Structure, thermodynamics, and relaxations in amorphous
phase-change materials
Shuai Wei*
Department of Chemistry, Aarhus University, Denmark*Email: shuai.wei@chem.au.dk

Amorphous phase-change materials (PCMs) (e.g. Ge-Sb-Te) belong to a family of functional
materials for novel electronic and photonic applications. PCMs can be rapidly and reversibly
switched between amorphous and crystalline phases. With large optical and electric property
contrast between the two phases, they can be used to encode data for non-volatile memory, photonic,and neuromorphic computing devices. However, due to their disordered structures and poor glass
forming ability, the amorphous phases are not well characterized, and a better understanding ofstructure-property relations is an urgency for the relevance of applications.
In this work, we demonstrate structure, thermodynamics, and relaxation behaviors of amorphous
PCMs over 14 orders magnitude in timescale and a thousand degree of temperature. In amorphoussolid, we show how thermal annealing and pressure may induce and diminish local distortions ofdefective octahedral structures (Peierls-like distortion), which drastically alter materials properties(e.g. stability, crystallization, compressibility, electronic density of states). Above the glasstransition temperature, materials enter a metastable supercooled liquid state, where thermodynamic
anomalies are uncovered and associated with dynamic crossovers and switching behaviors. We
show the evidence of a liquid-liquid transition in the metastable state with femtosecond diffractions
using X-ray free electron lasers (XFEL). At higher temperature above the melting point, the density
fluctuations of liquid state exhibit ultrafast structural relaxations. We demonstrate an experimental
strategy to directly observe atomic-scale relaxations using femtosecond X-ray photon correlation
spectroscopy by harnessing the coherent X-rays of XFEL. The fast atomistic dynamics isunderlying the low viscosity and high fragility behaviour of PCM liquids. We will also briefly
discuss the Boson peak in low-temperature heat capacity and development of new PCM alloys. A
better understanding of amorphous PCMs may lead to the tailoring of materials properties for novel
applications in photonic, and neuromorphic computing devices.

References
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S. Wei, P. Lucas, and C. A. Angell, Phase-Change Materials: The View from the Liquid Phase and the Metallicity
Parameter, MRS Bulletin 44, 691 (2019).
T. Fujita et al., Pressure-Induced Reversal of Peierls-like Distortions Elicits the Polyamorphic Transition in GeTe andGeSe, Nat Commun 14, 1 (2023).
Y. Cheng et al., Highly Tunable β-Relaxation Enables the Tailoring of Crystallization in Phase-Change Materials,
Nature Communications 13, 7352 (2022).
P. Zalden et al., Femtosecond X-Ray Diffraction Reveals a Liquid–Liquid Phase Transition in Phase-ChangeMaterials, Science 364, 1062 (2019).
S. Wei, P. Lucas, and C. A. Angell, Phase Change Alloy Viscosities down to Tg Using Adam-Gibbs-Equation Fittings
to Excess Entropy Data: A Fragile-to-Strong Transition, J. Appl. Phys. 118, 034903 (2015).
S. Wei, Anomalous Liquids on a New Landscape: From Water to Phase-Change Materials, Journal of NonCrystalline Solids: X 14, 100094 (2022).

Shuai Wei is tenure-track Assistant Professor in Materials Chemistry and Centre of Integrated
Materials Research at Aarhus University, Aarhus, Denmark. He is a VILLUM Young Investigator and aformer AIAS Associated Fellow. His research interests are novel amorphous materials for innovativetechnologies, including phase-change materials (e.g. Ge-Sb-Te) for next generation non-volatilememory applications and 3D printed amorphous metals for structural applications.
Group homepage: https://chem.au.dk/AmorphousMatLab