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

Abstract

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 GexSe100-x glasses as x> 10% to acquire values observed earlier as alloying concentration approaches 20% and networks become spontaneously rigid.