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Nanostructurization-driven responses in calorimetric heat-transfer phenomena are compared for glassy arsenoselenides at different levels of their molecular network organization, namely in high-polymerized g-As5Se95 and low-polymerized g-As55Se45, employing multifrequency temperature-modulated DSC-TOPEM® method complemented with Raman scattering microspectroscopy. It is shown that high-polymerized network composed of AsSe3 pyramids interlinked by Se chains with small number of Se8 molecular units decoupled from this network prevails in melt-quenched and nanomilling-derived g-As5Se95. Transition to more polymerized network due to incorporation of destroyed Se8 molecules into glass backbone occurs in this glass under nanostructurization. As a result, nanostructurization-driven calorimetric response in g-As5Se95 dominates by size-induced glass-transition temperature depression. The low-polymerized structure of g-As55Se45 is built of As–Se network enriched with tetra-arsenic selenide molecular units decoupled from this network. Molecular-to-network transition owing to nanomilling-driven destruction of these cage molecules and their incorporation into newly polymerized glass-forming backbone occurs in g-As55Se45 resulting in strong increase in calorimetric glass-transition temperature.

Complete hierarchy of network amorphization scenarios initiated in AsxS100-x nanoarsenicals within As4S4-As4S3 cut-Sect. (50 ≤ x ≤ 57) is reconstructed employing materials-computational approach based on ab-initio quantum-chemical modeling code (CINCA). Under nanostructurization due to high-energy mechanical milling, the inter-crystalline transformations to nanoscopic β-As4S4 phase accompanied by appearance of covalent-network amorphous matrix are activated. General amorphization trend under nanomilling obeys tending from molecular cage-like structures to optimally-constrained covalent-bonded networks compositionally invariant with parent arsenical. The contribution of amorphization paths in nanoarsenicals is defined by their chemistry with higher molecular-to-network barriers proper to As4S3-rich alloys. The generated amorphous phase is intrinsically decomposed, possessing double-Tg relaxation due to stoichiometric (x = 40) and non-stoichiometric (x > 40) sub-networks, which are built of AsS3/2 pyramids and As-rich arrangement keeping (i) two separated As-As bonds derived from realgar-type molecules, (ii) two neighboring As-As bonds derived from pararealgar-type molecules or (iii) three neighboring As-As bonds in triangle-like geometry derived from dimorphite-type molecules. Compositional invariance of nanoamorphous phase is ensured by growing sequence of network-forming clusters with average coordination numbers Z in the row (As2S4/2,Z = 2.50) – (As3S5/2, Z = 2.55) – (As3S3/2, Z = 2.67). Diversity of main molecular-to-network amorphizing pathways in nanoarsenicals is reflected on the unified potential energy landscape specified for boundary As4S4 and As4S3 components.

International audience; X-ray diffraction and calorimetric investigations were performed for 80GeS(2)-20Ga(2)S(3), 82GeS(2)-18Ga(2)S(3)and 84GeS(2)-16Ga(2)S(3)chalcogenide glasses. It is shown that structure of these glasses is characterized by short range ordering. It is established that Ga(2)S(3)and GeS(2)phases can be crystallized in Ge-Ga-S system. Transformation of voids in crystallized (80GeS(2)-20Ga(2)S(3))(100-and x445;)(CsCl)(x),x = 0; 5; 10; 15 chalcogenide glasses was studied by positron annihilation lifetime spectroscopy. The CsCl content in GeS2-Ga(2)S(3)glassy matrix changed the defect-related component in positron lifetime spectra and confirmed the structural void agglomeration in comparison with the base glass. A larger amount of CsCl in (80GeS(2)-20Ga(2)S(3))(85)(CsCl)(15)glass resulted in void fragmentation due to loosening of the structure.

