Bimodal phase percolation model for the structure of Ge-Se glasses and the existence of the intermediate phase

A detailed nuclear magnetic resonance and Raman study of ${\text{Ge}}_x{\text{Se}}_{1-x}$ glasses indicate that the glass structure is composed of intertwined microdomains of ${\text{GeSe}}_2$ and ${\text{Se}}_{\text{n}}$. Static nuclear magnetic resonance spectra of glasses ranging from $0\le x\le \frac{1}{3}$ reveal the absence of Ge-Se-Se fragments in the structure. High temperature nuclear magnetic resonance showing considerable line narrowing confirms this observation. More importantly, the fraction of Se-Se-Se obtained by integration of nuclear magnetic resonance lines matches closely the percentage predicted for a bimodal phase model and is not consistent with the existence of Ge-Se-Se fragments. Raman spectra collected on the same glass also confirm the existence of ${\text{GeSe}}_2$ domains up to high selenium concentrations. The mobility of the ${\text{Se}}_{\text{n}}$ phase observed at high temperature while the ${\text{GeSe}}_2$ phase remains rigid is consistent with their respective underconstrained and overconstrained structural nature. The proposed bimodal phase percolation model is consistent with the original Phillips and Thorpe theory however it is clearly at odds with the intermediate phase model which predicts large amounts of Ge-Se-Se fragments in the structure. A calorimetric study performed over a wide range of cooling/heating rates shows a narrow composition dependence centered at $⟨\text{r}⟩=2.4$ in contrast with the wide reversibility window observed by Modulated Differential Scanning Calorimetry. This suggests that the observation of the reversibility window associated with the intermediate phase in Ge-Se glasses could be an experimental artifact resulting from the use of a single modulation frequency.