Reversibility Windows, Non-Aging and Nano Scale Phase Separation Effects in Bulk Germanium-Phosphorus-Sulfide Glasses

Raman scattering and Temperature modulated Differential scanning calorimetric measurements on binary P_xS_1-x and ternary Ge_yP_yS_1-2y glasses have been performed over the wide glass forming range (5% < x < 22%; 5% < y< 20%). MDSC measurements on binary P_xS_1-x glasses reveal the non-reversing enthalpy, ΔH_nr, associated with the glass transition T_g to vanish in the 16% < x < 19% composition range. Here the composition range across which ΔH_nr vanishes represents the reversibility window. Parallel experiments on ternary Ge_yP_yS_1-2y glasses reveal the reversibility window to reside in the 9% < y < 13.5% composition range. In analogy to work on several family of chalcogenide glasses, these reversibility windows, in conjunction with constraint counting algorithms serve to uniquely fix the three elastic phases, floppy, intermediate and stressed-rigid elastic phases in these glasses as follows:

P_xS_1-x - Floppy 5% < x < 16%, Intermediate 16% < x < 19% and Stresed-rigid 19% < x < 25%; Ge_yP_yS_1-2y - Floppy 2% < y < 9%, Intermediate 9% < y < 13.5 % and Stressed-rigid 13.5% < y < 25%.

Intermediate phase glasses are found not to age, but glass compositions in the floppy and stressed-rigid phases reveal the ΔH_nr term to evolve with waiting time or age in both glass systems studied. These results are in harmony with the notion that Intermediate Phase glasses are self-organized.

The intermediate phase onset near x = 16 atomic percent of phosphorus in P-S glasses may be compared to a value of x = 28% in corresponding P-Se glasses. The shift of the intermediate phase onset to a much lower concentration of P in P-S glasses is consistent with the notion that the P-bearing backbone composed of pyramidal P(X_1/2)₃ and quasi-tetrahedral X=P(X_1/2)₃ local structures is qualitatively decoupled from X = S-chain fragments in the sulfides but is actually part of the X = Se-chain fragments in corresponding selenide glasses. Raman scattering measurements show that the sulfide glasses are qualitatively more decoupled or nano-scale phase separated (NSPS) into small molecular units including S₈, P ₄S₁₀, P₄S₇, P₄S₄, P₄S₃ than their Selenide counterparts. Such NSPS is also responsible for the much narrower width of the intermediate phase in P-S glasses in relation to P-Se ones. On the other hand, the reversibility window and aspects of structure in ternary Ge_yP_yS_1-2y glasses bear a remarkable similarity to corresponding selenides, and highlights in a striking fashion that the presence of Ge as a cross-linker serves to qualitatively polymerize the network structure. These findings highlight, in general, the crucial role of NSPS on shifting the rigidity and stress elastic phase boundaries in network glasses.