## 3 Results and Discussion
The understanding of the relative rates helps in preparing the heat-treatment protocol (suitable temperature and time) for the optimal crystallization in the base glass, to gain a better control over the engineering of the GC properties, specifically, towards a control over the size and number density of the crystallites. The nucleation and growth-like curve for both lab and commercial melts were studied and are shown in Figure 2b,d. For identical chemical compositions, the role of the parent glass morphology resulting from the varied glass melting protocols on these curves is dramatic. The lab melt showed a clear separation between the nucleation and growth regime of the glass, which is important when engineering GCs with desired properties. Measurable nucleation is shown to start at 200 \({}^{\circ}\)C and reaches a maximum near 210-220 \({}^{\circ}\)C. Whereas, crystal growth is seen to begin at 240 \({}^{\circ}\)C and reaches its peak at 270 \({}^{\circ}\)C. This clear separation of \(\sim\)20-25 \({}^{\circ}\)C between the nucleation, and thegrowth regimes is desirable as it enables the material to selectively nucleate, and then grow crystallites with a narrow size distribution, thereby ensuring a lower level of scattering. The peak temperatures of the nucleation and growth-like curve were used to define the heat-treatment protocol. Despite having the same chemical compositions as the lab melt, the commercial melt exhibited markedly different relative nucleation and growth curves as shown in Figure 2d. Here, the onset of nucleation started at 200 \({}^{\circ}\)C and reached its maximum at 210-220 \({}^{\circ}\)C, which is similar to what was observed in the lab melt. However, the growth curve over the same temperature range (210-220 \({}^{\circ}\)C) suggests the earlier onset and growth of one phase, followed by a second phase that appears to start growing at 240 \({}^{\circ}\)C, reaching its peak growth at 270 \({}^{\circ}\)C. This is not an ideal case scenario for generating crystallites with a narrow size distribution, but it does gives us an opportunity to understand the mechanism of the crystallite formation. This data is consistent with the broad crystallization peaks shown in the DSC data of Figure 2a. The nucleation and growth curve exhibited by the commercial melt highlights the impact of the lack of phase separation, when all constituents, including the crystallization-inducing constituent Pb, are homogeneously dispersed in the parent glass matrix. This presence of Pb throughout the parent glass matrix, combined with the glass's much slower cooling  
Figure 2: **(a)** Thermal analysis of lab- and commercial melt 20 PbSe base glass, with exothermic direction up. **(b)** The nucleation and growth-like curves of the lab melt. **(c)** TEM images of the as prepared parent glass lab-melt (top) [45] and commercial melt (bottom) showing ~80 nm Pb-rich droplets in the lab melt while the commercial melt remained homogenous (TEM images have different scale bars). **(d)** The nucleation and growth like curve of the commercial melt. Dashed lines are guides to the eye and indicate the nucleation temperature, \(T_{N}\) = 220 \({}^{\circ}\)C (blue) and growth temperature, \(T_{G}\) = 270 \({}^{\circ}\)C (red) used to heat-treat both lab and commercial melts of 20 PbSe glass in the present study. Figure 2c (top) reproduced with permission from _IJAGS_10, 27 (2019), Copyright 2019 John Wiley and Sons. rate, encourages not only more nuclei formation during parent glass processing, but also more growth of either Pb- or As-containing crystallites upon heat-treatment. This information on the dispersion of batch elemental constituents and their impact on thermally induced crystallization is further exemplified in microscopy of the parent glasses.