Violation of the Stokes–Einstein relation in Ge2Sb2Te5, GeTe, Ag4In3Sb67Te26, and Ge15Sb85, and its connection to fast crystallization

Shuai Wei, Christoph Persch, Moritz Stolpe, Zach Evenson, Garrett Coleman, Pierre Lucas, Matthias Wuttig

Research output: Contribution to journalArticlepeer-review

3 Scopus citations

Abstract

Phase-change materials (PCMs) are already commercialized in optical and non-volatile memory devices. Yet, the dynamics of atomic rearrangement processes and their temperature dependence, which govern their ultrafast switching, are still not fully understood. Here we use quasi-elastic neutron scattering to investigate the liquid-state dynamics of four prevailing PCMs Ge2Sb2Te5, GeTe, Ag4In3Sb67Te26(AIST), and Ge15Sb85 above their respective melting points Tm. Self-diffusion coefficients and structural relaxation times on the timescale of picoseconds are extracted from dynamic structure factors. The results indicate an unusual systematic violation of the Stokes-Einstein relation (SER) for each PCM in high-temperature regions above Tm, where the atomic-mobility is high. This is likely related to the formation of locally favored structures in liquid PCMs. Absolute values of diffusivity in the supercooled liquid AIST are derived from crystal-growth velocity, which are almost one order of magnitude higher than that expected from the SER in the technologically relevant temperature range ~20% below Tm. This is relevant to understand the crystallization kinetics of PCMs as crystal growth is controlled by diffusivity. Furthermore, the instantaneous shear modulus is determined ranging from 2 to 3 GPa for liquid PCMs, which permits extracting viscosity from microscopic structural relaxations usually accessible to simulations and scattering techniques.

Original languageEnglish (US)
Pages (from-to)491-500
Number of pages10
JournalActa Materialia
Volume195
DOIs
StatePublished - Aug 15 2020

Keywords

  • Crystallization
  • Diffusion
  • Phase-change materials
  • Structural relaxation
  • Viscosity

ASJC Scopus subject areas

  • Electronic, Optical and Magnetic Materials
  • Ceramics and Composites
  • Polymers and Plastics
  • Metals and Alloys

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