Thermal-alteration interphase transformations in natural (realgar α-As4S4 of two mineral origins) and synthetic (commercial powdered high-temperature β-As4S4 modification synthesized from elemental constituents and subjected to high-energy mechanical ball milling) arsenic monosulfide polymorphs are studied exploring temperature-modulated DSC TOPEM® method. Specific heat capacity and non-reversing heat flow variations in realgar α-As4S4 demonstrate two endothermic events, these being ascribed to interphase α → β transformation at ∼(540–550) K, and melting of this newly-formed high-temperature β-As4S4 phase at 581–582 K. This polymorph originated from thermal alteration of mineral realgar possesses congruent melting in contrast to synthetic β-As4S4 polymorph, which shows non-equilibrium melting due to accompanied generation of compositionally-authentic amorphous phase. Calorimetric studies on synthetic β-As4S4 in powdered coarse-grained and milled states demonstrate complicated non-equilibrium melting in principally different crystalline-amorphous environments along with crystal-to-glass transformation. Structural-chemical heterogeneity of β-As4S4 crystallites results in incongruent double-peak melting through two endothermic events at ∼578 K and ∼588 K. The amorphous phase formed under high-energy milling of synthetic β-As4S4 possesses a dual nature due to stabilization of As-rich glassy substances with low- and high-temperature glass transition mid-points. This process in the powdered synthetic β-As4S4, identified as re-amorphization of initial amorphous phase and direct vitrification from β-As4S4 crystallites, was parameterized as compared to calorimetric thermal-alteration events in orpiment As2S3 mineral.

Temperature-modulated DSC TOPEM® method was applied to study amorphization in directly synthesized high-temperature polymorph of tetra-arsenic tetra-sulfide β-As4S4 affected to high-energy mechanical milling in a dry mode with 100–600 min−1 rotational speeds. The appeared amorphous phase is shown to possess dual nature, being related to As-rich glassy-like substances with low- and high-temperature glass transition midpoints. In respect of DSC TOPEM® studies, the crystalline–amorphous heterogeneity of chemical environment around β-As4S4 crystallites results in incongruent double-peak melting revealed through two endothermic effects at~ 305 °C and ~ 315 °C. Amorphous phase continuously generated under ball milling with increased rotational speed is identified as compositionally authentic to arsenic monosulfide, but different in medium-range order from stoichiometric As2S3. The overall amorphization in commercial arsenic sulfide prepared by direct synthesis from elemental constituents under high-energy ball milling occurs from two sources, these being high-to-low-Tg amorphous phase transformation and direct vitrification of β-As4S4 phase. These data testifies in favor of “shell” model treated solid-state amorphization in terms of defect generation in parent β-As4S4 phase, the amorphous substance being nucleated heterogeneously from grain boundaries followed by stretching into crystalline grain interior.

International audience; The non-isothermal crystallization kinetics of novel BixGa5Ge20Sb10-xSe45Te20 (x = 1,3,5,10) glasses are studied with differential scanning calorimetry method, and analyzed using Frazer-Suzuki fitting function. The applicability of Johnson-Mehl-Avrami (JMA) model to describe the crystallization kinetics in these materials is verified, and JMA exponent is determined for the relevant crystallization processes. It is found, that substitution of Sb with Bi in glass composition leads to a decrease in optical gap and thermal stability of the glass, and increase in the density and crystallization activation energy for glass-ceramics formation.

Crystallization ability of the 80GeS 2 -20Ga 2 S 3 , 82GeS 2 -18Ga 2 S 3 and 84GeS 2 -16Ga 2 S 3 chalcogenide glasses were studied using X-ray diffraction and calorimetric methods. It is shown that structure of these glasses is characterized by short range ordering. It is established that Ga 2 S 3 and GeS 2 phases can be crystallized in Ge-Ga-S system.

Thermochemical events recognized from heat flux DSC measurements involving dynamic and progressive step-cycling are identified in multi-nanophase substances exemplified by melt-quenched and nanomilling-derived AsxS100-x arsenicals enriched in As over As4S3 composition (As60S40, As62S38, As64S36). Thermal-alteration heat transfer in these alloys composed of normal- and plastic-crystalline phases supplemented by dual-nature amorphous substance are complicated by transformations in nanocrystalline dimorphite-type As4S3 and realgar-type As4S4 phases. Primary calorimetric events in the examined nanoarsenicals are shown to be as follows: (i) reduction in onset temperature from normal- (α′-As4S3) to plastic-crystalline (β-As4S3) phase; (ii) depression in pre-melting temperature of β-As4S3 phase; (iii) decrease in decomposition temperature of dimorphite-type phase supplemented by appearance of realgar-type β-As4S4 and amorphous phase, (iv) increase in crystallization temperature of β-As4S3 phase; (v) high-temperature shift in relaxation from liquid to stabilized heterogeneous state. The multiphase occurrence of nanoarsenicals results in unexpected increase in some heat transfer events detected in reheating modes.

