Abstract
Glasses are often described as supercooled liquids, whose structures are topologically disordered like a liquid, but nevertheless retain short-range structural order. Structural complexity is often associated with complicated electron-charge distributions in glassy systems, making a detailed investigation challenging even for short-range structural order, let alone their atomic dynamics. This is particularly problematic for lone-pair-rich, semiconducting materials, such as phase-change materials (PCMs). Here, this study shows that analytical methods for studying bonding, based on the electron-charge density, rather than a conventional atomic pair-correlation-function approach, allows an in-depth investigation into the chemical-bonding network, as well as lone pairs, of the prototypical PCM, Ge2 Sb2 Te5 (GST). It is demonstrated that the structurally flexible building units of the amorphous GST network, intimately linked to the presence of distinctly coexisting weak covalent and lone-pair interactions, give rise to cooperative structural-ordering processes, by which ultrafast crystal growth becomes possible. This finding may universally apply to other PCMs.
Highlights
Applications of phase-change materials (PCMs) make use of their ultrafast crystallization behavior topologically disordered like a liquid, but retain short-range and large electronic constructural order
The coordination number (CN) of atoms is investigated in terms of pair-correlation functions, i.e., the number of neighbors within a certain distance from an origin atom
Elliott Department of Chemistry University of Cambridge we consider the electronic-charge distribution via ab initio molecular-dynamics (AIMD) simulations, along with analytic methods based on local electronic structures, revealing new Lensfield Road, Cambridge CB2 1EW, UK E-mail: thl32@cam.ac.uk; sre1@cam.ac.uk findings, concealed in the conventional approach
Summary
Applications of PCMs make use of their ultrafast crystallization behavior topologically disordered like a liquid, but retain short-range and large electronic (or optical) constructural order. Www.advmat.de the number of electrons involved in forming bonds or lone pairs exceeds eight per atom in their Lewis structures, the (4,1) and (5,1) units of Ge or Sb atoms can be regarded as hypercoordinated units.
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