Exploring the origin of the breakdown of the Stokes-Einstein Relation above the melting temperature in metalloid glassforming systems

• Applicant: Dr. Shuai Wei
• Subject Area: Thermodynamics and Kinetics as well as Properties of Phases and Microstructure of Materials
• Term: since 2019
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 422219280

The central objective of the proposed research is to gain insight into the supercooled and equilibrium liquid-state behaviors of the metalloid glassforming systems, and to understand the origin of the breakdown of the Stokes-Einstein relation (SER) above the melting temperature Tm. Furthermore, we will explore the possible liquid-liquid transitions (LLTs) in these alloys and reach a better understanding of the relationship between the liquid-state behaviors (likely including LLTs), crystallization kinetics and glass transitions.We propose to use quasi-elastic neutron scattering (QENS) experiments for determining the temperature dependence of mean self-diffusion coefficients and the α-relaxation times in the liquid metalloid glassforming systems. The systems include those technologically important phase-change materials (PCMs) such as GeTe, Ge2Sb2Te5, Ge15Sb85, Sb2Te3, and AIST, as well as the non-PCMs metalloid systems such as GeSe, As2Te3 and As2Se3. By measuring the diffusivities and α-relaxation times, the validity of the SER can be studied experimentally near and above the Tm. The results will allow us to determine the circumstances of validity of the SER in PCMs and non-PCMs and gain insight into the liquid dynamics on different microscopic length-scales. In the cases of the SER breakdowns, the parameters for a modified ‘fractional’ SER model will be determined for describing experimental data. An anomalous breakdown of the SER near Tm would provide supporting evidence for the hypothesis of the existence of a LLT below Tm hidden by crystallization in PCMs. The latter may play an important role in the ultrafast phase switching for non-volatile memory devices applications.Furthermore, by investigating those systems which have both the SER breakdown and a LLT above Tm, we aim to establish a quantitative relation between the SER breakdowns and the LLTs. This helps understand the cases where the LLT is below Tm hidden by crystallization. We will clarify whether the SER breakdown associated with a LLT (or LLCP) has the same origin as the ones that happen at lower temperature close Tg. The investigation will be extended towards the undercooled liquid regime below Tm obscured by fast crystallization. Since the relaxation dynamics of the undercooled liquid is closely related to crystallization kinetics, the crystal growth velocity of those compositions, where a SER breakdown would be observed in the QENS experiments, will be determined using the time-resolved pump-and-probe laser reflectivity technique. The manifestation of a LLT below Tm (e.g. diffusivity and viscosity anomalies) should be reflected in the temperature dependence of growth velocity and detected by the proposed POT experiments. The results can help clarify the existence of LLTs (and fragile-strong transitions) in metalloid systems, allowing for identifying a signature of LLTs in crystallization kinetics and providing insight into the mechanism of the SER breakdowns.

• DFG Programme: Research Grants
• International Connection: Denmark, USA
• Cooperation Partners: Professor Dr. C. Austen Angell; Professor Dr. Pierre Lucas