Evidence for nanoscale phase separation of stressed–rigid glasses

Abstract

Ternary (Ge2X3)x(As2X3)1−x glasses with X = S or Se are of interest because they span a mean coordination number r in the 2.40 < r < 2.8 range that is characteristic of stressed–rigid glasses. We have examined X = S glasses in Raman scattering and T-modulated differential scanning calorimetry measurements over the 0 < x < 1.0 range. Glass transition temperatures, Tg(x), increase monotonically in the 0 < x < 0.8 range and decrease thereafter (0.8 < x < 1) to display a global maximum near x = 0.8. Raman scattering provides evidence of sharp modes of As4S4 and As4S3 monomers, with scattering strength of these modes showing a global maximum near x = 0.3 and 0.5 respectively. The results suggest that at low x (0 < x < 1/2), addition of Ge2S3 to the As2S3 base glass results in insertion of Ge(S1/2)4 tetrahedra in the As(S1/2)3-based backbone as compensating As-rich monomers segregate from the backbone to deliver the requisite S. At higher x (0.4 < x < 0.8), the Ge2S3 additive continues to enter the glass in a majority (As2S3)(GeS2) backbone and several minority nanophases including an ethane-like Ge2(S1/2)6 and a distorted rock-salt-like GeS. In the 0.8 < x < 1 range, the nanophases grow qualitatively at the expense of the backbone as Tg values decrease and the end-member composition (x = 1) is realized. Heterogeneity of glasses near x = 1/2 or mean coordination, r = 2.60 derives intrinsically from the presence of several minority nanophases and a majority backbone showing that stressed–rigid networks usually phase separate on a nanoscale.