Showing papers in "Journal of Non-crystalline Solids in 1981"

Topology of covalent non-crystalline solids II: Medium-range order in chalcogenide alloys and ASi(Ge)

Abstract: Characteristic order over distances of 15–30 A is indicated by various experiments on covalent non-crystalline solids. The data are reviewed and detailed structural models are developed containing 30–1000 atoms.

Raman and IR absorption spectroscopic studies on α, β, and amorphous Si3N4

Abstract: Vibrational excitations of α- and β-Si 3 N 4 were studied by Raman and infrared absorption spectroscopies. Group theory was employed to interpret the observed spectra of crystalline α- and β-Si 3 N 4 . Both Raman and IR spectra of amorphous Si 3 N 4 exhibit two broad peaks centered at about 400 and 900 cm −1 . Amorphous Si 3 N 4 was found to be of the non-molecular, random-network type.

Temperature dependence of dielectric losses in chalcogenide glasses

Abstract: The frequency (2 × 10 4 −2 × 10 7 Hz) and temperature (250–600 K) dependence of the imaginary part of the permittivity, ϵ′', of As 2 Se 3 and Tl 2 SnSe 8.61 is explained within a theoretical model. The dielectric premittivities of these compounds are correctly explained by means of a model of a hopping process over a potential barrier between localized sites. We have found that the glassy system of chalcogenide can exist in the form of dipoles. The theory shows, in agreement with the experimental results, that ϵ ′' = Aω m , where m is a function of temperature.

Description of chemical ordering in amorphous alloys

Abstract: Partial coordination numbers Z ij ∗ for a chemically disordered binary amorphous alloy A X A B X B are obtained in terms of total coordination numbers Z i and average coordination numbers 〈 Z 〉 Z ij ∗ =x j Z i Z j /〈Z〉 . Departures from complete chemical disorder are characterized by a short-range order coefficient with η AB max = x B Z B / x A Z A for X B Z B x A Z A and η AB max = x A Z A / x B Z B for x B Z B > x A Z A . For complete chemical disorder ν AB 0 = 0; for complete chemical order, i.e. with chemical preference for AB nearest neighbor pairs, ν AB 0 = 1. For departures from chemical disorder associated with clustering, i.e. chemical preference against AB nearest neighbor pairs, ν AB 0 η AB max is necessary to compare the degree of ordering in the metal-metalloid and metal-metal alloys.

Raman spectra of binary high-silica glasses and fibers containing GeO2, P2O5 and B2O3

Abstract: The Raman spectra of binary high-silica glasses have been studied. The main peaks at 808 cm −1 and 710 cm −1 in vitreous B 2 O 3 and vitreous P 2 O 5 , respectively, are greatly reduced in binary high-silica glass, whereas a peak at 425 cm −1 due to GeOGe vibration and a peak at 1320 cm −1 due to P = O vibration remain strong, increasing in intensity with decreasing SiO 2 concentration. In the stimulated Raman spectra of a P 2 O 5 -SiO 2 glass fiber pumped by a mode-locked and Q-switched Nd:YAG laser at 1.064 μm, strong Stokes emissions due to the P = O vibration have been observed at 1.24 μm and 1.48 μm. In the spectra for a GeO 2 -SiO 2 glass fiber, four narrow-width Stokes emissions due to the GeOGe vibration have been observed at 1.115, 1.172, 1.235 and 1.305 μm.

