On the reversibility window in binary As–Se glasses

Physical ageing effects caused by prolonged natural storage are recorded for binary As_xSe_1−x glasses with 0.2x<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.     

Physical ageing in glassy As–Se induced by above-bandgap photoexposure

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.     

Topological controversies in the adaptability concept for glassy germanium selenides

Cluster modelling based on ab-initio calculations testifies lack of intermediate optimally-constrained phase in binary Ge_xSe_100-x system within expected reversibility window (20 ≤ × < 26) in terms of global connectivity. Network of these glasses within 20 ≤ × < 26 compositional range can be composed of over-constrained “outrigger raft” structural motives built of two edge- and four corner-shared GeSe_4/2 tetrahedra interconnected via optimally-constrained ≡ Ge―Se―Se―Ge≡ bridges, extra Se atoms forming ring-like configurations instead of Se―Se dimers.     

A study of reversible gamma-induced structural transformations in vitreous Ge23.5Sb11.8S64.7 by high-resolution X-ray photoelectron spectroscopy

The structural origin of reversible gamma-induced effects in vitreous Ge(23.5)Sb(11.8)S(64.7) has been investigated by high-resolution X-ray photoelectron spectroscopy (XPS). The changes in valence band spectrum from gamma-irradiation suggest a decrease of sulfur lone pair electron concentration accompanied by changes in bonding states of S and Ge. The appearance of additional doublets in the core-level XPS spectra of Ge, Sb, and S atoms for gamma-irradiated sample is described by the formation of over- and under-coordinated charged defect pairs (Ge(3)(-)-S(3)(+)) as a result of radiation treatment. The results verify the switching of Ge-S covalent bonds into S-S bonds as the main microstructural mechanism for gamma-induced optical effects in this glass.     

FSDP-related correlations in γ-irradiated chalcogenide semiconductor glasses: The case of glassy arsenic trisulphide g-As2S3 revised

Influence of high-energy γ-irradiation on the FSDP in XRD pattern is investigated at the example of g-As₂S₃. Spectroscopic measurements in the fundamental optical absorption edge regions were used to characterize the value of γ-induced changes through the observed darkening effect. It is concluded that γ-irradiation with ~3 MGy dose does not produce any detectable changes in the FSDP parameters, but leads to the appearance of satellite peaks at the left and right sides of the FSDP. Detailed analysis allows attributing of these peaks to crystalline As₂O₃ and S phases, which appear at the surface of g-As₂S₃ after prolonged γ-irradiation. Some correlations with the value of γ-induced optical changes in g-As₂S₃ were detected only for pre-FSDP.     

Topology and chemical order in AsxGexSe1 − 2x glasses: A high-resolution X-ray photoelectron spectroscopy study

The evolution of topology and chemical order along the As_xGe_xSe_1 ? 2x composition-line within the As–Ge–Se glass-forming region is studied by high-resolution X-ray photoelectron spectroscopy. It is shown that cation–cation bond formation becomes a dominant process for the compositions with x > 0.09. The results explain the peculiarities observed around this composition recently in the temperature-modulated differential scanning calorimetry data. Substitution of two selenium atoms within constituent structural units (pyramids and/or tetrahedra) by corresponding cations explains a second peculiarity point at x > 0.16 compositions observed recently with the above technique. The present observations show segregation of As and then Ge at high concentrations of cations in the system (x > 0.20).     

Self-assembly of an Octa-uranate Cage Complex with a Rigid bis-Catechol Ligand

While the terminally protected tripeptide Boc-Phe-Gly-m-ABA-OMe I (m-ABA, meta-amino benzoic acid) is an excellent gelator of aromatic organic solvents, another similar tripeptide Boc-Leu-Gly-m-ABA-OMe II, where the Phe residue of peptide I is replaced by Leu, cannot form gels with the same solvents. The morphology of the gels of peptide I, characterised by the field-emission scanning electron microscopy and high-resolution transmission electron microscopy, reveals the formation of nanofibrous networks which are known to encapsulate solvent molecules to form gels. The wide-angle X-ray scattering studies of the gels suggest the β-sheet-mediated self-assembly of peptide I in the formation of a nanofibrous network, where π-stacking interactions of Phe play an important role in the self-assembly and gel formation. The dried gel of peptide I observed between crossed polarisers after binding with a physiological dye, Congo red, shows a bluish-green birefringence, a characteristic of amyloid fibrils.     

Structure of SbxGe40-xSe60 glasses around 2.67 average coordination number

The evolution of the structure of Sb_xGe_40-xSe₆₀ (x = 8, 12, 15, 18 and 20) chalcogenide glasses is determined by high resolution X-ray photoelectron spectroscopy as Sb is added and the average coordination number (Z) is varied from 2.60 to 2.72. For glasses with Z < 2.67 the structure consists of deformed tetrahedra and pyramids, in which at least one Se atom is substituted by Ge or Sb atoms. Increase in the concentration of cations beyond Z ≥ 2.67 leads to a strengthening of the above regularity and formation of the structure by shared pyramids and tetrahedra with two or more Se atoms substituted by cations. At the same time Se–Se dimers are present in all the investigated compositions while they are not expected in these Se-deficient glasses. Their formation dictates that there must be metal-rich regions somewhere in the structure.     

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.