## 3 Results and Discussion
High resolution TEM images of the commercial melt illustrating this homogeneous dispersion of constituents and lack of immiscibility were collected to both quantify if phase separation was present and to further substantiate if there are TEM-visible nano-phases responsible for two \(T_{S}\) shown in Figure 2c. Similarly, Figure 3a,b shows the TEM image of the base and the heat-treated lab melt as measured and analyzed in previous works [45; 52]. Figure 3c,d shows the TEM images of the base and the heat-treated commercial melt following identical thermal treatment. The heat-treatment protocol used in the present study matched that used in the previous work (Figure 3a,b), which includes nucleation at 220 \({}^{\circ}\)C-2 hr. and growth at 270 \({}^{\circ}\)C-30 min. The secondary crystalline phase that resulted can be seen embedded in the heat-treated amorphous matrix. The interplanar spacing was extracted from Figure 3e to identify the crystalline phase present in the glass. The extracted interplanar spacing of -4.4 A matches closely the reported value of (333) As\({}_{2}\)Se\({}_{3}\) lattice plane [53]. These results indicate that in the commercial melt, thermally-induced crystallization resulted in the formation of As\({}_{2}\)Se\({}_{3}\) as the principal secondary phase. The principal crystalline phase formed in the droplet rich 20 PbSe is a mixture of Pb-rich (Ge\({}_{0.1}\)Pb\({}_{0.9}\))Se and PbSe, which has been studied and reported elsewhere [19; 45; 52]. The variation in post-heat-treated microstructures seen in Figure 3 suggest a variation in resulting crystal phases formed under similar heat-treatment protocols. Thus, comparison of the XRD patterns of the post-heat-treated GC samples as they evolve with individual thermal protocols for both commercial  
Figure 3: Bright field transmission electron microscopy (BF TEM) image (**a,b**) of the droplet rich, phase-separated 20 PbSe base lab-scale glass and its heat-treated morphology (270 \({}^{\circ}\)C, 30 min) [20; 45]. (**c**) The base glass morphology of the homogenous commercial melt. (**d**) The heat-treated morphology of the commercial melt at 270 \({}^{\circ}\)C, 30 min. (**e**) The interplanar spacing extracted from a local region matching the reported value for the (333) As\({}_{2}\)Se\({}_{3}\) crystallite. Figure 3a reproduced with permission from IJAGS 10, 27 (2019), Copyright 2019 John Wiley and Sons. Figure 3b reproduced with permission from Adv. Funct. Mater. 29, 1,902,217 (2019), Copyright 2019 John Wiley and Sons. and lab melts reveals this variation, as shown in Figure 4a,b. Both the samples followed the same heat-treatment protocols, as can be seen by the labels shown in the side-by-side figures. The presence of Pb-rich droplets in the lab melt makes it more prone to crystallization than the commercial melt, which can be seen by the presence of crystallite diffraction peaks emerging earlier than that for the commercial melt (270 \({}^{\circ}\)C, 30 min). The volume fractions of the crystalline phases were calculated by preparing calibration curves of the individual crystals in the base glass at different concentrations; this protocol is discussed elsewhere [54]. The nucleation and growth of the lab-melted sample held at 270 \({}^{\circ}\)C (30 min) sample, showed new crystal peaks at 24, 28, 31, 31.5, and 42\({}^{\circ}\) which correspond to (Ge\({}_{0.1}\)Pb\({}_{0.9}\))Se, As\({}_{2}\)Se\({}_{3}\), PbSe, As\({}_{2}\)Se\({}_{3}\), and (Ge\({}_{0.1}\)Pb\({}_{0.9}\))Se crystallites, respectively. The XRD pattern of the commercial melt under same conditions showed crystallization peaks at 28, 31.5, and 42\({}^{\circ}\), where the first two peaks correspond to As\({}_{2}\)Se\({}_{3}\), and the last peak corresponds to (Ge\({}_{0.1}\)Pb\({}_{0.9}\))Se crystallites. Extended heat-treatment of the lab and commercial melt coupons at 270 \({}^{\circ}\)C (60, 90 and 150 min) showed new crystalline phases at 18, 30, 41, and 53\({}^{\circ}\), which corresponds to Se, Germanium (Ge), Se, and As\({}_{2}\)Se\({}_{3}\), respectively.