Intermediate Phase, Molecular Structure, Aging and Network Topology of Ternary Ge_xSb_xSe_100-2x Glasses

Bulk alloy glasses of Ge_xSb_xSe_100-2x composition were synthesized over the compositional range, 0% < x < 23%, and examined in modulated differential scanning calorimetric, Raman scattering and molar volume measurements. Raman vibrational density of states systematically evolve with glass composition ‘x’ displaying normal modes of characteristic building blocks, including Se_n polymeric chains, Corner-sharing (CS) Ge(Se_1/2)₄ , Edge sharing (ES)-Ge(Se_1/2)₄ and Pyramidal (PYR) Sb(Se_1/2)₃ units. Line shapes are deconvoluted in terms of requisite number of Gaussians, and mode scattering strengths and mode frequencies established. These data along with first principles cluster calculations are used to identify the mode assignments. Our data confirm that x = x_ct = 18.2% is the chemical threshold in these glasses, in harmony with the valence requirements of Ge and Sb, and with homopolar bonds proliferating above the threshold as expected.

Calorimetric measurements yield variation in glass transition temperature, T_g(x), and show a global maximum near x_ct = 18.2%, the chemical threshold. Stochastic agglomeration theory is used to analyze Tg(x) variation and shows that backbones at x > x_ct are demixed into Ge-rich (ethanelike Ge₂(Se_1/2)₆ ) and Sb-rich (ethylenelike Sb₂(Se_1/2)₄ ) local structures. We find evidence of a reversibility window in the 14.9% < x < 17.5% range, or 2.45 < r̄ < 2.55 mean coordination number range, wherein the non-reversing enthalpy (ΔH_nr(x)) at Tg shows a square-well like global minimum with ΔH_nr(x) term almost vanishing. Molar volumes independently show a local minimum in the reversibility window, confirming the space filling nature of networks formed in the window. Aging of glasses, studied at room temperature as a function of waiting time, shows the ΔH_nr(x) term to age for glass compositions outside the reversibility window but not in the window. These data show that glass compositions at x < 14.9% are flexible, those at x > 17.5% stressed rigid and demixed, while those in the 14.9% < x < 17.5% range, rigid but stress-free (Intermediate Phase, IP). Onset of the IP at r̄ = 2.45 in the present Sb bearing ternary instead of r̄ = 2.29 as found in the corresponding As-bearing ternary, suggests that Sb takes only a 3-fold coordination as in a pyramidal local structure, while As takes on both the pyramidal and quasi-tetrahedral coordination. An optical elastic power law of p = 1.44(1) is obtained from Raman vibration mode frequency up shift of CS tetrahedral units with x in the IP. The corresponding power-law in As-based ternary was found earlier to be p = 1.04(3).

By combining IP results on the Ge₇Sb_93-xSe_x ternary and Ge_xSe_1-x binary with the IP result on present Ge_xSb_xSe_100-2x ternary, we have constructed a global elastic phase diagram of Ge-Sb-Se system. These data suggest that if amorphous Ge-Sb-Te films possess a network structure similar to that of the present Ge-Sb-Se ternary, then the all important Ge₂Sb₂Te₅ phase change material ( x = 22%) should be stressed-rigid as well as demixed in its amorphous state.