Melt Homogenization and Self‐Organization in Chalcogenides‐Part I

Abstract

An FT‐Raman profiling method is developed to monitor growth of structural homogeneity of binary GexSe100‐x melts in real time (tR) non‐invasively as starting materials are reacted over days. Raman spectra of quenched melts were acquired along a one inch long column of a sample in a quartz tube. In the first step of reaction, tR < 2 days, Ge‐rich crystalline‐ and glassy‐ phases form and coexist with Se‐rich glasses. In the second step, tR extending up to 7 days, local structures characteristic of melts/glasses form, and steadily homogenize as Ge and Se atoms diffuse. The process terminates when all Raman lineshapes taken along the length of a sample coalesce into unique profile. Several factors contribute to the long reaction time tR for melts to homogenize, including liquid density difference of Ge and Se, diffusion controlled nanoscale mixing of melts, batch dryness, batch sizes, and laser spot size. Physical properties of such homogeneous GexSe100‐x glasses are found to be quite different from their inhomogeneous counterparts realized after the first step of reaction. Slow homogenization of chalcogenides melts may occur generally. Variations in physical properties of chalcogenide glasses possessing nominally the same composition may have their origin in structural heterogeneity and purity.

Topological Ordering and Viscosity in the Glass-Forming Ge–Se System: The Search for a Structural or Dynamical Signature of the Intermediate Phase

“…The pairs of vertical dashed (black) or chained (red) lines, and associated horizonal arrows, mark compositions for which ∆Hnr ≃ 0 as found in the present work or in the work of Boolchand et al (2001a), respectively. are not in quantitative agreement with the work of Bhosle et al (2012b). Nevertheless, the data sets of Bhosle et al (2012b), Feltz et al (1983), Ota et al (1978), and Yang et al (2013) point to a minimum value of V m in the interval 0.20 x 0.25 (Bhageria et al, 2014). The present results show a shallow minimum around x = 0.19(4) corresponding to V m = 17.95(5) cm 3 mol −1 .…”

“…The results give ∆H nr ≃ 0, which is the defining characteristic of the intermediate phase, for the composition range 0.175(8) ≤ x ≤ 0.235(8) (Figure 5). This composition range compares to previously reported "reversibility windows" of 0.225 ≤ x ≤ 0.230 (Feng et al, 1997), 0.20(1) ≤ x ≤ 0.26(1) (Boolchand et al, 2001a), or 0.195(5) ≤ x ≤ 0.260(5) (Bhosle et al, 2012b) for the Ge x Se 1−x system. The composition range found in the present work is therefore shifted to lower x, and its mid-range value of x = 0.205 (8) is in agreement, within the experimental error, with the expectation from mean-field constraint-counting theory of a rigid to floppy transition in the Ge x Se 1−x system at x = 0.20 wheren = 2.40 (Thorpe, 1983).…”

“…The results are compared to those obtained from other measurements in Figure 2. The comparison shows that systematically smaller densities were obtained in the work by Bhosle et al (2012b). In the latter, agreement is claimed with the molar volume V m values given by Mahadevan et al (1995), but the latter were incorrectly copied from the work of Feltz et al (1983).…”

“…FIGURE 2 | The dependence of the mass density at room temperature ρmass for the GexSe 1− x system on the composition x and mean coordination number n = 2(1 + x). The results from the present work at ≃22°C are compared to those taken from Andonov (1982), Avetikyan and Baidakov (1972), Azoulay et al (1975), Bhosle et al (2012b), Borisova (1981), Feltz et al (1983), Feltz and Lippmann (1973), Guin et al (2002b), Hafiz et al (1993), Ito et al (1988), Loehman et al (1972), Ota et al (1978), Senapati and Varshneya (1995), Sreeram et al (1991b), and Yang et al (2013). The solid (black) curves are drawn as guides for the eye.…”

“…The structural motifs of the intermediate phase are expected to yield N c = 3 such that the network is optimally constrained to avoid stress. Ge x Se 1−x (0 ≤ x ≤ 1) is a prototypical covalently bonded network-forming system for which the intermediate phase spans a wide composition window, usually reported as 0.20 x 0.26 (Boolchand et al, 2001a(Boolchand et al, , 2007Bhosle et al, 2012b).…”