Intermediate Phase, Molecular Structure, Aging and Network Topology of Ternary Ge<sub>x</sub>Sb<sub>x</sub>Se<sub>100-2x</sub> Glasses

Intermediate Phase, Molecular Structure, Aging and Network Topology of Ternary Ge<sub>x</sub>Sb<sub>x</sub>Se<sub>100-2x</sub> Glasses

Bibliographic Details
Main Author: Gunasekera, Kapila J.
Language:English
Published: University of Cincinnati / OhioLINK 2010
Subjects:
Materials Science
Intermediate Phase
Phase Change Materials
Chalcogenide glasses
Aging
Network Topology
Ge2Sb2Te5
Online Access:http://rave.ohiolink.edu/etdc/view?acc_num=ucin1277132558
id ndltd-OhioLink-oai-etd.ohiolink.edu-ucin1277132558
record_format oai_dc
collection NDLTD
language English
sources NDLTD
topic Materials Science
Intermediate Phase
Phase Change Materials
Chalcogenide glasses
Aging
Network Topology
Ge2Sb2Te5
spellingShingle Materials Science
Intermediate Phase
Phase Change Materials
Chalcogenide glasses
Aging
Network Topology
Ge2Sb2Te5
Gunasekera, Kapila J.
Intermediate Phase, Molecular Structure, Aging and Network Topology of Ternary Ge<sub>x</sub>Sb<sub>x</sub>Se<sub>100-2x</sub> Glasses
author Gunasekera, Kapila J.
author_facet Gunasekera, Kapila J.
author_sort Gunasekera, Kapila J.
title Intermediate Phase, Molecular Structure, Aging and Network Topology of Ternary Ge<sub>x</sub>Sb<sub>x</sub>Se<sub>100-2x</sub> Glasses
title_short Intermediate Phase, Molecular Structure, Aging and Network Topology of Ternary Ge<sub>x</sub>Sb<sub>x</sub>Se<sub>100-2x</sub> Glasses
title_full Intermediate Phase, Molecular Structure, Aging and Network Topology of Ternary Ge<sub>x</sub>Sb<sub>x</sub>Se<sub>100-2x</sub> Glasses
title_fullStr Intermediate Phase, Molecular Structure, Aging and Network Topology of Ternary Ge<sub>x</sub>Sb<sub>x</sub>Se<sub>100-2x</sub> Glasses
title_full_unstemmed Intermediate Phase, Molecular Structure, Aging and Network Topology of Ternary Ge<sub>x</sub>Sb<sub>x</sub>Se<sub>100-2x</sub> Glasses
title_sort intermediate phase, molecular structure, aging and network topology of ternary ge<sub>x</sub>sb<sub>x</sub>se<sub>100-2x</sub> glasses
publisher University of Cincinnati / OhioLINK
publishDate 2010
url http://rave.ohiolink.edu/etdc/view?acc_num=ucin1277132558
work_keys_str_mv AT gunasekerakapilaj intermediatephasemolecularstructureagingandnetworktopologyofternarygesubxsubsbsubxsubsesub1002xsubglasses
_version_ 1719433196332908544
spelling ndltd-OhioLink-oai-etd.ohiolink.edu-ucin12771325582021-08-03T06:14:05Z Intermediate Phase, Molecular Structure, Aging and Network Topology of Ternary Ge<sub>x</sub>Sb<sub>x</sub>Se<sub>100-2x</sub> Glasses Gunasekera, Kapila J. Materials Science Intermediate Phase Phase Change Materials Chalcogenide glasses Aging Network Topology Ge2Sb2Te5 <p>Bulk alloy glasses of Ge<sub>x</sub>Sb<sub>x</sub>Se<sub>100-2x</sub> 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<sub>n</sub> polymeric chains, Corner-sharing (CS) Ge(Se<sub>1/2</sub>)<sub>4</sub> , Edge sharing (ES)-Ge(Se<sub>1/2</sub>)<sub>4</sub> and Pyramidal (PYR) Sb(Se<sub>1/2</sub>)<sub>3</sub> 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<sub>ct</sub> = 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.</p> <p>Calorimetric measurements yield variation in glass transition temperature, T<sub>g</sub>(x), and show a global maximum near x<sub>ct</sub> = 18.2%, the chemical threshold. Stochastic agglomeration theory is used to analyze Tg(x) variation and shows that backbones at x > x<sub>ct</sub> are demixed into Ge-rich (ethanelike Ge<sub>2</sub>(Se<sub>1/2</sub>)<sub>6</sub> ) and Sb-rich (ethylenelike Sb<sub>2</sub>(Se<sub>1/2</sub>)<sub>4</sub> ) local structures. We find evidence of a reversibility window in the 14.9% < x < 17.5% range, or 2.45 < <i>r̄</i> < 2.55 mean coordination number range, wherein the non-reversing enthalpy (ΔH<sub>nr</sub>(x)) at Tg shows a square-well like global minimum with ΔH<sub>nr</sub>(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<sub>nr</sub>(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 <i>r̄</i> = 2.45 in the present Sb bearing ternary instead of <i>r̄</i> = 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). </p> <p>By combining IP results on the Ge<sub>7</sub>Sb<sub>93-x</sub>Se<sub>x</sub> ternary and Ge<sub>x</sub>Se<sub>1-x</sub> binary with the IP result on present Ge<sub>x</sub>Sb<sub>x</sub>Se<sub>100-2x</sub> 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 Ge<sub>2</sub>Sb<sub>2</sub>Te<sub>5</sub> phase change material ( x = 22%) should be stressed-rigid as well as demixed in its amorphous state.</p> 2010-08-03 English text University of Cincinnati / OhioLINK http://rave.ohiolink.edu/etdc/view?acc_num=ucin1277132558 http://rave.ohiolink.edu/etdc/view?acc_num=ucin1277132558 unrestricted This thesis or dissertation is protected by copyright: all rights reserved. It may not be copied or redistributed beyond the terms of applicable copyright laws.

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