Glassy Forms Research Articles

The constant-photocurrent method applied to chalcogenide materials

Optical absorption edges in glassy and crystalline forms of As2S3 and Se have been evaluated using the constant-photocurrent method, and the results are compared with transmission spectra. In glassy and crystalline As2S3 and crystalline Se, the photocurrent method can determine the absorption coefficient down to 10 - 2-10- 1 cm- 1. In contrast, in glassy Se, a substantial difference exists between the photocurrent and the transmission spectra. The origins of the discrepancy and other features are discussed.

Temperature dependence of Raman spectra in amorphous, crystalline and liquid GeSe 2

We have investigated the dynamical processes of glass transition, crystallization and melting in GeSe 2 by analyzing Raman spectra of glassy, crystalline and liquid GeSe 2 as a function of temperature in a range between room temperature and 730°C. Raman vibration bands related to short- and medium-range structures are observed in glassy and liquid forms. The result suggests that there is a topologically crystalline layer-like fragments in these two forms. The change of the spectral shape around the glass-transition temperature is explained by the decrease of interlayer interaction between the neighboring layer-like fragments consistent with the decrease of the viscosity.

Structural investigations of phosphate glasses: a detailed infrared study of the x(PbO)-(1−x) P2O5 vitreous system

The results and detailed discussion of an extensive experimental study of infrared spectra of the x (PbO)-(1−x)P2O5 vitreous system (x=0.3–0.75) together with a brief review of infrared spectra of phosphate compounds, are presented. Theoretical models employed in the interpretation of infrared spectra of glasses have been reviewed. The frequency ranges of various infrared bands belonging to PO 4 3− and P2O 7 4− , observed in different phosphate compounds, are discussed. The glassy and quenched samples were prepared from PbO and NH4H2PO4 by the rapid quenching technique. The infrared spectra of the constituents of the system, PbO and P2O5, in their polycrystalline and glassy forms, have been discussed. The intensity and wavenumbers of the infrared bands around 1600 and 3300 cm−1, assigned to the bending and stretching modes in H2O trapped by the hygroscopic glasses, have been followed for different compositions with x<0.5. The changes observed in these infrared bands established the role of water as an additional glass modifier. The intensity and frequency variations of the infrared bands have been followed through all the compositions for characteristic phosphate group frequencies including P=O, P-O-P stretching and bending modes and P-O bending mode. The results clearly suggest that the x(PbO)-(1−x)P2O5 system undergoes gradual structural changes from metaphosphate (x=0.5), to pyrophosphate (x=0.66) and to orthophosphate (x=0.75). The continuing presence of the infrared band, in varying intensity, in the region 1200–1280 cm−1 attributed to P=O, suggests that the glass-forming ability of the binary system is extendable at least up to x=0.66 composition, and that no complete rupture of P=O bond by Pb2+ takes place. The ionic character of the phosphate groups, P-O(−), PO 4 3− is well revealed by significant changes with the PbO content in the spectral features of the infrared bands around 1120 and 980 cm−1 respectively. The maximum intensity of the P-O(−) band at 1120 cm−1 for 55 mol% PbO suggests a partial breakdown of the covalent vitreous network of the phosphates and formation of a crystalline phase consisting of ionic groups PO 4 3− , P2O 6 2− and P2O 7 4− for PbO greater than 55 mol%. The observed pattern of variation in the intensity of the infrared bands in the 940–1080 cm−1 region attributed to the v3-mode in PO 4 3− , suggests a gradual transformation of PO 4 3− units to PO 3 − groups in lead meta-phosphate glass and then their restoration to PO 4 3− groups of pyro- and ortho-phosphate quenched samples. The results indicate a gradual decrease in the number of bridging oxygens and increase in the resonance behaviour of non-bridging oxygens as the mole percentage of metal oxide (PbO) increases in the glass. The infrared spectra of several binary phosphate glasses have been reviewed in the context of the study of effect of the cation on the infrared spectra. It is found that the influence of the cation on the infrared spectra of phosphate glasses does not show any striking regularity. Theoretical calculations of these band frequencies were found to agree well only in the case of pure stretching (P=O and O-H) vibrations and pure bending (P-O-P and O-H) vibrations. The disagreement in the case of P-O(−), P-O-H and other modes of P-O-P groups, has been attributed to the mixed nature of modes occurring in glasses. The changes in the positions of the characteristic bands and their relative intensities are strongly dependent on the structural units and PbO content in the phosphate glasses and the results emphasize the role of PbO as a network modifier.