Complete solid-state amorphization from parent crystalline-amorphous As 45 S 55 alloy was first detected under high-energy milling in a dry mode. The amorphized alloy exhibits stretched X-ray diffraction halos proper for glassy substances with well-expressed first sharp diffraction peak near reciprocal space scattering vector of 1.16 A −1 , which corresponds to 5.42 A in a real-space interplanar distance and 22.11 A in a correlation length. These peak parameters are in good respect with compositional extrapolation for melt-quenched As-rich compositions in As-S system. At the basis of comparison between first sharp diffraction peak and most intensive interlayer reflexes in overstoichiometric molecular As-S polymorphs, the network of amorphized As 45 S 55 is assumed as random packing of multifold cycle-type entities proper to these polymorphs, where the role of realgar As 4 S 4 cage formations is notably most important. Double- T g relaxation effect identified by multifrequency temperature modulated differential scanning calorimetry is shown to be character for As 45 S 55 alloy amorphized under high-energy milling.

The obtained results show that kinetics of light-assisted physical ageing in S-rich glasses can be well fitted with stretch-exponential Kohlrausch-type function, in which exponent β-values and the effective time relaxation constant τ depend on the wavelength of incident photons. The obtained β-values exhibit well-expressed minimum for the structural relaxation stimulated by light with energy of quanta comparable with the optical gap of the material. This effect is found to be similar to Se-rich glasses.

Compositional features of enthalpy losses ΔH(t) caused by long-term physical aging at normal conditions are studied at the example of Se-rich chalcogenide glasses As20Se80, As30Se70 and Ge5Se95 obeying “chain-crossing” structural model. The observed relaxation kinetics in general are described by a stretched exponential behavior, but can be also parameterized in terms of multi-step single exponential decays. Microstructural mechanism of natural physical aging in the studied glasses is explained by accepting their structural–topological specificity with a decisive role of preferential chemical environment around Se atoms. The characteristic time constants of aging are shown to increase in more topologically constrained structural fragments capable to relaxation. Independent relaxation stages originated from different chemical environments in As10Se90 and Ge5Se95 glasses allow observation of plateau-like behavior. However, plateaus are found to be smoothed in As20Se80 glass because of overlap of different structural complexes responsible for relaxation.

Nanonization accompanied by amorphization processes driven by high-energy mechanical milling are studied in AsxS100-x alloys from higher-crystallinity domain (51 Principal disproportionality in these arsenicals under milling comprises molecular-to-network transformations resulting in β-As4S4 polymorph supplemented by isocompositional amorphous phase. Nanosized state is signalized in broadened Raman-active bands of β-As4S4 crystallites (~40–50 nm), the milling-driven nanonization being accompanied by disappearing of other crystalline phases (pararealgar, χ-phase As4S4, low-temperature dimorphite α-As4S3). At high As content in studie alloys, milling facilitates disappearing of low-temperature α-As4S3 at cost of nanosized high-temperature β-As4S4 and α′-As4S3 phases. These transformations are clarified with heat capacity measurement showing depressing and splitting trend in melting and interphase-transition temperatures from normal-crystalline α′-As4S3 to plastic-crystalline β-As4S3. Intermediate-range ordering of the amorphized AsS phase is enhanced under milling, this phase dominating in network derivatives from As4S4 cage-like molecules, while generation of competitive As-rich phase being expected at higher As content.

Homogeneous As x S 100 − x glasses (25 ≤ x ≤ 42) are investigated with conventional differential scanning calorimetry. Enthalpy losses during structural relaxation are determined for the glasses aged at different temperatures in the dark and under the exposure with 620 nm light. It is shown that light exposure accelerates structural relaxation in chalcogen-rich glasses, but does not modify the kinetics of As 40 S 60 and As 42 S 58 compositions. The results are analyzed from the temperature-dependent constraint theory point of view.

Theory of chaos is shown to be adequately applied to analyze kinetics of natural physical aging revealed in structural relaxation of Se-rich As–Se glasses below glass transition. Kinetics of enthalpy losses induced by prolonged storage in natural conditions is used to determine phase space reconstruction parameters. The observed chaotic behavior (involving chaos and fractal consideration such as detrended fluctuation analysis, attractor identification using phase space representation, delay coordinates, mutual information, and false nearest neighbors) reconstructed via TISEAN is treated within potential energy landscape as diversity of multiple transitions between different basins–metabasins towards more thermodynamically equilibrium state, minimizing the free energy of the system. Natural physical aging, facilitates further atomic shrinkage of the glass backbone.

