Structure property relations in (As₂Se₃)_x(GeTe₄)_100-x glasses

Bulk (As₂Se₃)_x(GeTe₄)_100-x glasses have been prepared over the entire composition range 0 ≤ x ≤ 100. In this tie-line, the average coordination number (Z_av) of all the glasses is 2.40 at which the constraints acting on the network and the number of degrees of freedom are balanced. Glass transition (T_g) and non-reversing heat flow (ΔH_nr) determined from DSC and MDSC measurements exhibit interesting variations with composition. Based on these variations the structural network can be divided into three regions: (I) 0 ≤ x ≤ 20 (II) 25 ≤ x ≤ 45 and (III) 50 ≤ x ≤ 100. Both T_g and ΔH_nr show a decreasing trend in the region I and remain invariant in the region II. In the region III, T_g and ΔH_nr show an increasing trend. From the Raman measurements we infer that the network in the region I is dominated by the GeTe_4/2 tetrahedral units. The addition of As₂Se₃ initially depolymerizes the network due to which a decrease in Tg is observed in the region I. The region II is dominated by the AsTe_3/2 pyramidal units and Te-Te chains. In this region, the network starts polymerizing and at the same time there is a decrease in the mean bond energy. These two factors compete with each other and hence both T_g and ΔH_nr remains invariant. The region III is rich with As₂Se₃ and the structural network is dominated by AsSe_3/2 structural units. Both network connectivity and the mean bond energy go hand in hand in this region and there is an increase in the T_g and ΔH_nr.  For 25 ≤ x ≤ 45, ΔH_nr almost vanishes indicating non aging of the glasses in the region II. This study underlines the effects of chemical composition and the mean bond energy variations in a critically coordinated covalent network.