Short-range Order In Glasses Research Articles

ХАЛЬКОГЕНІДНІ СТЕКЛА: СТРУКТУРНІ І ОПТИЧНІ ВЛАСТИВОСТІ (ОГЛЯД)

Structural properties of chalcogenide glasses mainly on the example of binary As-S(Se) and Ge-S(Se) systems and ternary Ge-As-S(Se) systems, structural models, parameters of short range order of glasses obtained using diffraction methods, EXAFS and Raman spectroscopy are considered. Raman spectra of binary As-S(Se) and Ge-S(Se) systems and ternary Ge-As-S(Se) systems, structural models that are used for interpretation of Raman spectroscopy results are considered. Optical properties of chalcogenide glasses and optical absorption edge in binary and multicomponent systems are discussed. The refractive index and its wavelength dependence, other optical properties are among important parameters that determine the suitability of materials as optical media. Refractive and absorption indexes, optical band gap of chalcogenide glasses can be changed by doping of different elements. The results suggest a combined effect of chemical ordering and topological in such glasses (parameters dependence on average coordination number, composition, nanophase separation, etc.). Importance of study of interrelation of structural and physico- chemical properties is stated. As frequently pointed out by various researchers, chalcogenide glasses are promising materials for various applications because they are transparent over a wide range of wavelengths in the infrared region, they possess high linear and non-linear refractive indices, number of photoinduced effects, low phonon energies and are easy to fabricate. Applications of chalcogenide glasses cover wide range, among them: IR optics, recording and storage of information, xerography, thermoplastic and holographic media, inorganic resists, optical filters, diffraction optical elements, non-linear elements, fiber and integrated optics, etc. Composition-structure-properties correlations are convenient to tailor the physical, optical and other properties of chalcogenide glasses and provide an important reference for the further development of new chalcogenide glasses taking into account their possible applications.

Effect of B2O3 concentration on the local atomic structure of lanthanum in lanthanum-borate glasses: XANES study and the principle of crystal-chemical similarity of the short-range order in glasses and crystals

Dependence of lanthanum local atomic structure in borate glasses La2O3-Nb2O5-B2O3 upon component composition was studied combining XANES, computer simulations using crystal chemical similarity (CCS) of a short-range order in glass and crystal, and multidimensional interpolation of spectra as the function of structural parameters. By this approach, the values of La-O bond lengths and angles in chains O-La-O were determined. Comparison of the obtained radial and angle distribution functions of oxygen atoms around La in the studied glasses with the same functions of the reference crystals revealed that within the existing CCS of La local structure in glasses and in B3LaO6, there is a decrease in the average length of La-O bonds by ∼ 0.1 Å and decrease of FWHF by ∼ 0.2 Å in the radial distribution of nearest O atoms, under retaining angle distribution of these atoms. It was shown that the ratio of intensities of the first two peaks in experimental La L3-edge XANES is an effective fingerprint of the number of nearest oxygen neighbors of lanthanum. Using the dependence of this ratio upon the component composition of glasses, the changes in the local structure of La under decreasing of B2O3 fraction from 72.5 to 47.5% were determined.

The properties and structure of Ge[sbnd]Se[sbnd]Te glasses and thin films

Ge30Se70−xTex bulk glasses and thin films for x=0, 10 and 20 were prepared. The properties and structure were characterized by differential scanning calorimetry (DSC), Raman spectroscopy and also by 77Se and 125Te MAS NMR spectroscopy. The optical properties were also measured. The absence of crystalline phases was verified by XRD, while DSC indicated that the glasses were not phase separated. Although the glass transition temperature of both Ge30Se70 glass and thin film is the same, different ΔCp reveal a higher disorder of the thin film. Both the glass transition temperature and ΔCp of Te glasses as well as crystallization temperatures decrease when Te increases due to increasing GeTe2/2 instead of GeSe4/2 structural units thereby reducing 3D interconnection of the glassy network. Based on the 77Se MAS NMR results, the short-range order model was determined as a suitable structural model for Ge30Se70 glass. 77Se and 125Te MAS NMR confirmed that Ge in alloys with Te only reaches oxidation state +II in agreement with GeTe phase diagram and therefore no GeTe4/2 tetrahedra can be present in these glasses and/or films. Raman spectroscopy confirmed that the short-range order of glasses and films is basically the same. The refractive index of thin films increases with tellurium content. The optical band gap of the thin films strongly decreases from 2.02eV to 1.36eV with increasing Te content and simultaneously the increasing structural disorder and width of the band tails increase. Reduction of the oxidation state of germanium to GeII caused by a reaction with tellurium ultimately leads to a reduction in 3D connectivity of the non-crystalline matrix and the loss of the glass-forming ability of the GeSeTe system.

