Dynamics of Ge-Se glasses at stiffness transition

Using the Raman scattering technique, we have investigated the thermal relaxation processes at various ranges of temperature in Ge(x)Se(1-x) glasses. Below 100 K, the reason why the peak position of the A, mode of GeSe(4/2) tetrahedra shows a positive shift with temperature is that the network relaxes, caused by a thermal excitation of soft bending modes related to Se, (n > 2) chain structures. This strengthens the remaining stretching modes. For rigid glasses (of average coordination number, [r] > 2.4, or x > 0.20), few Se(n) (n >2) units contained in the structure result in well mixed local vibrational modes. At the stiffness transition, the changes in peak positions with temperature in 100-300 K are the same, although the local modes differ markedly in frequency. In the temperature range 300-750 K including the glass transition temperatures (T(g)), relaxational modes, quasi-elastic contributions in the low-frequency region of 10-80 cm(-1) appear around the Tg only in the floppy (x less than or equal to 0.20) glasses, but it is undetectable in the rigid glasses (x greater than or equal to 0.23). We comfirm that the quasielastic contribution originates from the fast (beta) process of relaxation. The compositional dependence of the quasielastic intensity is well explained by the constraint counting theory under the assumption that the origin of the relaxational modes is from the floppy modes. We attribute the relaxation modes mainly to the damping or jumping motions of the rotating Se(n) segments, which could be regarded as the microscopic picture of the floppy modes in the Se-contained network glasses.