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) degrees C to 1.5(2) degrees C and the non-reversing enthalpy of relaxation (Delta H-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 A(1) 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 Se-8 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 Tg increases beyond 15 degrees C in binary GexSe100-x glasses as x > 10% to acquire values observed earlier as alloying concentration approaches 20% and networks become spontaneously rigid. Published by AIP Publishing.