Angstrom-beam electron diffraction of amorphous materials

We have developed an Angstrom-beam electron diffraction (ABED) technique for structure characterization of amorphous materials using a state-the-of-art spherical aberration-corrected scanning transmission electron microscope. The focused electron beam with a diameter of ~0.4nm, comparable to the size of short range order in glasses, enables us to directly detect local atomic structure of disordered materials in a diffraction mode. In this paper we briefly introduce the basic principle of ABED and preliminary applications in structural characterization of metallic glasses and oxide glasses. We also discuss the effect of sample thickness and the method to study medium range order of glassy materials using ABED.

The local structure and magnetic interactions between Fe3+ and V4+ ions in lithium–phosphate glasses

Electron Paramagnetic Resonance (EPR) provides a useful tool not only as a probe of local structure and short range order in glasses, but also of magnetic interactions in the glasses containing suitable magnetic ions. We have analyzed the spectra of xFe2O3·(100 − x)[P2O5·Li2O] and x(Fe2O3·V2O5)·(100 − x)[P2O5·Li2O] glass systems, with 0 < x ≤ 50 mol %. [P2O5·Li2O] stands for 50Li2O·50P2O5 glass composition. For samples x > 50 mol % a study indicates the presence of crystalline α Fe2O3 in the glasses. Observed spectra have resonance lines centered at g ∼ 4.3 and g ∼ 2.0 typical for Fe3+ and V4+ ions present in the oxide glasses. For low contend of transition metal (TM) oxides (Fe2O3 or V2O5·Fe2O3) the spectra present a hyperfine structure typical for isolated V4+ ions. With the increasing of TM content, the EPR absorption signal showing hyperfine structure superposed by a broad line without hyperfine structure characteristic for clustered ions. At high TM content, the vanadium hyperfine structure disappears and only the broad line can be observed in the spectra.

Structural and thermal properties of some zinc borate glasses containing gadolinium ions

Glasses in the system (Gd2O3)x·(B2O3)(60−x)·(ZnO)40 (0 ≤ × ≤ 15 mol%) have been prepared by the melt-quenching technique. The local structure and the thermal behaviour of the obtained glasses were investigated using X-ray diffraction (XRD), differential thermal analysis (DTA) and Fourier transform infrared (FTIR) spectroscopy. The XRD pattern of the glasses confirmed their amorphous nature. Compositional dependence of the glass transition (Tg), fragility index (F), activation energy of the glass transition (Eg), activation energy for the crystallization process (Ep) were determined by DTA investigations. FTIR data revealed that the short-range order of glasses is affected by the addition of the Gd2O3 to the host glass matrix. The compositional dependence of the different structural units which appear in the investigated glasses was followed. FTIR data suggest that the gadolinium ions play the network modifier role in the studied glasses.

Electron diffraction study of the short-range order in glasses of the system TeO2B2O3

The short-range order in vitreous TeO2 and B2O3 and in TeO2B2O3 glasses has been studied using electron diffraction. The amorphous samples investigated were prepared by two methods. Powdered samples were produced either by rapid quenching using the roller technique (TeO2 glass) or by slow cooling (B2O3 and TeO2B2O3 glasses) and amorphous TeO2 and TeO2B2O3 thin films were deposited by vacuum evaporation with resistive heating. According to the electron diffraction data, the TeO4 polyhedron is shown to be the main structural unit in the TeO2 glass. The boron atoms in the B2O3 glass are found to be threefold-coordinated with respect to the oxygen and the presence of B3O6 groups is indicated. In the binary glasses mainly TeO or BO distances are resolved, depending on their composition. The results for the powdered and thin film samples are compared and a change in the TeO distances for the films is established.