Model quasi-molecule Si2O as an approach in the IR spectra description glassy and crystalline framework silicates

In MIR spectra of framework silicates, both in crystalline and glassy forms, three characteristic group of bands appear. These bands can be found in spectra of different compounds in almost the same wavenumbers range and having similar intensity rations. In this work new approach in the IR spectra description of framewoek silicates has been demonstrated. A bent quasi-molecule Si2O is used as a monomeric unit of structure. In model of Si2O unit three main bands in spectra are due to internal vibrations bridge SiOSi. There are stretching vibrations (1100–1100cm−1), bending (∼800cm−1) and rocking (∼470cm−1). Spectra obtained at low temperatures have been presented. Changes in spectra with decrease of temperatures contribute proposed model of quasi-molecule Si2O. A series of calculations and simulation of spectra has been also curried out. The study are based on the spectra of crystalline and glassy silica and potassium aluminosilicates.

Microscopic dynamics of ortho-terphenyl in its polycrystalline and glassy forms

The microscopic dynamics of polycrystalline ortho-terphenyl are investigated by means of the concurrent use of harmonic lattice dynamics calculations and Raman spectroscopy. In order to reproduce the spectra and thermodynamics of the crystal up to about 70 K, the motions of several internal modes are considered in detail. An empirical force field, able to account for the observed crystal structure, thermodynamics and Raman spectra up to such temperature, is then developed. Estimates of the anharmonic effects are derived from the temperature dependence of vibrational frequencies of those modes of collective or hybrid character (below 6 THz). The Raman spectra of the glass are finally discussed by comparison with the polycrystalline case. The implications of the present findings regarding studies on the glass transition of complex materials are discussed.

Nanometer‐scale reversible recording using STM

Data storae using the scanning tunneling microscope has come one step nearer. Instead of using changes in surface morphology for recording as previously proposed, changes in tunneling current are produced by locally induced phase transitions between crystalline and glassy forms of avanadate composite material. This has the advantages of being reversible, a room‐temperature technique, and less susceptible to defects. The recording mechanism is though to the pulse‐voltage‐induced migration of sodium ions along the electric field.

The possibilities for glassy clusters: (KCl)32

The (KCl)32 cluster is used as a model system to study the possibilities for clusters to exhibit amorphous or glassy solid forms. The problem has two aspects: first, whether the potential surface of the cluster supports a myriad of locally stable, disordered structures, the ensemble of which would constitute the glassy state, and second, whether an ensemble of amorphous clusters can be prepared under laboratory conditions. Molecular dynamics studies give an emphaticyes to the first issue, and an equally emphaticno to the second, for cooling rates up to 1012 K/s, a thousand-fold faster than the fastest rates yet reported. However, if the long-range Coulomb interaction of the ions is replaced by a shielded Coulomb (Debye or Yukawa) potential, the secondary minima are sufficiently stabilized and the saddles, sufficiently high, that disordered equilibrium structures can be reached by cooling at fast, but still conceivably attainable rates. The implication is that while alkali halide clusters probably cannot form glasses, binary clusters with shorter-range forces, such as those of II–VI and III–V compounds, probably can form glasses. The highly disordered structures of (KCl)32 are perhaps the most disordered forms yet seen for solid matter.

Structure-conductivity relationships in divalent polymer electrolytes

The potential influence of averaging caused by spherulite morphology on local structures deduced from EXAFS studies of crystallisable films is discussed. Evidence is presented to show that only high-melting, polymer-salt complexes interfere significantly with studies of the ionic environment within the amorphous conducting region. The possible modifications to the amorphous matrix that could result from the proximity of spherulites are considered. Results are presented for EXAFS studies of the local environment surrounding Zn in PEO n·ZnBr 2 ( n=4, 8) prepared by three different drying procedures, at room temperature, -20°C and -70°C. Structural differences between amorphous and glassy forms were observed. The results suggest the presence of neutral ZnBr 2 entities.

