Local structure of semiconductor glasses in the germanium-sulfur and germanium-selenium systems

¹²⁹Te(¹²⁹I) and ¹¹⁹Sn Mössbauer investigations have revealed that the structure of Ge_{100 ? x} S _x and Ge_{100 ? x} Se _x glasses enriched in the chalcogen (x ≥ 0.66) is built up of structural units containing twofold-coordinated chalcogen (X) atoms in chains of the

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and

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types, whereas germanium atoms in these glasses have the oxidation state of only +4 and are fourfold-coordinated so that the local environment of the germanium atoms includes only chalcogen atoms. The structure of the glasses depleted in the chalcogen consists of structural units containing twofold-coordinated chalcogen atoms in chains of the

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type and threefold-coordinated chalcogen atoms in chains of the

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type, whereas germanium in these glasses is stabilized both in the oxidation state of +4 and the fourfold-coordinated state and in the oxidation state of +2 and the threefold-coordinated state so that the local environment of the germanium atoms includes only chalcogen atoms.

     

Temperature of nanometer-scale structure appearance in glasses

The structural network of glass is known to be partitioned into quasiordered nanometer regions (clusters). It is considered that the formation of nanometer-scale inhomogeneities occurs still in a liquid state in the process of cooling and substantially affects the ability of the substance to glass transition. In this work, we discuss the problem of determining the temperature at which a glass nanometer-scale structure is formed. Based on the analysis of the inhomogeneous broadening of Raman spectrum lines and the Landau-Placzek ratio, we show that the formation of an inhomogeneous structure starts near the temperature of the transition from the Arrhenius behavior of the relaxation time (viscosity) to non-Arrhenius behavior. This temperature can be determined using the derivative analysis.     

Anisotropic structure of alkali metaphosphate glasses

Orientation anisotropy, which is well known in organic polymers with appropriate network structures, is less common in oxide glasses. We present the intermediate-range order in anisotropic alkali metaphosphate glass which consists of oriented PO₄ tetrahedral chains and intervening alkali cations along the elongation direction. The X-ray total structure factor S(Q) indicates that the inter-chain spacing depends on the size of alkali cations and varies from 5.03 to 6.28 Å. The mixed alkali effect is primarily related to an increase of the separation. The total correlation function T(r) provides the first definite evidence that the anisotropic structure is composed of phosphorus-bridging oxygen bonds (P–O_B) lying along the elongation direction and phosphorus-non-bridging oxygen bonds (P–O_T) oriented perpendicular to the elongation direction. The present result unveils fundamental aspects of the anisotropic structure of an oxide glass and provides essential information for the development of oxide glasses to control structural anisotropy.     

Computational insights into the structure of barium titanosilicate glasses

Understanding the role of TiO₂ in BaO-TiO₂-SiO₂ (BTS) glasses is one of the keys to develop new glasses and glass-ceramics for different technological applications. For the first time, molecular dynamics simulations were conducted to get new insights into the atomic structure of the BTS glasses and their elastic moduli. Various compositions are studied where SiO₂ have been replaced by TiO₂. The calculated mechanical properties of our models are observed to depend linearly on TiO₂ content. However, the structure-induced changes are far from such dependence. The structural results indicate that BTS glasses are mainly built on four types of basic units: SiO₄, TiO₄, TiO₅ and TiO₆. This high structural heterogeneity induced by the three coordination states of Ti is found to have an impact on the medium range order by increasing the rings number, the polymerized regions, and by transforming Q₃-Q₄ and Q₂ without neglecting the increase in Q₅ and Q₆ species. Those structural modifications of the BTS glass network features have been found to be consistent with available experimental data.     

Ion-resistive effect in surface layers of glasses and glass-ceramics in the Cu-As-Se and Cu-As-Te systems

A linear logarithmic dependence of the electrical resistance of glasses and glass-ceramics in semiconductor chalcogenide systems in cupric salt solutions on the cation content in the solution is established for the first time. The revealed effect can be used in chemical analysis for both fundamental and practical applications, in particular, for the development of chemical analysis methods.     

