Structural relaxation of glassy GeSe$_2$ during isothermal annealing below and above $T_g$

Raman scattering, X-ray diffraction, and differential scanning calorimetry in the temperature range 300–800 K are used to show that, upon isothermal annealing of glass GeSe$_2$ powders below the glass-softening temperature ($T_g$ = 635 $\pm$ 2 K), fragments of high-temperature polymorphic modifications having no long-range order (HTPM polymorphoids) decompose and transform into low-temperature polymorphoids (LTPM) with an exothermic effect. A high concentration of LTPM polymorphoids in the glass and their stabilization upon annealing below $T_g$ favor ordering and the appearance of coherent domains of low-temperature $\alpha$-GeSe$_2$ (3D form). Upon annealing above $T_g$, the reverse LTPM $\to$ HTPM polymorphoid transformation accompanied by an endothermic effect takes place, which results in the predominance of HTPM $\beta$-GeSe$_2$ polymorphoids (2D form) in glassy GeSe$_2$ and their crystallization.