Phase stability of glasses

Allowing rearrangements effected by covalent bond rupture it appears that all three-dimensional materials are more stable in some crystalline (or quasi-crystalline) state than in glassy form. The scaled glass temperature, T rmrg (= T g/ T l, where T g is the actual glass and T l the liquidus temperature) might be taken as one index of the thermodynamics stability of the glass relative to its crystallized form. Reported values of T rg range from 0.45, for certain metallic glasses, to 0.87, for fused silica scaled with the quartz liquidus. The effects of covalent connectivity, molecular asymmetrization, and homophase compositional changes on T rg will be surveyed. The thermodynamic properties of some selected materials in their glassy and crystalline forms will be compared. The effects of relative glass stability on glass forming tendency, a property determined by the crystallization kinetics, will also be reviewed.

The Influence of Structure on the Corrosion of Glassy Copper-Zirconium Alloys

Abstract Differences in structure between crystalline and glassy copper-zirconium alloys lead to subtle increases in the corrosion rate of crystalline compared to glassy material. Glassy alloys of Cu60Zr40 were melt spun and subsequently devitrified, yielding single phase polycrystalline material. Comparing the potentiodynamic polarization response of these structural types revealed no significant difference in ϕcorr. ba, bc, icorr. and io. Other alloys, Cu58Zr42 and Cu55Zr45, compared in glassy and crystal-line forms revealed that inhomogeneity, caused by precipitation of a second phase, did not alter the corrosion behavior from that of single phase Cu60Zr40. Corroded glassy surfaces were generally smooth except for scattered hemispherical features, while crystalline surfaces were densely populated with these features. Corrosion of copper-zirconium alloy occurs by the selective dissolution of zirconium in the vicinity of ϕcorr, whereas, at potentials noble to −0.1 VSCE, copper contributes to the anodic current. Above approximately 0.0 VSCE the alloys passivate. Results of potentiodynamic polarization studies suggest that for these alloys, unlike transition metal-metalloid glasses, devitrification does not significantly affect the corrosion kinetics of copper-zirconium alloys.

Gamma Radiolysis of Aqueous Media and Its Effects on the Leaching Processes of Nuclear Waste Disposal Materials

AbstractThe effect of the presence of moderate gamma doses (6×104 Gy) on the leaching of glassy and ceramic waste forms has been investigated. It is found that the leach rates are enhanced by <50% in the case of alumina-free glasses, by a factor of 3 to 4 in the case of glasses that contain 5 to 8% Al2O3, and by a factor of 20 to 100 in the case of SYNROC-D (20% A32O3). Buffer studies show the enhancement to be almost entirely due to a decrease in pH, and the composition dependence of the enhancement factor is interpreted in terms of the sharp rise in both alumina solubility and leach rates of alumina-containing materials with increasing acidity. The radiation-induced pH decrease is partially due to the formation of nitric acid but formic and oxalic acid are also observed to be produced. The concentration of carboxylic acids is as large as that of HNO3. A mechanism is proposed that assumes HNO3 is produced due to the oxidation of dissolved nitrogen, while formic and oxalic acids result from the reduction ...

Trapped electrons in amorphous solids: A method for estimating the concentration of molecular vacancies

The densities for n-propanol and n-butanol in their glassy and crystalline forms have been measured at 77 K. The density difference of ca. 0.11 cm 3 g −1 is comparable with that normally found between liquids and solids at their melting points. The maximum yield of trapped electrons in the γ-irradiated glassy alcohols has been estimated and, if it is assumed that they are exclusively trapped at molecular-sized vacancies present only in the glasses, then such vacancies account for no more than ca. 1.0% of the volume difference between the crystalline and glassy materials. Much of the remaining difference can be accounted for in terms of long and bent hydrogen bonds in the glasses.

Comparison ofγ−radiolysis of hydrocarbons in glassy and polycrystalline states

Abstract The effects of the glassy and polycrystalline states of methylcyclohexane (MCH), isopentane (IP), isohexane (IH) and 2,3-dimethylbutane (DMB) on the yields of the heavy products from the γ-radiolysis has been investigated. Total G values are the same in glassy and polycrystalline solids for MCH and IP, but G values for IH and DMB are larger in the polycrystalline states than those in the glassy forms. Comparison of the relative distribution of stable products always shows matrix effects. Differences in esr spectra between either states have only been observed for irradiated IH and DMB. The results from radical scavenging methods are interpreted in terms of different probabilities of radical processes in the glassy and polycrystalline states.

