R. Golovchak

Atomistic model of physical ageing in Se-rich As–Se glasses

Thermal, optical, X-ray excited and magnetic methods were used to develop a microstructural model of physical ageing in Se-rich glasses. The glass composition As10Se90, possessing a typical cross-linked chain structure, was chosen as a model object for the investigations. The effect of physical ageing in this glass was revealed by differential scanning calorimetry, whereas the corresponding changes in its atomic arrangement were studied by extended X-ray absorption fine structure, Raman and solid-state 77Se nuclear magnetic resonance spectroscopy. Straightening–shrinkage processes are shown to be responsible for the physical ageing in this Se-rich As–Se glass.

Step-wise kinetics of natural physical ageing in arsenic selenide glasses.

The long-term kinetics of physical ageing at ambient temperature is studied in Se-rich As-Se glasses using the conventional differential scanning calorimetry technique. It is analysed through the changes in the structural relaxation parameters occurring during the glass-to-supercooled liquid transition in the heating mode. Along with the time dependences of the glass transition temperature (T(g)) and partial area (A) under the endothermic relaxation peak, the enthalpy losses (ΔH) and calculated fictive temperature (T(F)) are analysed as key parameters, characterizing the kinetics of physical ageing. The latter is shown to have step-wise character, revealing some kinds of subsequent plateaus and steep regions. A phenomenological description of physical ageing in the investigated glasses is proposed on the basis of an alignment-shrinkage mechanism and first-order kinetic equations.

Structural paradigm of Se-rich Ge–Se glasses by high-resolution x-ray photoelectron spectroscopy

The structure of binary GexSe100−x chalcogenide glass family (0≤x≤30) is determined by high-resolution x-ray photoelectron spectroscopy (XPS). On the basis of compositional dependences of fitting parameters for Ge and Se core level XPS spectra, the ratio between edge- and corner-shared tetrahedra is determined. We find that this ratio for glasses with 20≤x≤30 is almost constant with a value same as for the high-temperature crystalline form of GeSe2.

Structural model of homogeneous As–S glasses derived from Raman spectroscopy and high-resolution XPS

The structure of homogeneous bulk As x S100− x (25 ≤ x ≤ 42) glasses, prepared by the conventional rocking–melting–quenching method, was investigated using high-resolution X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. It is shown that the main building blocks of their glass networks are regular AsS3/2 pyramids and sulfur chains. In the S-rich domain, the existence of quasi-tetrahedral (QT) S = As(S1/2)3 units is deduced from XPS data, but with a concentration not exceeding ∼3–5% of total atomic sites. Therefore, QT units do not appear as primary building blocks of the glass backbone in these materials, and an optimally-constrained network may not be an appropriate description for glasses when x < 40. It is shown that, in contrast to Se-based glasses, the ‘chain-crossing’ model is only partially applicable to sulfide glasses.

Positronics of subnanometer atomistic imperfections in solids as a high-informative structure characterization tool.

Methodological possibilities of positron annihilation lifetime (PAL) spectroscopy applied to characterize different types of nanomaterials treated within three-term fitting procedure are critically reconsidered. In contrast to conventional three-term analysis based on admixed positron- and positronium-trapping modes, the process of nanostructurization is considered as substitutional positron-positronium trapping within the same host matrix. Developed formalism allows estimate interfacial void volumes responsible for positron trapping and characteristic bulk positron lifetimes in nanoparticle-affected inhomogeneous media. This algorithm was well justified at the example of thermally induced nanostructurization occurring in 80GeSe2-20Ga2Se3 glass.

Long-Term Physical Ageing in As-Se Glasses with Short Chalcogen Chains

Long-term physical ageing of chalcogenide glasses, which occurs over tens of years, is much less understood than the short-term ageing. With Se-rich underconstrained As30Se70 glass as a model composition (consisting of Sen chains with n≤3 on average), a microscopic model is developed for this phenomenon by combining information from differential scanning calorimetry, extended x-ray absorption fine structure, Raman, and 77Se solid state nuclear magnetic resonance spectroscopies. The accompanying changes in the electronic structure of these glasses are investigated by x-ray photoelectron spectroscopy. The data suggest ageing from cooperative relaxation, presumably involving bond switching or reconfiguration of As–Se–Se–As fragments.

Structure of Se-rich As-Se glasses by high-resolution x-ray photoelectron spectroscopy

To establish the validity of various proposed structural models, we have investigated the structure of the binary As{sub x}Se{sub 100-x} chalcogenide glass family (x{<=}40) by high-resolution x-ray photoelectron spectroscopy. From the composition dependence of the valence band, the contributions to the density of states from the 4p lone pair electrons of Se and the 4p bonding states and 4s electrons of Se and As are identified in the top part of the band. The analysis of Se 3d and As 3d core-level spectra supports the so-called chain crossing model for the atomic structure of Se-rich As{sub x}Se{sub 100-x} bulk glasses. The results also indicate small deviations ({approx}3-8%) from this model, especially for glass compositions with short Se chains (25<x{<=}40). For example, the presence of As-As homopolar defect bonds in the stoichiometric As{sub 40}Se{sub 60} and of Se-Se-Se fragments in a glass with composition x=30 is established.

In search of energy landscape for network glasses

Quick scanning extended x-ray absorption fine-structure spectroscopy is used to obtain in situ structural information on the real-time response of network glasses at the nanoscale level of atomic organization to the temperature ramp through the glass transition range. The results testify nonlinear, real-time temperature response indicative of nanoscale dynamic heterogeneity in disordered systems with intermediate fragility, related to the intermetabasin transitions within potential energy/enthalpy landscape.

Effect of Co60 γ-irradiation on the optical properties of As-Ge-S glasses

Abstract The influence of Co 60 γ-irradiation on the optical properties of chalcogenide semiconducting glasses from As 2 S 3 –Ge 2 S 3 cut-section is analyzed taking into account the accompanying spontaneous thermal annealing of the samples in the irradiation chamber. It is established that essential thermal heating of the investigated glasses during high-doses irradiation leads to the rough changes in compositional dependences of radiation-induced total (unrelaxed) and static (relaxed) optical effects. An attempt to describe dose dependence of the observed optical changes is made on the basis of stretched–exponential relaxation function.

Complex structural rearrangements in As-Se glasses.

Structural relaxation of As-Se glasses through the glass-to-supercooled liquid transition interval is studied with temperature-modulated differential scanning calorimetry. It is shown that connectivity of glass network and long-term physical ageing change not only the full width at half maximum of the out-of-phase component of complex heat capacity, which is conventionally used for analysis, but also its asymmetry value. The latter is shown to carry very important information on the dynamic heterogeneity in glasses. Raman spectroscopy is used as complementary technique to reveal possible structural rearrangements in the investigated glass network.