Icosahedral order in glass: Electronic properties.

Icosahedral short-range order in glass is modeled by a crystalline packing of atoms in ${S}^{3}$, known as polytope 120. We diagonalize realistic electronic Hamiltonians using the symmetry group of polytope 120. We predict band structures for amorphous semiconductors and transition metals. Icosahedral order in silicon produces a gap in the center of the valence band. The absence of d-level splitting at the center of the Brillouin zone distinguishes icosahedrally ordered transition metals from their fcc counterparts. We use the d-band density of states of polytope 120 to understand band ferromagnetism in amorphous transition metals.

Structural characterization of soft-core and hard-core glasses by Delaunay tessellation

Methods of the Delaunay and Voronoi tessellation are applied to investigate significant structures and structural differences among liquids, glasses, and crystals of the soft-core (inverse 12th power potential) and hard-core models for which molecular dynamics (MD) techniques are used. With particle configurations obtained by MD simulations the Delaunay tessellation for the three dimensional systems defines uniquely a network of tetrahedra. Density dependent properties of the irregularity distribution which measures the magnitude of distortion of each Delaunay tetrahedron (DT) from regular shape (equal edge lengths) and of the length of edges of DT are examined in detail for both models. Clear differences between the hard-core and soft-core systems are observed. Short range correlation of DT is calculated on the pair distribution function of center-to-center distances of the circumspheres of DT, which turns out to be useful to distinguish the short range order of glasses from that of crystals or liquids. It is also shown that there are no clear differences in the distribution of various types of Voronoi polyhedron (VP) between liquids and glasses. However, among various types of polygons (i-edged faces) of VP, the contribution from i=5 increases rather rapidly with increasing density around the glass transition point.

The vibrational spectra of crystalline W 2O 3(PO 4) 2 and related tungsten phosphate glasses

IR and Raman spectra were measured and analyzed for crystalline W 2O 3(PO 4) 2. Factor group analysis indicates that the observed vibrational spectra can be interpreted better with the earlier determined space group P2 1 m−C 2 2h than with the space group P2 1− C 2 2. Band assignments for this material are consistent with those made earlier for related materials. IR spectra indicate that the short range order of glasses with compositions close to W 2O 3(PO 4) 2 crystals are similar to that of the latter crystalline material. As the WO 3 concentration increases in the glasses, the number of WO 6-octahedra possessing only W-O single bonds increases in the glass structure. PO bonds are found in the tungsten phosphate glasses containing higher P 2O 5 contents.

X-ray diffraction studies of glasses in the TeO 2V 2O 5 system

The short-range order in glasses of the TeO 2V 2O 5 system is studied with the aid of X-ray diffraction. It is found that the main structural units in most of the glasses are VO 5- and TeO 3-groups. The assumption is made that their structures gradually change; sheet complexes are formed close to V 2O 5; chains - around 2TeO 2V 2O 5, and a tridimensional aperiodic network in glasses from TeO 2.

Polymorphic transition in silica glass

THE short range order in glasses is now more or less generally accepted to have a dimension of 20 A or slightly more1,2. In the case of silica glass we have established3, on the strength of infrared absorption bands, that those regions (clusters, substructures) have the high-cristobalite structure, which is metastable at room temperature and normal pressure. It has been possible to relax a polymorphic transformation in pure silica glass into the stable low temperature form of cristobalite by heating a powdered sample at 483 K for 27 h (ref. 3). Heating relaxed the existing strains in the clusters by adjusting their structure to the low temperature conditions. A report by Wagstaff4 suggested another possible way of provoking the transition—by increasing the strains in the high-cristobalite clusters in silica glass by cooling it to liquid helium temperature.