FWF projects

P28232-N36

Abstract

The movement of single atoms is a fundamental issue in materials science. The fabrication, specific properties and the stability of materials can be significantly improved with knowledge about atomic movement. Numerous properties of materials can be attributed to the single atom motion. Studies of diffusion mechanisms on the atomic scale are challenging despite a number of well-established methods which can be applied to experimentally investigate diffusion in solid state systems. Drawing conclusions from macroscopic measurements on the actual microscopic dynamics is a highly indirect procedure. Due to the relatively long timescales where atomic motion takes place, scattering techniques that rely on a high energy-resolution can only resolve fast processes. The greatest challenge is, however, that only a small number of selected isotopes are accessible to these methods which strongly limits their use. A new method not restricted to certain isotopes and capable of detecting slow diffusion is therefore required. As we have shown in our previous projects these requirements are met by atomic-scale X-ray photon correlation spectroscopy (aXPCS). Former grants served the implementation of XPCS as the preferred method for studying atomic dynamics mainly in crystalline phases. The current project is devoted to studying another very intriguing form of matter, namely the family of glasses. 

Glasses are an active and promising field of research. Understanding disordered solids on a fundamental level and particularly understanding ion conduction in these materials still presents a fundamental challenge. The random formation of structural networks is an important model for explaining the glass forming ability of many materials. The dynamics of basic network components play a key role in decoding puzzling properties of this form of matter like ionic dynamics. Considerable progress with solid-oxide fuel cells, batteries and supercapacitors, in electrochemical sensors and functional polymers has been achieved recently. Even so, the basic concept of ion transport in disordered materials remains poorly understood. This is due to the fact that there is no simple, widely accepted model of transport. Considering the current strong interest in the field and the plethora of experimental and theoretical works it is striking that, in contrast to transport properties in crystalline matter, there is no general consensus on several fundamental questions. It is our aim to shed light on the motion of ions on the atomic level in ionic conducting boron and silica glass and we are confident that the insights gained can be transferred to other ionic glasses.

In our project we will profit from the continuously increasing brilliance of existing synchrotron sources like the upgrade of the ESRF in Grenoble, which was just completed and from new sources like PETRA III in Hamburg.

For in-depth information about the project, please visit its respective page.

P22402-N20

Abstract

Understanding atomic motion in solids is a fundamental issue in synthesis and stability of technically important materials; this is even more critical in nanomaterials. The timescale of atomic motion is usually uncomfortably long for experimental techniques that rely on high energy resolution (e.g. inelastic neutron scattering or Mößbauer spectroscopy), forcing measurements at unduly high temperatures. Moreover, only a small number of selected isotopes are accessible to these methods. There exists, of course, plenty of information about atomic diffusion from tracer measurements. However, drawing conclusions from tracer measurements on the actual microscopic dynamics is a highly indirect procedure, often contradictory, and always based on assumptions. Hence, a nonresonant method not restricted to certain isotopes and capable of detecting slow diffusion is extremely desirable.

We have shown in our previous project the answer to these requirements: X-ray photon correlation spectroscopy (XPCS). This method monitors the temporal variations of the coherently scattered intensity as a function of the wavevector, that is, it measures dynamics directly in the Fourier domain. It has been applied since the emergence of high-energy synchrotron sources in the mid-90s for studies of slow dynamics on the nanometer scale, but had never been practiced before on single atoms. We have taken XPCS to its limits in terms of resolution by using it to reveal the slow atomic dynamics in a copper-gold alloy. The project at hand follows up on this success and establishes XPCS as the preferred method for studying atomic dynamics. Specifically, we have studied technically important high-temperature structural materials like Ni-Pt alloys, metallic glasses, and oxide glasses. These studies give a direct microscopic picture of the mechanisms leading to atomic transport, which can be very intricate in the case of ordered alloys. They also yield the temperature dependence of diffusion (i.e. the activation energy) in the low-temperature region.

In our project we profit from the continually increasing brilliance of existing synchrotron sources (cp. imminent upgrade of the ESRF in Grenoble) and from emerging new sources like PETRA III and the European XFEL in Hamburg.

For in-depth information about the project, please visit its respective page.