On crack branching angles in glasses and ceramics

Cracks branch when they run unstably at high velocities and there is sufficient energy to create new fracture surfaces. Observations of crack branching angles are useful in component fractographic analysis since they can be used to estimate the stress state in a part when it breaks. Uniaxial loading causes branch angles of 30–45 degrees, although scatter can be large. Biaxial tensile loading causes larger branching angles. The first quantitative trend of crack branching angle with stress state was shown by Frank Preston in a brief paper in 1935. Over the decades many researchers have obtained smaller branching angles than those shown by Preston. A review of the literature and some new data in this work show that Preston’s 1935 trend must be corrected. A new version is presented.     

Crystallization behavior and IR structure of yttrium aluminosilicate glasses

The crystallization of four Y₂O₃-Al₂O₃-SiO₂ (YAS) glasses were investigated to prepare YAS glass ceramics precipitated singly/mainly Y₂Si₂O₇ or Y_4.67(SiO₄)₃O apatite, and to explore the crystallization difference between the stoichiometric parent glass (SPG) and non-stoichiometric parent glass (NSPG). The DSC results revealed that glass locating at the higher liquidus surface temperature has lower crystallization peak temperature, which indicating that the corresponding glass has higher crystallization potential to crystallize easily. Crystallization of the NSPG samples is along surface and caused by phase separation, while SPG sample is the surface crystallization at the first exothermic peak temperature and overall crystallization at the second exothermic peak temperature. Glass ceramics only containing y-Y₂Si₂O₇ or Y_4.67(SiO₄)₃O apatite are obtained successfully, and which are illustrated by fitting FTIR spectra. These results can provide technical guide for controlling the crystallization process and the types of precipitated crystals in YAS glass for different application potentials.     

Atomic structure and dissolution properties of yttrium-containing phosphate glasses

We have conducted classical molecular dynamics simulations of three compositions of yttrium-containing phosphate glasses, to study the atomic structure around yttrium, and understand how yttrium incorporation will affect the glass dissolution rate. The Y-O bond length is about 2.2 Å and the coordination number is 6.3. To avoid effects due to different network connectivities, our compositions were chosen to keep the Qⁿ distribution and network connectivity roughly constant, which was confirmed through direct calculation. For these compositions, the structure of the phosphate network is comprised of finite-length chains of PO₄ tetrahedra bound to the network modifiers. We showed that yttrium bonds to 4.2-4.3 of these chains, compared to 3.8 for calcium, and 3.1-3.2 for sodium. This implies that yttrium will bond more parts of the glass to the same place, and therefore, that yttrium incorporation will strengthen phosphate glass against dissolution, making it potentially suitable for radiotherapy applications where a durable glass is required.     

Effect of irradiation on the atomic structure of borosilicate glasses

Borosilicate glasses incorporating high-level nuclear waste are exposed to high-energy radiations during their storage in the deep geological repositories. However, the effect of radiation on the atomic structure of borosilicate glasses remains poorly understood. Herein, using molecular dynamics simulations, we study the irradiation-induced structural changes of a series of calcium-sodium borosilicate glasses with varying Si/B molar ratios—ranging from pure silicate to pure borate glasses. We observe that irradiation leads to an increase in disorder, both in the short- and medium-range, as evidenced by the enthalpy, coordination number, and ring distribution. In particular, the impact of the change in the atomic structure (due to radiation) on the glass volume is investigated. Interestingly, we observe a composition-dependent transition in the volumetric response of borosilicate glasses under irradiation—wherein borate-rich compositions tend to swell, whereas silica-rich glasses tend to densify. Through a detailed analysis of the structure, we demonstrate two competing mechanisms contributing to the volume change, i.e., a decrease in the coordination number of boron atoms and a reduction in the average silicon inter-polytope angle. We also show that the increase in the disorder in the medium-range order may play a major role in governing the volumetric changes in the irradiated structure in a non-trivial fashion. Altogether, the present study highlights that irradiation has a non-trivial effect on borosilicate glasses, which, in turn, could impact their corrosion kinetics.     

On the durability of glasses in the range of low stresses

An asymptotic relationship for the average durability of glasses in the range of low stresses, where the Zhurkov formula of the average durability becomes invalid, has been obtained in the framework of the structural and statistical theory of brittle fracture of glasses.     