Thermoluminescent behavior of crystalline and glassy lithium borate

The thermoluminescence of the crystalline and glassy forms of Li 2O:2B 2O 3 and of the crystalline and glassy forms of Li 2O:2.7B 2O 3, doped with various concentrations of divalent and tetravalent manganese, was investigated. Changing sample stoichiometry changes TL sensitivity, with the greatest TL intensity being found for the crystalline Li 2O:2B 2O 3 containing 0.1% manganese impurity. Among the glasses, the Li 2O:2.7B 2O 3 composition has the greatest TL intensity. The thermoluminescence vs. exposure curves are linear between 30 and 10 5R for all glasses tested, a fact that could be useful in high dose measurements.

Photoemission studies of crystalline palladium and the metallic alloyPd0.775Cu0.06Si0.165in its glassy and crystalline forms

We report x-ray and ultraviolet photoemission measurements of the levels and valence bands of palladium and of the metallic alloy ${\mathrm{Pd}}_{0.775}$${\mathrm{Cu}}_{0.06}$${\mathrm{Si}}_{0.165}$ in both its glassy and crystalline forms. The x-ray photoemission measurements show that there is little significant difference between the relative peak positions of the various levels, and that only the copper core level widths in the glass alloy are broader than those in the crystal. The anomalously large width of the $4p$ levels, which is often more sensitive to the chemical environment than the chemical shifts, is also the same for all three samples. A large variation with angle was observed in the peak positions of the $3d$ core levels of the crystalline palladium sample. In the valence band the ultraviolet photoemission spectra of both forms of the alloy are quite similar. However, the valence band for the alloy is narrower than that for the pure palladium, and the density of states at the Fermi level is smaller. The results of these measurements are compared with the various theories of metallic glass stability, and it is suggested that chemical bonds do not contribute to the enhanced glass-forming tendency or stability against crystallization of this alloy. The results are consistent with the nearly-free-electron model of metallicglass stability.

Petrology of the lunar soil and geophysical implications

The Apollo 11 soil sample consists of particles derived from two rock suites, basaltic and anorthositic. Crystalline, glassy, and brecciated forms of each rock type are present. The basaltic suite (95% of identifiable soil particles) must be representative of mare rock; the anorthosites appear to derive from the lunar highlands. An anorthositic crust ∼10 km thick is required beneath the highlands to support their relief above the mare surfaces isostatically. To form such a crust via crystal fractionation requires that magma derived from initial partial melting of a substantial fraction of the moon (one-third the volume, for a chondritic moon) be collected at the lunar surface in early times. Such a layer could not be kept hot and molten for very long; hence the young basalts collected by the Apollo 11 and 12 missions must have been produced by subsequent melting at depth, attributable to internal radioactive heating, and are not products of the early magma system that gave rise to the anorthositic crust. Basaltic lava, being less dense than solid basalt, is capable of rising into topographic basins (maria) that are already isostatically equilibrated with their higher surroundings. A lava overfilling of this type would constitute a mascon, giving rise to a positive gravity anomaly.

Studies of Chemical Bonding in Glasses by X‐Ray Emission Spectroscopy

Silicon and aluminum Kβ band X‐ray emission spectra of a series of lithia‐aluminosilica (LAS) glasses were determined using the X‐ray spectrometric systems of commercially available electron microprobes. Comparison with spectra of reference materials, including glassy and crystalline forms of silica, high‐quartz‐phase crystals of the L:A:S‐1:1:2 (mol ratio) composition (high‐eucryptite), and 6‐ and 4‐coordinated aluminum oxides, led to tentative assignment of spectral peaks to specific electronic transitions. Molecular orbital theory and crystallographic structural data were used to assign bands in crystalline materials. The LAS glass spectra indicated progressive bond weakening with decreasing silica content; the Kβ peak shifts permitted approximate calculation of the decreases in Si–O and Al–O bond energies, which were as great as 30 kcal/mol relative to SiO2 (for the L:A:S‐1:1:2 glass).