## 3 Results and Discussion
Figure 5a,c illustrates a correlation between the Vickers hardness and the volume fraction of the crystalline phases in the lab melt and commercial melt, respectively. An increase in hardness was observed in both lab and commercial melts as the crystallization progressed towards an extended period of 150 min. The heat-treatment at 270 \({}^{\circ}\)C (150 min) results in nearly 40% of the parent  
Figure 4: **(a,b)** XRD patterns of the (left) commercial melt (right) lab melt, following the heat-treatment at 250 \({}^{\circ}\)C (30 min), 260 \({}^{\circ}\)C (30 min), and 270 \({}^{\circ}\)C (30, 60, 90, and 150 min). **(c)** Average PbSe crystal size formed in lab and commercial after following the same heat-treatment protocol. **(d)** Average As\({}_{2}\)Se\({}_{3}\) crystal size formed in lab and commercial after following the same heat-treatment protocol. **(e)** The size distribution of the PbSe and As\({}_{2}\)Se\({}_{3}\) crystalline phases calculated as FWHM in both lab and commercial melts. glass (for both lab and commercial melt) being converted to crystallite-containing glass (i.e., a GC). This conversion results in a modified residual glass matrix chemistry, depleted of these crystal- forming constituents. While similar, as shown below, the types and fractions of the formed crystals (and thus depleted species) were not identical in the two glasses. In the lab melt, a slight increase in hardness was observed during this extended heat-treatment at 270 \({}^{\circ}\)C (150 min). In the post-heat-treated lab-melt data in Figure 5b, crystallization of both the cubic Pb-rich phases and As\({}_{2}\)Se\({}_{3}\) phases were observed during the 30-60 min heat-treatments. However, during the extended (90 additional minutes) heat treatment, an increase of only the Pb-rich phases was observed, suggesting a significant depletion in the early forming As-species in the matrix. Figure 5b also corroborates the fact that after 60 min, a consistent crystallization of the Pb-rich phase was observed. Converse to these observations in the commercial melt, the anisotropic As\({}_{2}\)Se\({}_{3}\) phase is the dominant phase contributing to the initial increase in hardness and total volume fraction of crystals formed (at 30 and 60 min), but after \(\sim\)60 min, the PbSe and (Ge\({}_{0.1}\)Pb\({}_{0.9}\))Se phase fractions appears to grow at a more rapid rate than the (perhaps) depleted As-containing phase. Since hardness does not appreciably increase in this window, and instead the volume fraction of cubic crystals continues to increase, this validates the assumption that the crystal phase responsible for the hardness increase is dominated by the anisotropic crystalline phase and is changing with time as shown in Figure 5d. The volume fraction of various heat-treated sample is also shown in Supplementary Information, Table S1. Figure 5: (**a**) Correlation between the hardness, volume fraction of the crystals and heat-treatment time in the lab [20 PbSe] melt. (**b**) Volume fraction of the crystalline phases in the lab melt. (**c**) Correlation between the hardness, volume fraction of the crystals and heat-treatment time in the commercial melt. (**d**) Volume fraction of the crystalline phases in commercial melt. These findings support our hypothesis that as the As is initially partially depleted during As\({}_{2}\)Se\({}_{3}\) formation, it is subsequently thus easier for Se to now participate in crystal formation of Pb-containing phases rather than 'finding' As available for further crystallization. Stoichiometrically, this makes sense as the number of atoms needed to form the crystal are greater (2-As and 3-Se) versus 1 Pb and Se in the cubic crystals. Se tends to crystallize (as it is present in excess) in small amounts initially but at a much more extensive rate later in the heat-treatment process.