On the critical displacement of excited kinetic units in liquids and glasses

The critical displacement of an atom (a group of atoms) in inorganic glasses Δr _m, which corresponds to the maximum of the interatomic attractive force, is calculated using available data on the surface tension and elastic constants. It is found that the critical atomic displacement Δr _m is close in order of magnitude to the linear dimension of the activation volume of atomic excitation v _h ^1/3 for glasses in the As-S and Ge-As-S systems with a chain structure and is considerably less than the value of v _h ^1/3 for alkali silicate glasses and glasses in the Cd-As system with a structure involving ionic sublattices. A relationship for calculating the activation volume of the atomic excitation from data on the glass transition temperature and elastic constants is derived within the model of an excited state.     

Study of the properties of bismuth-borate systems toward low-melting lead-free glasses

Data on glass formation, crystallization, and physicochemical properties in the binary (Bi₂O₃-B₂O₃) and ternary (PbO-Bi₂O₃-B₂O₃, ZnO-Bi₂O₃-B₂O₃, and BaO-Bi₂O₃-B₂O₃) systems are presented. The feasibility of glasses with high bismuth oxide content (>80 mol %) is addressed. The formation of a glass network of octahedral [BiO₆] structural units rather than [BiO₃] ones in such systems is shown.     

Unveiling crystallization mechanism for controlling nanocrystalline structure in glasses

Controlling nanocrystalline structure in glasses renders the exploration of new composite multiphase (glass-ceramic) materials with novel functionalities that determined by the precipitated nanocrystals and residual glassy matrix. Previous microstructural investigation of glass-ceramics focused only on one aspect of nanocrystalline structures, e.g., nano-polycrystalline or single nanocrystalline. The recognition of the microscopic mechanism of nanostructure formation in glasses is absent. Here, we use advanced microscopic techniques to show the formation of different nanocrystalline structures composed of nano-polycrystals and single nanocrystals in 80GeS₂·20In₂S₃ and 72.5GeS₂·14.5Sb₂S₃·13RbCl glasses, respectively. Crystallization mechanism for controlling the nanocrystalline structure in glasses was revealed to depend on whether the glass network former participates in crystallization process. The results may shed light not only on glass crystallization mechanism, but also on the fundamental nature of the network structure of chalcogenide glasses.     

Influence of water on the structure and kinetics of structural relaxation in sodium borate glasses

The influence of the water content on the microinhomogeneous structure of sodium borate glasses containing 2.5 and 5.0 mol % Na₂O and on the kinetics of structural relaxation in vitreous boron oxide and sodium borate glasses is studied by the small-angle X-ray scattering (SAXS) technique. It is demonstrated that an increase in the water content brings about an increase in the size of the inhomogeneity regions and a decrease in the mean square of the difference between the electron densities 〈(Δρ)²〉 for the microinhomogeneous structure. The inference is made that water has a catalytic effect on the aggregation of alkali borate structural units. The kinetic dependence of the intensity of small-angle X-ray scattering by thermal density fluctuations is described by the Kohlrausch equation. The exponent β in the Kohlrausch equation is close to 1.0 for “dry” glasses and decreases to 0.5 for glasses containing 0.7–1.0 mol % H₂O.     

IR spectroscopic study of water in the structure of binary alkali borate glasses

The influence of the synthesis conditions and composition of binary sodium and potassium borate glasses on the spectral absorption in the wavelength range 2.5–4 µm due to the presence of structurally bound water is investigated. It is demonstrated that water in the structure of alkali borate glasses, as in the structure of alkali silicate glasses, can exist in the form of so-called free and bound hydroxyl groups that are incorporated into the glass network in different ways. The ratio between the numbers of these forms of structural water in borate glasses is determined by the synthesis conditions and composition of the glass. The synthesis under different conditions makes it possible to prepare glasses not only with different water contents but also with different ratios between the numbers of free and bound hydroxyl groups.Original Russian Text Copyright © 2005 by Fizika i Khimiya Stekla, Golubeva, Pavinich.