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

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 Sex/Sc 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, Sex/Sc = 1.47. The proportionality between the configurational and total excess entropies supports the non-existence of the Adam-Gibbs equation paradox. Finally, using Sc 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.