Interstice correlation functions; a new, sensitive characterisation of non-crystalline packed structures

Abstract: A dense random packing of soft spheres is developed as a model of an “ideal” single component metallic glass. This structure can be considered as an assembly of distorted octahedra and tetrahedra, as can simple close-packed crystal structures. The more complex polyhedra (Bernal holes) previously identified in hard sphere models are largely squeezed out as a result of the additional free volume conferred by the soft potential. The local correlations of these basic tetrahedral and octahedral building blocks are discussed as possible characterisations of non-crystalline packed structures, together with their organisation around the component atoms. These characterisations of local correlations seem more sensitive and more reliably interpreted than the Voronoi polyhedra, particularly in detecting local icosahedral structures. Extensions of the descriptive techniques developed include examinations of weighted interstice connectivity, using data on the sizes of the interconnecting largely triangular “necks” as weighting factors. Departures from this idealised framework of linked tetrahedra and octahedra can be considered as defects, or distortions from the idealised structure, and the relative populations of local groupings provide a possible route to configurational entropy calculations.

On mechanisms of structural relaxation in a Pd48Ni32P20 glass

Abstract: Structural relaxation processes are investigated calorimetrically for a pre-conditioned Pd48Ni32P20 glass over a wide temperature range from well below to just above the glass transition. The low temperature anneals further stabilize the glassy structure. Upon heating, the annealed sample shows an excess endothermic specific heat ΔCp above the annealing temperature and completely recovers the initial enthalpy before any manifestation of glass transition, Tg. Significantly the ΔCp peak evolves in a continuous manner with annealing time. A physically reasonable activation energy spectrum N 0 ∗ (Q) is obtained with the proper choice of coupling constants which are dependent on annealing temperature. Results suggest the existence of localized relaxation modes which do not contribute to macroscopic flow. A concept of distribution in glass transition temperatures H(Tg,m) is conceived to account for the reversible relaxation with temperature. A model glass transition based on percolation theory is proposed and is found to reproduce the calorimetric relaxation phenomena well.

Analysis of CaOSiO2 and CaOSiO2CaF2 glasses by Raman spectroscopy

Abstract: Raman spectra of CaOSiO2 and CaOSiO2CaF2 glasses were measured to analyze three states of oxygen, i.e. bridging, non-bridging and free oxygen, in a similar way to the previous work on PbOSiO2 glasses. The stretching vibrational modes of the SiO bond in CaOSiO2 and CaOSiO2CaF2 glasses corresponded to the bands at 880, 920, 975 and 1050 cm−1. It is suggested from the comparison of the Raman spectra of the glasses with the Raman and infrared absorption spectra of crystalline silicates that these bands would arise from the SiO4 tetrahedron with four, three, two and one non-bridging oxygens, respectively. The fractions of bridging, non-bridging and free oxygens were calculated from the intensities of the four Raman bands and the composition of the glasses. They were in agreement with those obtained from the thermodynamical model for the CaOSiO2 glasses. When the content of CaF2 was smaller than 15–20 mol.% and the CaO SiO 2 ratio was smaller than unity, CaF2 contributed to the breakage of some SiO bonds.

Formation and stability of nickel-zirconium glasses☆

Abstract: Glasses may be formed in two composition ranges: 33–42 and 60–76 at.% Zr. The crystallisation temperatures (Tx), activation energies (Qx), crystallisation morphologies, densities, Young's moduli and peak in the X-ray diffraction pattern (Qp) are reported as a function of composition. Tx, Qx and Qp do not interpolate smoothly between the two composition ranges. Maximum thermal stability occurs at 36 at.% Zr, well away from the value predicted by the free electron approach of Nagel and Tauc. The glasses are denser than would be expected from a simple dense random packing of hard spheres. Some implications for the structure of all-metal glasses are discussed.

Photoluminescence in as-drawn and irradiated silica optical fibers: an assessment of the role of non-bridging oxygen defect centers

Abstract: The spectral and temporal characteristics of the photoluminescence in as-drawn and irradiated silica and doped silica fiber-optic waveguides have been investigated. The extended pathlength available with a fiber-optic geometry has offered the opportunity to make high sensitivity emission measurements on high silica glasses under both steady state and pulsed laser excitation. The analyses of the fiber data coupled with emission studies on selectivity doped bulk glasses suggest that the dominant emission band centered near 650 nm is intrinsic to defects in the SiO network, specifically, dangling non-bridging oxygens ions which can be generated by irradiation, fiber drawing or by the introduction of network modifying ions such as alkali.

