Molecular origin of aging of pure Se glass: Growth of inter-chain structural correlations, network compaction, and partial ordering.

Glass transition width W of pure Se narrows from 7.1(3) °C to 1.5(2) °C and the non-reversing enthalpy of relaxation (ΔHnr ) at Tg increases from 0.23(5) cal/g to 0.90(5) cal/g upon room temperature aging for 4 months in the dark as examined in modulated differential scanning colorimetry (MDSC) at low scan rates. In Raman scattering, such aging leads the A1 mode of Sen -chains (near 250 cm-1 ) to narrow by 26% and its scattering strength to decrease as the strength of modes of correlated chains (near 235 cm-1 ) and of Se8 rings (near 264 cm-1 ) systematically grows. These calorimetric and Raman scattering results are consistent with the "molecular" chains of Sen , predominant in the fresh glass, reconstructing with each other to compact and partially order the network. Consequences of the aging induced reconstruction of the long super-flexible and uncorrelated Sen -chains are also manifested upon alloying up to 4 mol. % of Ge as revealed by a qualitative narrowing (by 25%) of the Raman vibrational mode of the corner-sharing GeSe4 tetrahedra and a blue-shift of the said mode by nearly 1 cm-1 in 194 cm-1 . But, at higher Ge content (x> 6%), as the length of Sen chain-segments across Ge cross-links decreases qualitatively (⟨n⟩ < 8), these aging induced chain-reconstruction effects are suppressed. The width of Tg increases beyond 15 °C in binary Gex Se100-x glasses as x> 10% to acquire values observed earlier as alloying concentration approaches 20% and networks become spontaneously rigid.