Structural relaxation of glassy GeSe2 during isothermal annealing below and above Tg

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 GeSe2 powders below the glass-softening temperature (Tg = 635 ± 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 Tg favor ordering and the appearance of coherent domains of low-temperature α-GeSe2 (3D form). Upon annealing above Tg, the reverse LTPM → HTPM polymorphoid transformation accompanied by an endothermic effect takes place, which results in the predominance of HTPM β-GeSe2 polymorphoids (2D form) in glassy GeSe2 and their crystallization.