Colloidal versus polymer gels and monolithic transformation in glass-forming systems

Abstract: There are three basic types of precursor solution used to form oxides and glasses through gellation: solutions of metallic salts, colloidal sols, and solutions of polymerizing species. Although an intermediate state in all three cases is called a gel, there are fundamental differences in the gelling mechanism and nature of the gel in these systems. In colloidal systems gelling is a result of electrolytic effects which also determine the interparticle distance at the gelling point, thus influencing the capability of the gel to remain monolithic during the transformation to an organic-free state. On the other hand, gelling in polymerized systems occurs as a result of polymerization by chemical reaction. The nature and kinetics of these reactions determine the properties of the gel and resultant inorganic polymer.

Optical properties of Nd3+ in metaphosphate glasses

Abstract: Absorption and fluorescene spectra and fluorescence lifetimes of Nd 3+ ions were measured for the following polymeric metaphosphate glasses: [ M (PO 3 ) 2 ] n , where M = Mg, Ca, Sr, Ba, Zn, Cd, and Al(PO 3 ) 3 . Judd-Ofelt intensity parameters for f-f transitions were derived from the integrated absorption spectra and used to calculate the spontaneous emission probabilities from the 4 F 3 2 state. Metaphosphate glasses exhibit systematic variations of refractive indices, optical intensity parameters, fluorescence lifetimes and linewidths, and stimulated emission cross sections with alkaline earth. The origin of these variations and their implications for tailoring spectroscopic properties by compositional changes are discussed. Neodymium laser action in metaphosphate glasses is also considered.

On the compositional dependence of the optical gap in amorphous semiconducting alloys

Abstract: It is found that the optical gap EAB for amorphous A-B alloys can be determined by the energy gap EA for element A and EAB for the element B in the equation EAB(Y) = YEA + (1−Y) EB where Y is the volume fraction of element A. Calculations based on a random bond network agree with experiments for SiGe, SbSe, and AsTe films (class A). Calculations based on a chemically ordered bond network which tends to form microscopic molecular species gree with experimental results for the AsSe, AsS, GeTe and Sb2Se3As2Se3 systems (class B). In contrast to the above systems, agreement with experiment is not obtained for the TeSe, As2Te3As2Se3 and GeTe2GeSe2 systems which contain atoms of both Te and Se (class C). The classification into three types (classes A, B and C) is consistent with the calculation based on effective medium percolation theory which interprets the compositional dependence of the conductivity of chalcogenide glasses.

B11 nuclear magnetic resonance studies of Li2OB2O3 glasses of high Li2O content

Abstract: A B 11 NMR study has been made of lithium borate glasses spanning the composition region from 375–65 mol% Li 2 O Values were obtained as a function of R = mol% Li 2 O/mol% B 2 O 3 for the fraction of boron in BO 4 units ( N 4 ), the fraction of symmetric BO 3 units ( N 3S ) which have either zero or three non-bridging oxygens (NBO's), and the fractions of BO 3 units with one ( N 3A1 ) NBO's A structural model is presented for the region 1 ⩽ R ⩽ 213 which involves the destruction of metaborate and loose BO 4 units as Li 2 O is added This extends an earlier model for the region 05 ⩽ R ⩽ 1 involving the destruction of diborate units to form metaborate and loose BO 4 units The data are in generally good agreement with model predictions in the entire range (0 ⩽ R ⩽ 186) for which glasses can be made

Continuous cooling approximation for the formation of a glass

Abstract: Non-isothermal equations describing the liquid-crystal transformation are derived using the isothermal Avrami equations. A theoretical expression for the critical cooling rate for the formation of a glass is found. Calculations based on this expression are in better agreement with experimental values than those derived from TTT (time-temperature-transformation) curves. A study performed on typical glass forming materials enables the glass forming ability (GFA) to be determined by experimentally measuring crystallization temperatures at different cooling rates which are easily accessible with commonly available technology. The behaviour of the rate constant for crystallization is also obtained from the same data in the experimental range considered. In both cases no previous knowledge of the parameters involved is needed. With some assumptions the values of the viscosity in the crystallization temperature range can be estimated. Although the study was performed for an Avrami index of 4 an extension to other values of n is made under some restricted conditions and a more general treatment is outlined.

