Microscopic Structure Studies to understand the Thermal Stability of Amorphous Copper-Telluride-Phases

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Two different groups of amorphous Ge-Cu-Tellurides (GCT) are currently regarded to comprise great potential for possible technical applications. While compositions Cux+yGe20 xTe80-y (0≤x≤20, 0≤y≤10) show excellent thermoelectric properties, mixtures on the (FeTe) – (CuTe) quasi-binary line in the GCT-phase diagram, were identified as Phase-Change-Materials (PCM) and are candidates for non-volatile digital data storage-applications. These two material groups are located relatively close to each other in the GCT phase diagram yet, a fundamentally different thermal stability is observed with varying Cu-concentration. While the crystallization temperature of the thermoelectric materials (TE) drops strongly when Copper is constantly added, a significant rise is observed in the PCMs up to relative high Cu-concentrations, where it finally starts dropping however remaining always above the TE-values. Especially for the TE-systems a high crystallization temperature, implying high thermal stability, would be desirable for their use in technical applications. The origin for this different behavior is yet unidentified. More generally, the reasons determining crystallization temperature and related thermal stability in multi component glasses in general are still widely unknown although such an understanding is highly desired for the thermal optimization of technical glasses.Only one structural study yet exists for each of the systems, both carried out with EXAFS, at completely different Cu-concentrations. Only information about the chemical short range order was obtained and no notable cognition on the thermal stability or other physical properties was acquired. In the requested project we would like to systematically study the variation of the microscopic structure of both systems with rising Cu-content to identify the structural origin for the different stability behavior. For this we will us Anomalous X-ray scattering (AXS) combined with Reverse Monte Carlo simulations, which allows to gain atom-specific structural information on the chemical short range order but also on larger length scales. This allows characterizing structural correlations between chemically ordered short range structures. It will be investigated if such structural correlations, or even more specific the resulting similarities to the corresponding crystal phase, affects the stabilities of the amorphous phases. This is indicated by results obtained from a preceding study on amorphous Ge-Sb-Te-compositions. It would be an important first step towards a universal understanding of the thermal stability in amorphous multi component systems if such a relation could be verified and therefore also highly important for directed progress in the development of new amorphous materials.