Aging-Induced Structural Evolution of a GeSe₂ Glass Network: The Role of Homopolar Bonds

The structural evolution of GeSe₂ glass during aging is studied using Raman spectroscopy and density relaxation measurements. The Raman spectra indicate volume- and entropy-driven changes in the relative concentrations of the corner-sharing (CS) and edge-sharing (ES) GeSe₄ tetrahedra and in the degree of chemical order of the tetrahedral network during aging at 65 °C below the nominal glass transition temperature. The attendant structural changes involve a progressive increase in the CS:ES ratio and in the chemical order that can be expressed in the form of a reaction Ge-Ge + Se-Se → 2 Ge-Se, which shifts to the right, with lowering of fictive temperature. The isothermal relaxation of both the structure and density during aging displays rather similar stretched exponential kinetics with a stretching exponent β ∼0.54 and an average relaxation time of ∼13.5 h. In situ high-temperature Raman spectroscopic measurements indicate that structural relaxation does not affect the anharmonicity of the vibrational potential wells in the energy landscape of GeSe₂ glass.