Frank L. Galeener

Raman and ESR studies of the thermal history of amorphous SiO2

Abstract The room temperature structure of vitreous SiO 2 is known to change subtly with thermal history. This paper reviews Raman and ESR spectroscopic observations that reveal some of these changes and show that the rate of structural relaxation depends very strongly on trace water (OH) content.

Thermal equilibration of raman active defects in vitreous silica

Abstract We have used Raman spectroscopy to characterize the thermal treatment of several types of vitreous silica samples by monitoring the intensity of a strongly polarized mode at 606 cm−1. Room-temperature measurements were made on quenched samples as they were progressively equilibrated at the desired fictive temperatures. As the annealing temperature or network terminator concentration increased, the times required for equilibration decreased. The equilibrium intensity of the 606 cm−1 Raman line is exponentially dependent on the inverse fictive temperature, but independent of the impurity (such as OH) concentration. We reconcile this latter disagreement with other published results.

Raman studies of Al coordination in silica-rich sodium aluminosilicate glasses and some related minerals

Abstract Polarized Raman spectra have been obtained for a series of sodium aluminosilicate glasses and selected minerals, in order to investigate changes in the glass structure with composition. Particular attention is paid to characterization of the Al coordination, and to test Lacys hypothesis concerning the role of “triclusters” in Al-rich glasses (Al/Na = R > 1.0). A comparison of five glasses ranging from nearly sodium trisilicate composition glass (R = 0.03) to albite composition glass (R = 1), and beyond to a glass of R = 1.61, leads to the following observations with increasing R: the polarized bands previously assigned to T-O (non-bridging) vibrations (950, 1110 cm−1) decrease in intensity, while those assigned to T-O (bridging) vibrations (485, 980 cm−1) either remain the same or increase in intensity. The narrow band at 495 cm−1 and a somewhat wider one at 595 cm−1, may be related to Raman active “defects” that have been observed in vitreous SiO2 at essentially the same frequencies. Our observations indicate a general increase in tetrahedral polymerization and random incorporation of Al into tetrahedral sites. Comparison of the Raman spectra of mullite (which contains (Si, Al)3O10 triclusters) with sillimanite (which contains no triclusters) shows that the bands at 418 and 1200 cm−1 in mullite are absent for sillimanite. Assuming that these “extra” bands are due to the triclusters in mullite, we have to conclude that similar triclusters do not exist in any of the glasses we examined. If these “extra” bands in mullite are not due to triclusters, we can draw no such conclusion. Our study indicates that observed changes in physical properties of sodium aluminosilicate glasses as R increases are not due to an Al coordination change.

Local structure and vibrational spectra of vAs2O3

Abstract We discuss the atomic displacements of the optically active vibrational modes of vitreous (v-)As2O3 through comparisons of the infrared (ir) and Raman response with the corresponding spectra of the two crystalline polymorphs, claudetite, a layer crystal similar to orpiment, and arsenolite, a molecular crystal based on the As4O6 molecule. We conclude from these comparisons that the structure of the glass is composed of AsO 3 2 pyramidal units that are corner connected to form a continuous random network. The character of the strong ir and polarized Raman modes in the vitreous form, suggests that the interconnection of these pyramidal units cannot be described by a random distribution of dihedral angles, but rather has peaks at angles characteristic of the different ordering in two-dimensional macromolecular layer basis of claudetite, and the As4O6 molecule. The comparisons are extended to v-As2S3 and vAs2Se3 where we conclude the dihedral angle distributions characteristic of an As4O6-like local geometry are less prevalent.

Intermediate range order in amorphous solids

Abstract Information about local atomic arrangements in amorphous solids is derived by a variety of techniques, X-ray RDFs, vibrational spectroscopy, NMR and EXAFSs; however, there is a scarcity of information about intermediate range order, in particular the distribution of dihedral angles. We show that this information can frequently be obtained from an analysis of polarized features in the Raman response.

The Raman spectra of defects in neutron bombarded and Ge-rich vitreous GeO2

Abstract This paper reports the polarized Raman spectra of three forms of vitreous GeO2: the pure glass, neutron irradiated pure glass and unirradiated Ge-rich glass of composition Ge1.1O2. The data reveals that the line seen at 520 cm−1 in the pure glass is due to a network defect that is not a GeGe bond and very probably also not an OO bond. Comparison with spectra of fused silica suggests that the 606 cm−1 defect line seen in v-SiO2 is not due to SiSi or OO bonds.

Raman and inelastic neutron scattering by vitreous ZnCl2

Abstract Polarized Raman and inelastic neutron scattering spectra are reported for pure vitreous ZnCl 2 . The HH and HV Raman spectra were obtained from 20 cm −1 to 800 cm −1 using 514.5 nm incident radiation. The dominant Raman line is seen at 230 cm −1 in the HH spectrum and is strongly polarized, with HH/HV ∼ 28. The neutron data were obtained for energy transfers from 70 cm −1 to 450 cm −1 , and reveal two broad peaks in the density of vibrational states with maxima at ∼85 and ∼270 cm −1 . The results support a structural model consisting of corner-connected ZnCl 4 tetrahedra; they also are consistent with recent observations concerning the relation between polarized Raman spectra and the total density of vibrational states of several other AXd 2 tetrahedral glasses.

A model for the distribution of dihedral angles in SiO2-like glasses

Abstract An analytical model for the distribution of dihedral angles in AX2 tetrahedral glasses is discussed and compared with available data. The comparison with experiment for vitreous SiO2 is promising, but deteriorates for glasses like GeSe2 which have much smaller intertetrahedral angles and more regular rings than vitreous SiO2.

Central force model for the high frequency vibrational bands of glasses

Abstract We have set up a simple model for the high frequency vibrational modes of A n X m glasses (i.e. those modes which are probed most easily in infrared absorption and Raman scattering experiments). This model involves using only nearest neighbor central forces and is a generalization of previous work by Sen and Thorpe for AX 2 glasses in which the vibrational spectrum is derived from the connectivity matrix of the structure. Simple algebraic expressions are obtained for the band edges and applied to the spectra of glassy GeS 2 , GeO 2 and P 2 O 5 . The dominant features of the spectra are associated with particular band edges in the central force model, taking due account of selection rules governing polarized Raman scattering in network glasses. Errors to be expected from the omission of non-central forces and long-range Coulomb forces are discussed.

On the thermal history of Libyan Desert glass

Abstract Libyan Desert glass shows polarized Raman spectra that are virtually identical to those of pure fused silica. We use the time-temperature dependence of the D2 = 606 cm−1 “defect” line to gain information about the thermal history of one of the naturally occurring samples. The effective fictive temperature of several samples is (1000±50)°C. From measured annealing rates for the D2 defect we conclude that our samples did not originally cool from the melt in seconds, nor did they cool from high temperatures over geological periods of time.