The quenching and electrochemical behavior of Li2OV2O5 glasses

Abstract: Rapid quenching from the melt produced glass in V2O5 and in Li-containing compositions up to Li6V4O13 The oxygen content was a function of the quench temperature The crystallization paths of these glasses were studied by differential thermal analysis and powder X-ray diffraction The electrochemical reactions of these glasses with lithium were studied in Li/LiAsF2-propylene carbonate/vanadium glass cells LiV3O8 glass, as well as the mixture first to crystallize from this glass, was comparable to crystalline LiV3O8 with an initial, reversible storage capacity of about 15 Li per formula weight The other glasses, including V2O5 quenched from various temperatures, exhibited inferior extent and rate of cell reaction compared to those of their crystalline analogs

Use of regular polytopes for the mathematical description of the order in amorphous structures

Abstract: The local order is well defined in amorphous material, but filling all the space with a unit cluster involves distortions. This is due to an incompatibility between the local order symmetry and the properties of euclidian space. Nevertheless curved spaces can be tessellated with a given cell; in this paper we discuss the case of tetrahedral packing. It is well known that tetrahedra cannot fill euclidean space, but they can be packed together to build a regular structure (called a polytope) in a space of uniform positive curvature. The disorder in amorphous materials is then described in terms of defects occurring during the mapping of curved spaces on the euclidian space. This method of describing an amorphous structure provides a mathematical tool, similar to the crystallographic laws used to describe crystals.

Glass transition temperature dependence on heating rate and on ageing for amorphous selenium films

Abstract: The study, by differential thermal analysis (DTA) of the glass transition of thermally evaporated amorphous selenium, allows the effect of ageing on structural properties to be observed. An enthalpic diagram is used to explain the shift of the glass transition region, with heating rate, and also with ageing. These shifts are directly dependent on structural relaxation in amorphous selenium. The model used, to express the change of the structural time relaxation with temperature, and with the departure of thermodynamic equilibrium leads to a slightly different expression for the dependence of T g on heating rate than that previously published (by Moynihan et al.). The activation enthalpies of the studied phenomena compared with other results, seem to show that structural relaxation in a-Se principally involves bond breaking and reformation of chains.

Density and microhardness of GeSbSe glasses

Abstract: The results of measurements of microhardness and density on families of glasses of the GeSbSe system are reported. Microhardnes shows a monotonic increase with Ge content for all the families of glasses studied. The stoichiometric composition in each family exhibits a minimum in density and a maximum in molar volume as compared with the Se-rich (Ge-rich) glasses of the corresponding family. A calculation based on the volume additivity of the individual elements and compounds satisfactorily explains the observed dependence of density on composition. The density results further indicate that the molar volume occupied by each of the glasses with composition (GeSe2)1−C (Sb2Se3)C is nearly constant and is equal to 18.05 ± 0.06 cc/mol.

