Fragility and aging behavior of Si_xSe_1-x glasses and liquids

The composition dependence of the fragility of Si_xSe_1-x liquids with 0.05 ≤ x ≤ 0.33 is determined using the calorimetric method and is found to be rather similar to that characteristic of their Ge analogues. In addition, the nature and the time scale of the structural relaxation of the Si₂₅Se₇₅ glass during aging at 40 K below T_g are measured using Raman spectroscopy. The structural relaxation in this glass, which belongs to the so-called intermediate phase, involves progressive conversion of the doubly edge-shared SiSe_4/2 tetrahedra E² into singly edge-shared E¹ and corner-shared E⁰ tetrahedra upon lowering of temperature. This tetrahedral speciation can be expressed in the form of the reaction 2 E² → E⁰ + E¹. The time scale of this tetrahedral conversion reaction corresponds well with that of shear relaxation. This result is inconsistent with the claim made previously in the literature that intermediate phase compositions do not undergo aging. Moreover, when taken together, the fragility and the structural relaxation results suggest that the constraint counting scheme typically adopted in the literature for edge- vs. corner-shared tetrahedra in chalcogenide networks may need to be revised. A rigid-polytope based constraint counting approach is shown to be more consistent with the experimental results.