Study of the glass transition region in amorphous selenium by EXAFS

EXAFS spectra of amorphous bulk selenium and Se80Te20 alloy have been analysed in the temperature range of the glass transition region (-15 to 37oC). a-Se shows an unexpected behaviour becouse the modulated part of the X absorption increases with the temperature, irrespectively of the action of the EXAFS Debye-Waller factor from which one expects an opposite behaviour. Analogous features do not appear in the Se80Te20 alloy, probably because of the role played by the intramolecolar covalent Se-Te bond strength, weaker than the corresponding Se-Se. In the absence of suitable models for the temperature dependence of the microscopic processes occuring in glasses near Tg, a tentative explanation is proposed in terms of the temperature dependence of the backscattering amplitude function F(k)