To establish the validity of various proposed structural models, we have investigated the structure of the binary ${\mathrm{As}}_x{\mathrm{Se}}_{100-x}$ 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 ${\mathrm{As}}_x{\mathrm{Se}}_{100-x}$ bulk glasses. The results also indicate small deviations $(\sim 3–8\%)$ 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 ${\mathrm{As}}_{40}{\mathrm{Se}}_{60}$ and of Se-Se-Se fragments in a glass with composition $x=30$ is established.

Physical aging effects caused by prolonged natural storage $\left(\sim 22\text{years}\right)$ in binary ${\text{As}}_x{\text{Se}}_{100-x}$ glasses are probed by temperature-modulated differential scanning calorimetry. It is shown that all aged samples with $x<40$ reveal nonzero out-of-phase component of complex heat flow testifying physical aging effect. The first composition, which can be attributed to a so-called self-organized phase characterized by the absence of natural physical aging effect, is the stoichiometric ${\text{As}}_2{\text{Se}}_3$ glass; lack of aging within the so-called reversibility window, $28<x<38$, is not found. High-resolution x-ray photoelectron spectroscopy fails to show any evidence of fourfold-coordinated As atoms, which is consistent with the aging ability of glasses with $x<40$.

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.

The structure of homogeneous bulk As _x S_{100− 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 AsS_3/2 pyramids and sulfur chains. In the S-rich domain, the existence of quasi-tetrahedral (QT) S = As(S_1/2)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.