Sigmoid behavior of natural physical ageing in glassy As10Se90 reveals multi-step-wise growing kinetics of enthalpy losses. Phenomenological description of this kinetics can be adequately developed in terms of first-order relaxation processes, tending atomic structure from initial towards more thermodynamically equilibrium state. This kinetics is shown to obey characteristic stretched exponential behavior originated from a number of growing steps, attributed to the interconnected processes of chalcogen chain alignment and cooperative shrinkage of glass network.

A fundamental understanding of glass relaxation under ambient temperatures, as well as under the external influences is vital to the glass and polymer science communities. Our results show that kinetics of light-assisted physical aging in Se-based glasses can be well fitted with stretch-exponential Kohlrausch type function, which exponent β and the effective time relaxation constant τ depend on the wavelength of incident photons. The obtained β values for Se-rich glasses group around 3/7 and 1/3 values, predicted by Phillips field-free and field-forced axiomatic diffusion-to-traps models.

Principal possibility of the using of chalcogenide glasses (on the example of Ge18As18Se64) as active media for temperature sensors is considered in this work. Differential scanning calorimetry testing of the investigated glasses shown that 2 years of natural storage does not lead to the drift of their DSC-parameters (glass transition temperature and endothermic peak area). Investigation of the temperature dependence of optical transmission spectra shows the linear character of optical band-gap changes with a temperature. Temperature sensitivity index β for Ge18As18Se64 is estimated to be equal to the ~1.2·10-3 eV/°C.

Kinetics of physical aging at different temperatures is studied in situ in arsenic selenide glasses using high-precision differential scanning calorimetry technique. A well-expressed step-like behaviour in the enthalpy recovery kinetics is recorded for low aging temperatures. These fine features disappear when the aging temperature (T a) approaches the glass transition temperature (T g). The overall kinetics is described by stretched exponential function with stretching exponent close to 3/5 at T a > ~0.95 T g almost independent on glass composition, and 3/7 when the aging temperature drops to ~0.9 T g. These values are consistent with the prediction of Phillips' diffusion-to-traps model. Further decrease in aging temperature to ~0.85 T g leads to the appearance of step-like behaviour and stretching exponent of 1/3 for the overall kinetics, which is the limiting value predicted by random walk on the fractal model. Such behavior is explained as crossover from homogeneous cooperative relaxation of non-percolating structural units to high-dimensional fractal relaxation within hierarchically-arranged two-stage physical aging model.

International audience; An atomistic view on Johari-Goldstein secondary β-relaxation processes responsible for structural relaxation far below the glass transition temperature (Tg ) in network glasses is developed for the archetypal chalcogenide glass, As20Se80, using positron annihilation lifetime, differential scanning calorimetry, Raman scattering and nuclear magnetic resonance techniques. Increased density fluctuations are shown to be responsible for the initial stage of physical ageing in these materials at the temperatures below Tg . They are correlated with changes in thermodynamic parameters of structural relaxation through the glass-to-supercooled liquid transition interval. General shrinkage, occurred during the next stage of physical ageing, is shown to be determined by the ability of system to release these redundant open volumes from the glass bulk through the densification process of glass network.

Physical aging induced by an exposure of As–Se, As–S, and Ge–Se glasses to the light of different discrete wavelengths is studied using differential scanning calorimetry technique. The value of this effect is compared to the physical aging caused by natural storage in the dark. It is shown that a choice of As or Ge atoms does not influence significantly the spectral dependence of light-assisted physical aging, whereas substitution of Se with S causes drastic changes in the magnitude of the effect. The mechanism of the observed light-induced phenomena is discussed in terms of transient and metastable displacements of network chalcogen atoms.

The origin of structural relaxation in As 40 Se 60 glass at different annealing temperatures is studied by differential scanning calorimetry (DSC) and in situ extended X-ray absorption fine structure (EXAFS) methods. Strong physical aging effect, expressed through the increase of endothermic peak area in the vicinity of T g , is recorded by DSC technique at the annealing temperatures T a > 90 ° C . EXAFS data show that the observed structural relaxation is not associated with significant changes in the short-range order of this glass. An explanation is proposed for this relaxation behavior assuming temperature-dependent constraints.

