Contribution of floppy modes to configurational and excess entropy in chalcogenide glasses

Abstract

In this work, following Naumis' ideas [G.G. Naumis, Phys. Rev. B 61 (2000) R9205], we include the floppy modes as a free energy in order to obtain the configurational and excess entropy as well as the jump of the heat capacity of the chalcogenide glasses as function of the coordination number < r >. Theoretically, we find that S-ex/S-c ranges from 1.5 for strong liquids (< r > = 2) to 2 for fragile ones (< r > = 2.4). These results are consistent with the values reported by Angell and Borick [C.A. Angell, S. Borick, J. Non-Cryst. Solids 307&310 (2002) 393], who find experimentally for selenium, < r > = 2, S-ex/S-c = 1.47. The proportionality between the configurational and total excess entropies supports the non-existence of the Adam-Gibbs equation paradox. Finally, using S, in the Adam-Gibbs equation we obtain a VFT-like equation as function of < r >, predicting that when < r > increases, D decreases as well, as it has been seen in previous work. (c) 2006 Elsevier B.V. All rights reserved.