Intermediate phases, boson and floppy modes, and demixing of network structures of binary As-S and As-Se glasses

Binary As_xS_1-x glasses are studied in modulated-DSC, Raman scattering, IR reflectance and molar volume experiments over a wide range (8% < x < 41%) of compositions. Calorimetric experiments reveal a reversibility window, which permits fixing the three elastic phases; flexible at x < 22.5%, intermediate in the 22.5% < x < 29.5% range, and stressed-rigid at x > 29.5%. Raman vibrational density of states supported by first principles cluster calculations show features of both pyramidal (PYR) [As(S_1/2)₃] and quasi-tetrahedral (QT) [S=As(S_1/2)₃] local structures. The QT unit concentrations maximize in the intermediate phase (IP) and both PYR and QT local structures contribute to the width of the IP. The IP centroid in the sulfides is shifted to lower As content x than in corresponding selenides, a feature identified with excess chalcogen partially segregating from the backbone in the sulfides, but forming part of the backbone in selenides. These ideas are corroborated by the proportionately larger free volumes of sulfides than selenides, and the absence of chemical bond strength scaling of T_g’s between As-sulfides and As-selenides. Low-frequency Raman modes (Boson modes) increase in scattering strength linearly as As content decreases from x = 20% to 8%, with a slope that is close to the floppy mode fraction in flexible glasses predicted by rigidity theory, showing that floppy modes contribute to the excess vibrations observed at low frequency, the Boson modes.

Long term aging, extending from months to several years, is studied on several families of chalcogenide glasses including the Ge-Se, As-Se, Ge-P-Se and Ge-As-Se systems. Our results show all samples display sub-T_g endotherms typically 10°C to 70°C below T_g in glassy networks possessing a mean coordination number r in the 2.25 < r < 2.45 range. Special attention is given to the As-Se binary and XRD, m-DSC and Raman scattering experiments are undertaken. Two sets of As_xSe_1-x samples aged for 8 years were compared, set A consisted of slow cooled samples aged in the dark, and set B consisted of melt quenched samples aged at laboratory environment. Samples of set B in the As concentration range, 35% < x < 60%, display a pre-T_g exotherm, but the feature is not observed in samples of set A. The aging behavior of set A presumably represents intrinsic aging in these glasses, while that of set B is extrinsic due to presence of light. The reversibility window persists in both sets of samples but is less well defined in set B. These findings contrast with a recent study by Golovchak et al., which finds the onset of the reversibility window moved up to the stoichiometric composition (x = 40%). The upshifted window is better understood as due to nanoscale phase separation (NSPS) of As₄Se₄ and As₄Se₃ molecules from the backbone. We attribute sub-T_g endotherms to compaction of the flexible part of networks upon long term aging, while pre-T_g exotherms to NSPS.