Research ArticleMATERIALS SCIENCE

Uncovering β-relaxations in amorphous phase-change materials

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Science Advances  10 Jan 2020:
Vol. 6, no. 2, eaay6726
DOI: 10.1126/sciadv.aay6726
  • Fig. 1 Powder mechanical spectroscopy.

    (A) Schematic drawing of the experimental setup with powder samples inside the holder. (B) The storage modulus E′ and the loss modulus E″ of Ge15Te85 using powder mechanical spectroscopy. T* represents the temperature before the calibration. a.u., arbitrary units. (C) The storage modulus E′ and loss modulus E″ of bulk Ge15Te85 samples measured using conventional DMS. (D) Comparison of peak temperatures in E″ for a range of frequencies between bulk and powder samples. Solid dots represent the data for powders after temperature calibration by subtracting 18 K to match the values found by conventional DMS. The fragility m can be extracted from the measurements of powder and bulk samples. The same information about the shape of E″(T) and temperatures can be derived from powder mechanical spectroscopy as from conventional DMS.

  • Fig. 2 Identification of β-relaxations in the non-PCMs (GeSe, Ge15Te85, and GeSe2) and PCMs (GeTe, Ge2Sb2Te5, and AIST) using powder mechanical spectroscopy.

    The loss modulus E″ is measured as a function of temperature at 1 Hz (top panels in each sub-figure) at a heating rate of 3 K/min. The corresponding DSC scans at a heating rate of 20 K/min are shown below each E″ curve. The non-PCMs GeSe (A), Ge15Te85 (B), and GeSe2 (C) exhibit a symmetric peak in E″, which can be modeled well with a Gaussian peak function (solid line). No clear excess wing is present in non-PCMs. By contrast, the PCMs GeTe (D), Ge2Sb2Te5 (E), and AIST (F) show a clear asymmetry in the E″ peak with a pronounced excess wing on the low-temperature flank of the main peak. The solid lines represent a Gaussian peak function fitting to the symmetric part of the main peak. The shadowed area visualizes the excess wings, indicating the presence of β-relaxations in PCMs. By contrast, the excess wing is vanishingly small for chalcogenide-based non-PCM glasses.

  • Fig. 3 The ratios of the excess wing area to the entire area are compared for PCMs (blue columns) and non-PCMs (orange columns).

  • Fig. 4 β-Relaxations as a function of composition.

    (A) Normalized loss modulus E″ at 1 Hz along the pseudobinary line (GeTe)1−x(GeSe)x (0 ≤ x ≤ 1). (B) The excess wing ratio remains nearly the same with increasing GeSe content until about 60%, where a sudden drop in excess wing ratio is observed. With higher GeSe content, the excess wing diminishes. (C) Electrical conductivity of amorphous solids at different temperatures along the same pseudobinary line decreases with increasing GeSe content. The solid dots represent data of this work; the open symbols represent the data taken from (60).

  • Fig. 5 A schematic illustration for α- and β-relaxation behaviors in PCM and non-PCM covalent glasses.

    The α-relaxation time (red line) shows a typical Vogel-Fulcher-Tammann (non-Arrhenius) temperature dependence in the supercooled liquid regime (T > Tg) and an Arrhenius behavior in the glassy state (T < Tg). β-Relaxation represents a secondary faster relaxation process, which is structurally distinct from the slower α-relaxation at lower temperatures and remains fast even in the glassy state. Inset: In non-PCM glasses, atomic motions are spatially constrained within the covalently bonded network. In amorphous PCMs, the local relaxation processes (i.e., β-relaxation) associated with the faster atomic motion (orange circles) are readily available.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/6/2/eaay6726/DC1

    Section S1. X-ray diffraction

    Section S2. Comparison between powder and conventional mechanical spectroscopy

    Section S3. Storage modulus E′ and loss modulus E″ measured using powder mechanical analyzer

    Fig. S1. X-ray diffraction results of as-deposited samples.

    Fig. S2. Testing powder mechanical spectroscopy using Y58Ni30Al12 metallic glasses.

    Fig. S3. Temperature-dependent DMS of Zr46Cu42Al8Ag4 metallic glasses with the testing frequency of 1 Hz.

    Fig. S4. Full scans of the non-PCMs (GeSe, Ge15Te85, and GeSe2) and PCMs (GeTe, Ge2Sb2Te5, and AIST) using powder mechanical spectroscopy.

  • Supplementary Materials

    This PDF file includes:

    • Section S1. X-ray diffraction
    • Section S2. Comparison between powder and conventional mechanical spectroscopy
    • Section S3. Storage modulus E′ and loss modulus E″ measured using powder mechanical analyzer
    • Fig. S1. X-ray diffraction results of as-deposited samples.
    • Fig. S2. Testing powder mechanical spectroscopy using Y58Ni30Al12 metallic glasses.
    • Fig. S3. Temperature-dependent DMS of Zr46Cu42Al8Ag4 metallic glasses with the testing frequency of 1 Hz.
    • Fig. S4. Full scans of the non-PCMs (GeSe, Ge15Te85, and GeSe2) and PCMs (GeTe, Ge2Sb2Te5, and AIST) using powder mechanical spectroscopy.

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