Physical ageing caused by 25-years natural storage is studied in glassy As25S75 by conventional DSC technique. The observed effect was recorded as a significant increase in the value of glass transition temperature and endothermic peak area in the vicinity of glass-to-supercooled liquid transition. This result testifies, that marginal values of non-reversible heat flow in temperature-modulated DSC (ΔHnr), observed for as-prepared (or short-term aged) AsxS100-x samples within 22.5 < x < 29.5 compositional range [Ping Chen et al 2008 Phys. Rev. B78 224208], can significantly increase after longer period of physical ageing. Owing to these and previous results on collapse of the reversibility window in AsxSe100-x glasses after long-term physical ageing, we have concluded that marginality of non-reversible heat flow is not a sufficient criterion for express identification of the optimally-constrained self-organized phases in chalcogenide glasses. To make a conclusion on the marginality of ΔHnr, the long-term component of physical ageing should be taken into account. A comparison between conventional or temperature-modulated DSC scans recorded for long-term aged and rejuvenated glass samples is considered as a suitable criterion for physical ageing in glasses and, therefore, for the possibility of their network to be optimally-constrained. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Radiation-induced physical aging effects are studied in binary As x S100−x and As x Se100−x (30 ≤ x ≤ 42) glasses by conventional differential scanning calorimetry (DSC) method. It is shown that γ-irradiation (Co60 source, ~3 MGy dose) of glassy As x S100−x caused a measurable increase in glass transition temperature and endothermic peak area in the vicinity of glass transition region, which was associated with acceleration of structural relaxation processes in these materials. In contrast to sulfide glasses, the samples of As–Se family did not exhibit any significant changes in DSC curves after γ-irradiation. The observed difference in radiation-induced physical aging between sulfides and selenides was explained by more effective destruction-polymerization transformations and possible metastable defects formation in S-based glassy network.

Temperature dependence of optical transmission in the fundamental optical absorption edge region is studied through the glass transition interval for the typical representative of fragile As–Se glasses. Near-linear temperature dependence of the optical characteristics is observed for the non-aged As 10 Se 90 glass through the whole investigated range of temperatures, while a well-expressed non-linear peculiarity in the vicinity of glass transition is recorded for the 1 year and 20 years aged samples. It is associated with restoration of the optical properties of supercooled liquid during glass-to-supercooled liquid transition and is directly observed in the temperature range where the endothermic peak in DSC curves is observed. It is shown that long-term physical ageing at normal conditions results in a bleaching effect. Observed phenomenon is discussed in terms of the microstructural model of natural physical ageing developed recently.

Effect of Co 60 γ-irradiation on physical ageing in binary Ge x Se 100– x glasses (5 ⩽ x ⩽ 27) is studied using conventional differential scanning calorimetry method. It is shown, that high-energy irradiation leads to additional increase in the glass transition temperature and endothermic peak area near the glass transition region over the one induced by isochronal storage of these glasses at normal conditions. This γ-induced physical ageing is shown to be well-pronounced in Se-rich glasses ( x x ⩽ 27 compositions. The effect under consideration is supposed to be associated with γ-activated structural relaxation of the glass network towards thermodynamic equilibrium of supercooled liquid.

Effect of long-term physical ageing (~18–21 years) in vitreous germanium selenides is studied using differential scanning calorimetry method. It is compared with short-term physical ageing observed in these glasses earlier. Low compositional limit of the non-ageing ability determined using old samples coincides with the onset of reversibility window obtained using short-term aged samples of Ge–Se system by temperature modulated differential scanning calorimetry technique.

Physical ageing induced by above-bandgap photoexposure is studied in As–Se glasses using the differential scanning calorimetry technique. The kinetics of this effect are compared with those caused by natural storage in the dark. Acceleration of physical ageing processes is recorded for Se-rich glasses having more than three Se atoms in their chains.

Physical ageing effects caused by prolonged natural storage are recorded for binary AsxSe1−x glasses with 0.2 ⩽ x 0.4 . Their origin is associated with pure “shrinkage” of under-constrained glass networks. It is shown that only stoichiometric As2Se3 composition, which network is formed by a so-called self-organized phase, possesses the ideal non-ageing ability.

