studies. Raman spectra were recorded over the full spectral range including external (librational, translational) and internal (bond bending and bond stretching) modes. Drastic changes are observed as regards the peak frequencies, relative intensities and band splitting of degenerate modes. The main outcome of the present study is the observation of a “structural” transition at ∼ 16 GPa manifested as slope changes of certain frequencies and sudden relative intensities changes. The present findings are discussed in the context of previous pressure Raman studies and comparison with existing X-ray diffraction as well as ab initio molecular dynamics results is attempted.