Angelos G. Kalampounias

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...

Temperature induced changes on the structure and the dynamics of the “tetrahedral” glasses and melts of ZnCl2 and ZnBr2

Glassy, supercooled, and molten ZnCl2 and ZnBr2 have been studied by Raman spectroscopy over the broad temperature range −196 to 800 °C in an effort to follow in detail the structural changes caused by temperature variation. A systematic study has also been undertaken for the corresponding crystalline polymorphs showing that each material exists in only one crystalline phase if water traces are not present. The reduced isotropic and anisotropic Raman spectra of the ZnCl2 and ZnBr2 glasses and melts are isomorphous. Unusually drastic changes of the relative intensities of particular bands occur with temperature in the reduced isotropic spectra. A comparison between the spectral features of crystals, glasses, and melts has revealed that the network structure of the glasses and melts consists of ZnX4/2 (X=Br,Cl) tetrahedra bound to each other by apex- and edge-bridged halides. The substructure of the glass/melt is formed by mixing a variety of tetrahedra participating in “open,” “cluster,” and “chain” networ...

Short-time dynamics of glass-forming liquids: Phenyl salicylate (salol) in bulk liquid, dilute solution, and confining geometries

Raman spectroscopy has been used to investigate picosecond vibrational dynamics of the molecular glass-former phenyl salicylate (salol). The study has concentrated on the elucidation and comparison of molecular dynamics of salol subjected to different local environments, namely in bulk, in a dilute CCl4 solution, and under spatial confinement in nanoporous sol–gel glasses. Contrary to most of the previous picosecond dynamics studies performed by means of Raman line profile analysis, we have employed in this paper a novel approach that enables the extraction of valuable short-time dynamics information through spectra fitting in the frequency domain. As a result, strongly overlapping vibrational lines can be put in focus. The analysis has shown a systematic dependence of vibrational relaxation on the local environment around a salol molecule. The magnitude of the vibrational relaxation time τV decreases in the sequence dilute solution→bulk liquid→confined liquid, indicating the modification of interactions ...

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).

A high-temperature Raman spectroscopic investigation of the potassium tetrasilicate in glassy, supercooled, and liquid states.

Raman spectra of K2Si4O9 were measured over a broad temperature range including the glassy, supercooled, and molten states in an effort to follow the structural changes caused by temperature variation. Potassium tetrasilicate glass has been prepared using a containerless method and a CO2 laser for heating and melting the samples and thus avoiding contamination induced by the walls of the crucibles. Systematic Raman intensity measurements caused by temperature variation have been performed in order to elucidate the induced structural changes in the high-frequency stretching and in the three- and four-membered ring breathing vibration regions. The high-frequency symmetric stretching vibrations of the nonbridging Si-O bond are associated to the presence of two distinct types of tetrahedral units with terminal oxygen atoms. The low-frequency Raman spectra reveal the, well resolved, presence of the boson peak at temperatures above the melting point. The temperature dependence of the boson peak energy has also been determined and compared with that of the sound velocities of potassium tetrasilicate. The results are discussed in the context of recent experimental and theoretical works.

Structure and vibrational modes of sulfur around the λ-transition and the glass-transition

Abstract In this paper we present a detailed temperature-dependence of Raman spectroscopic study of the vibrational modes of elemental sulfur around its polymerization transition or λ-point (Tλ) in order to elucidate the structural transformations. The considerable supercooling that liquid sulfur exhibits makes it possible to extend the light scattering study to ∼50 °C below melting point as well as to glassify sulfur by fast quenching the high temperature melt. Despite the apparent simplicity of sulfur, the determination of its structure has proved considerably puzzling. The results are discussed within the context of existing structural models for liquid sulfur.

Raman spectra and microscopic dynamics of bulk and confined salol

In this paper we employ a new method, which enables one to calculate time correlation functions of vibrational relaxation by fits in the frequency domain, in the study of picosecond vibrational dynamics, that is, vibrational dephasing and vibrational frequency modulation. This method is applied to the molecular glass former salol in bulk, in a dilute CCl4 solution and in restricted geometries after confining it in nanoporous silica glasses of various pore sizes. The important finding is that the vibrational dynamics of the confined molecules becomes faster with decreasing the pore size. An attempt is made to rationalize this effect by invoking the cooperativity issue related to the sluggish dynamics as the glass transition is approached. The removal of many-body effects by trapping the molecules in less-crowded environments seems to be the key factor. The obtained results are described in the light of recent spectroscopic, X-ray diffraction and molecular dynamics studies performed on this liquid.

“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...

Inelastic light scattering from xCaO-(1-x)SiO2 glasses

A Raman scattering study is presented in an effort to determine the structural changes that take place when modifying silica glass with an alkaline earth oxide modifier. Glasses have been prepared from the binary system xCaO-(1 - x)SiO 2 (x = 0, 0.3, 0.35, 0.4, 0.45, 0.5, 0.55, 0.6, where x is the molar ratio of CaO) using a containerless method to melt and quench the samples. In the concentration region 0.3 0.45 structural changes take place as indicated in modifications of peak relative amplitudes and Raman shifts. The frequency of the Boson peak and the defect peak (D 2 ) change when crossing the x = 0.45 point. The obtained results are discussed in the framework of existing structural models of silica and modified silica glasses and are suggestive of a microphase separation near the peritectic composition of the binary system.

PROGRESSIVE FORMATION OF HALLOYSITE FROM THE HYDROTHERMAL ALTERATION OF BIOTITE AND THE FORMATION MECHANISMS OF ANATASE IN ALTERED VOLCANIC ROCKS FROM LIMNOS ISLAND, NORTHEAST AEGEAN SEA, GREECE

Occurrences of halloysite-rich material in altered volcanic rocks, principally trachyandesites, dacites, and tuffs, extend over an area of ∼1 km2 in the southwestern part of Limnos, Island, northeast Aegean Sea, Greece. The present study was designed to investigate the alteration processes which acted on the biotite in these volcanic rocks, to describe in detail the mechanism of formation of the halloysite, and to specify the mechanisms of formation of anatase during the alteration processes. Samples were examined using polarized-light microscopy, X-ray powder diffraction, scanning electron microscopy, scanning electron microscopy-energy dispersive spectroscopy, and Fourier-transform-Raman techniques. The extensive alteration of the parent rocks, triggered by the circulation of hydrothermal fluids through faults and fractures, resulted in the alteration of biotite to halloysite. Six stages of alteration were recognized. Nanoparticles of halloysite were initially formed on the mica layers, which progressively grew through short-tubular to well formed tubular halloysite, with increasing alteration. In the most altered samples, laths and interconnected laths with the composition (Al3.96Fe0.04)Si4O10(OH)8, were the dominant halloysite morphologies. Anatase was encountered as an alteration product of both ilmenite and biotite. Ilmenite was altered to anatase and Fe oxides. The altered ilmenite crystals constrained most of the newly formed anatase within the space occupied previously by ilmenite, leading to the formation of skeletal anatase. The layered structure of the micas was the main factor governing the morphology of newly formed anatase developed outside ilmenite margins in the form of layers parallel to those of mica. An unusual ring-like structure of anatase was thought to be the result of the uncommon alteration of inner parts of mica folia to tubular halloysite oriented perpendicular to the mica layers. The detachment of the halloysite tubes by circulating hydrothermal fluids was considered to be the reason for the creation of holes which were subsequently surrounded by the anatase ring forms.