Glass formation and crystallization in highly undercooled Te-Cu alloys

Abstract: The large undercoolings required for glass formation have been achieved by the slow cooling (10-20°C/min) of liquid Te-Cu alloys in the form of a fine droplet emulsion. Within the region of glass formation, between 19 and 39 at.% Cu, DTA measurements indicate that the glass ( T g ) and crystallization ( T c ) temperatures during heating exhibit a broad maximum at the eutectic. During slow cooling of Te-rich alloy droplets, the maximum undercooling for nucleation increases from 213°C for pure Te to 264°C for Te-12.5 at.% Cu. An enhanced depression of the nucleation ( T n ) temperature compared with the change of the liquidus develops in Te-rich alloys upon approaching the glass forming composition range and can be a useful feature in assessing the glass forming tendency. Thermal cycling experiments indicate that even at an undercooling of 181°C crystallization in an eutectic Te-29 at.% Cu alloy is limited by an inadequate nucleation rate in clean droplet samples. For a eutectic alloy, at undercoolings in excess of 200°C crystal nucleation does develop in the droplet samples, but complete crystallization is hindered by a rapidly rising liquid viscosity with increased undercooling.

Interstitial structure and diffusion in DRP metallic glasses

Abstract: Interstitial diffusion in dense random packed structures is discussed and the literature on the description of interstitial space in these structures reviewed. The numbers and sizes of the principle sphere configurations at saddle points and interstitials are determined for three model structures: (1) the Finney hard sphere model; (2) the Finney model relaxed under a Morse potential; and (3) the Scott model relaxed under a Lennard—Jones potential. The results obtained are in good agreement with recent work by other authors. Finally, the implications of the results for interstitial diffusion are discussed and crude estimates for the activation energy obtained.

Ultrasonic and brillouin scattering study of the elastic properties of vitreous silica between 10 and 300 K

Abstract: The velocity and attenuation of longitudinal and transverse waves have been measured for frequencies ranging between 5 MHz and 35 GHz on the same sample of vitreous silica. The main features of the ultrasonic results are described on the basis of the usual model assuming a coupling of phonons with a thermally activated relaxation. In contrast, the frequency dependence of the attenuation peak and velocity minimum are in quantitative disagreement with the calculations based on this model. Above 100 K, at least half of the attenuation at high frequencies has to be assigned to another physical origin, most likely anharmonic three-phonon interactions. Furthermore, the minimum in the hypersonic velocity seems to occur as the result of a combination of dispersion processes more than from the frequency-temperature shift of a relaxational dispersion.

Optical and magnetic properties of first-row transition metal ions in lead-silicate glass

Abstract: First-row transition metal ions in lead-silicate glasses of composition: 37.9% mol PbO, 61.8% mol SiO2 have been studied with regard to the oxidation state and the coordination by means of optical and magnetic measurements. Optical spectra have been recorded at 300 and 77 K. All the observed bands are assigned as being due to d-d transitions in Td and/or Oh fields. Magnetic moments are of the order of the spin-only values. By ESR measurements we reveal and quantify titanium, vanadium and manganese oxidation states not detectable by the other techniques. It turns out that the various metal ions have the following oxidation states: Ti3+ and Ti4+, V4+ and V5+, Cr3+ and Cr6+, Mn2+ and Mn3+, Fe3+, Co2+, Ni2+, Cu2+. The coordination is 4-fold for Ti4+ and V5+ and 6-fold for Mn3+ and Cu2+. Moreover it has been found that Fe3+, Co2+ and Ni2+ are both 4-fold and 6-fold coordinated and that Cr3+ and Cr6+ oxidation states coexist in the same sample.

A model of photostructural changes in chalcogenide vitreous semiconductors: 1. Theoretical considerations

Abstract: Light-induced processes in chalcogenide vitreous semiconductors are considered in the framework of a configurational model of two stable states (ground and metastable) of atomic units with optical and thermal transitions between these states. Probabilities of these transitions as functions of photon energy and temperature are calculated. an expression for the metastable state population kinetics is given.