Extended free-volume defects trapping positrons are studied in inorganic glasses and functional ceramics using positron annihilation lifetime spectroscopy. Obtained results are treated in terms of general macroscopic approach grounded on physical–chemical properties of the tested materials which depend on the value of free volume content. At the example of chalcogenide glasses of ternary Ge–Sb–S system and spinel-type mixed transition-metal manganite Cu 0.4 Co 0.4 Ni 0.4 Mn 1.8 O 4 and magnesium aluminate MgAl 2 O 4 ceramics, it is shown that strict correlation exists between numerical values of free volumes of potential positron traps and lifetime parameters determined within two-state positron trapping model.

The effect of γ -irradiation (Co 60 sources) on 20 years stored As 10 Se 90 glass is studied using high-resolution X-ray photoelectron spectroscopy (XPS) technique. It is shown that high-energy irradiation treatment stimulates studied chalcogenide glasses (ChG) towards a more thermodynamically equilibrium state which cannot be attained when making glass by the conventional melt-quenching method.

Effect of external influences on the physical ageing processes in As–Se chalcogenide glasses is studied at the example of Co60 γ-irradiation. The increase in glass transition temperature and endothermic peak area near glass transition region for Se-enriched glasses is shown by differential scanning calorimetry method as a result of radiation treatment. The observed changes are associated with additional γ-activated structural relaxation of glass network towards thermodynamic equilibrium of supercooled liquid. This relaxation is connected with boundary displacements of inner Se atoms within –Se–Se–Se– cells followed by shrinkage processes.

The structure of nanoscale void-species has been studied in glassy arsenic selenide g-As2Se3 by combining the first sharp diffraction peak in X-ray diffraction (FSDP-related XRD), treated within Elliott’s void-based model, and positron annihilation lifetime spectroscopy (PALS) within two-state positron trapping model. By suggesting that the same nanovoids are responsible for both FSDP and PALS data, the analytical correlation relationship between the FSDP position, Q1, and nanovoid diameter, D, has been presented in the form of Q1 = 2.3 · π/D.

Influence of high-energy Co 60 gamma-irradiation on natural physical ageing in vitreous As 13.5 Ge 4.5 Se 82 is studied. It is shown that slow relaxation processes that occur in this material after 10 yr of natural storage can be significantly affected by irradiation. The increase in glass transition temperature and endothermic peak area in the vicinity of glass transition is recorded after gamma-irradiation and during an additional 1 yr period of natural storage of the irradiated glasses. It is associated with gamma-activated relaxation of As 13.5 Ge 4.5 Se 82 glass network towards a new thermodynamic state, which is closer to thermodynamic equilibrium of super-cooled liquid. The process occurred under stationary radiation field conditions is considered as pure γ -induced physical ageing , and those observed after radiation treatment—as post-radiation physical ageing .

A meaningful interpretation of positron lifetime characteristics for glassy-like g-As2Se3 is developed taking into account theoretical calculations of Jensen et al. [J. Non-Cryst. Solids 170 (1994) 57] for positrons trapped by so-called free-volume vacancy-type defects (vacancies and vacancy-type clusters) in orthorhombic As2Se3 and void volume distribution for 146-atoms layer-biased and random computer models of amorphous As2Se3 performed by Popescu yet in 1980. It is concluded that the obtained experimental data can be divided into two groups with close lifetime characteristics but different positron trapping coefficients. This feature can be well explained in terms of average positron lifetime by applying two-state positron trapping model for mathematical treatment of the experimental data.

An approach to structural characterization of chalcogenide glasses based on the study of void distribution is discussed. The results of positron annihilation lifetime spectra measurements for glassy-like g-As2Se3 are compared with nano-void distribution data obtained from Monte Carlo simulation. In this consideration perspectives to involve the parameters of nano-voids calculated from the first sharp diffraction peak in the framework of known Elliott’s model are analyzed.

A meaningful interpretation of positron lifetime characteristics for glassy-like g-As2Se3 is developed taking into account calculations of Jensen et al. (J. Non-Cryst. Solids 170 (1994) 57) for positrons trapped by free-volume extended defects in orthorhombic As2Se3 and void volume distribution for 146-atoms layer-biased model of amorphous As2Se3 presented by Popescu (J. Non-Cryst. Solids 35–36 (1980) 549). The obtained results are compared for samples having different thermal pre-history. Two groups of experimental results with close lifetime characteristics are distinguished for each of the investigated samples. This feature is explained in terms of average positron lifetime by applying two-state positron trapping model for mathematical treatment of the obtained spectra. r 2003 Elsevier B.V. All rights reserved.

