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

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 (ΔH_nr) at T_g 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 Se_n-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 Se_n, 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 Se_n-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 Se_n chain-segments across Ge cross-links decreases qualitatively (⟨n⟩ < 8), these aging induced chain-reconstruction effects are suppressed. The width of T_g increases beyond 15 °C in binary Ge_xSe_100-x glasses as x> 10% to acquire values observed earlier as alloying concentration approaches 20% and networks become spontaneously rigid.