Low temperature synthesis of non-crystalline solids of the system SrO-SiO2

Abstract: Non-crystalline solids within the liquid-liquid immiscibility region in the system SrO-SiO 2 have been prepared from a gel obtained by the hydrolysis of silicon tetramethoxide with an aqueous solution of strontium nitrate. The gel which was porous and translucent at room temperature increased in transparency with heating due to the collapse of micropores until it became completely clear. The gel became opaque again due to the precipitation of α-quartz at higher temperatures. The critical temperatures below which clear solids were obtained fell on a line connecting the glass transition temperatures of vitreous silica and those of SrO-SiO 2 glasses prepared by melting. The density and refractive index of the pore-free, clear glassy solid, changed continuously with the SrO content along lines connecting those of vitreous silica and SrO- SiO 2 glasses of high SrO content prepared by melting. The maximum amount Sr 2+ which could be introduced using an aqueous solution as the starting material corresponded to a composition of 10 SrO · 90 SiO 2 by weight.

The structure of germanosilicate glasses, studied by X-ray photoelectron spectroscopy

Abstract: O1s photoelectron spectra of several alkali silicate, germanate and germanosilicate glasses are given. The concentration ratio of bridging and non-bridging oxygen atoms is determined from these spectra. In contrast to silicate glasses, in germanate glasses non-bridging oxygen atoms are only observed when the alkali oxide mole fraction exceeds 0.18. The introduction of SiO 2 in the germanate glasses drastically alters the ratio of fourfold coordinated to sixfold coordinated germanium atoms.

New transition metal fluoride glasses isolated in the PbF2MtIIF2MtIIIF3 systems

Abstract: New glasses have been prepared in the PbF 2  M t II F 2  M t III F 3 systems ( m t II = Mn 2+ , Fe 2+ , Co 2+ , Ni 2+ , Cu 2+ , Zn 2+ ; M t III = Fe 3+ , V 3+ , Cr 3+ , Ga 3+ ). The extent of the vitreous area is shown in a PbF 2 MnF 2 FeF 3 diagram. Thermal properties have been measured for all samples. Some of these glasses are very transparent over a wide range of wavelengths (from 250–12 000 nm). The sixfold coordination of transition metal ions has been established by spectroscopic investigations. The structure of the glasses is discussed on the basis of a random corner-sharing of MF 6 octahedra.

An exafs study of the structure of lithium germanate glasses

Abstract: The extended X-ray absorption fine structure (EXAFS) of a series of lithium germanate glasses were obtained using synchroton radiation. Structural parameters were extracted from the data using an iterative fitting algorithm designed to minimize the ‘sum of squares’ difference between the experimental data and an ab initio theoretical calculation of the unapproximated (‘curved wave’) EXAFS expression. We identify two distinct co-ordination sites in the glasses with germanium-oxygen bond lengths which correspond closely to the bond lengths found in the crystalline forms of germanium dioxide. With the additional assumption that these sites are four-fold and six-fold co-ordinated, we conclude that the ratio of six-fold to four-fold sites increases with the addition of alkali oxide, up to a maximum of around 34%.

Structures, spectra and related properties of group IVB-doped vitreous silica

Abstract: Vibrational spectra and related physical properties were analyzed for titanium-, zirconium- and hafnium-containing vitreous silicas on the basis of glass structure. Linear functional relationhips were found between both partial molar volume and refractivity, and the dopant concentration throughout the dopant range examined. Analysis of the data suggested four-fold coordination with oxygen atoms by titanium atoms with greater coordination by zirconium and hafnium atoms. Raman and difference infrared spectra for these glasses showed an intensity decrease of the v3 (F2) band at 1068 cm−1 accompanied by the emergence of a band at circa 950cm−1 whose exact wavenumber was defined by the dopant species ( gv Hf > gv Zr > gv Ti ) and, in the case of TiO2- containing glasses, a band at 1110 cm−1. The intensities of these bands were proportional to the dopant concentration. Empirical spectral analysis suggested that upon TiO2 addition Ti-atoms bonded to a single oxygen atom of a SiO4-unit, while upon zirconia and hafnia addition each metal atom bonded to two oxygen atoms of a SiO4-unit. These structural models were consistent with ion coordination predictions based upon the physical property measurements.