The modified model of native and radiation-induced microvoid-type positron traps in vitreous chalcogenide semiconductors is developed to explain compositional features of positron annihilation lifetime measurements in stoichiometric As2S3–GeS2 and non-stoichiometric As2S3–Ge2S3 chalcogenide glasses before and after γ-irradiation.

A model of radiation-induced coordination topological defects with an associate open volume is developed in order to explain the experimental results on positron annihilation lifetime measurements in γ-irradiated vitreous chalcogenide semiconductors of the ternary As – Ge – S system. It is shown that, contrary to the native open-volume microvoids frozen technologically at thermodynamic equilibrium near a glass transition, the principally different part of void-type defects can be created as a result of 60 Co γ-irradiation. They are associated mainly with structural transformations (atomic displacements) at the medium-range ordering level in the nearest vicinity of some metastable atomic configurations, especially those containing the negatively charged coordination topological defects. 1. Introduction Vitreous chalcogenide semiconductors (VChSs) or, in other words, wide-gap semiconducting compounds of chalcogen atoms (S, Se or Te, but not O) with some elements from the IV-th and V-th groups of the Periodic Table (typically As, Ge, Sb, Bi, etc.) [1], are unique solid state materials, showing the effect of changes in their physical properties under the influence of high-energetic ionizing radiation, such as 60 Co γ-quanta with an average energy of more than 1 MeV [2, 3]. These changes are caused by specific radiation-induced structural transformations, connected with so-called coordination topological defects (CTDs) [4, 5]. These defects are created in the form of diamagnetic pairs of over- and under-coordinated atoms in a glassy-like network with positive and negative electrical charge, respectively [1, 4, 5]. The microstructural mechanism of this process is well studied in vitreous v –As2S3 [2, 3]. However, the similar transformations were not considered previously neither for v –GeS2, nor for more complicated cross-linked VChSs with two-, three- and four-fold coordinated atoms.

Kinetics of physical ageing (PhA) far below the glass transition temperature (Tg) is studied by positron annihilation lifetime and differential scanning calorimetry techniques for vitreous As20Se80 as typical representative of network glasses. The increased density fluctuations are shown to be responsible for the initial stage of PhA in this glass at below-Tg temperatures. These fluctuations are correlated with changes in thermodynamic parameters of structural relaxation through the glass-to-supercooled liquid transition interval. General shrinkage, occurred during the next stage of PhA, is shown to be determined by the ability of system to release these redundant open volumes from the glass bulk through densification process of glass network.

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.

Physical aging effects are shown to be typical of chalcogenide glasses with an under- or overconstrained network and to influence their physicochemical properties. The natural physical aging of glasses can be accelerated by gamma irradiation or exposure to light.

Effects of long-term physical ageing (∼20 years) studied in vitreous arsenic selenides using differential scanning calorimetry are compared with conventional short-term physical ageing (up to 1 year). It is shown that these effects differ not only by their amplitudes (increase in glass transition temperature and endothermic peak area near the glass transition), but also by compositional dependences. These results clearly testify in a favour of sufficient difference in microstructural origin for short- and long-term physical ageing in chalcogenide glasses.

This chapter covers state of the art in physical ageing of chalcogenide glasses. The thermodynamic origin of this phenomenon, experimental possibilities for its investigation, up-to-date results, general phenomenology and observed regularities are critically reviewed. The influence of various external factors, such as elevated temperatures, light exposure, high-energy irradiation, on the physical ageing effect in chalcogenide glasses are considered and analyzed from the constraints theory point of view. Possible mechanisms of structural relaxation caused by these factors are discussed thoroughly.

Modified model for positron annihilation in vitreous chalcogenide semiconductors is developed to explain a number of previously obtained results on positron lifetime measurements in glassy As–Ge–S of stoichiometric As2S3–GeS2 and non-stoichiometric As2S3–Ge2S3 cut-sections.

Physical aging effects caused by prolonged natural storage ({approx}22 years) in binary As{sub x}Se{sub 100-x} glasses are probed by temperature-modulated differential scanning calorimetry. It is shown that all aged samples with x

International audience; 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.

International audience; 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.