Diffusion in glasses studied with X-Ray Photon Correlation Spectroscopy

Diffusion in glasses studied with X-Ray Photon Correlation Spectroscopy

FWF Project NumberP28232-N36
Project Leaderao. Univ. Prof. Dr. Bogdan Sepiol
Duration1.1.2016–31.12.2018

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.

Ongoing work

The current work focuses on obtaining the structure factors for different alkali borates from total scattering experiments in order to evaluate and interpret the available XPCS data. This functions are needed in order to compare the results from XPCS measurements to models and additionally yields further information about the atomic arrangements in the sample via the pair distribution function (PDF). The latter quantity is basically obtained via Fourier transformation which is performed by the programs PDGgetX2 or PDFgetX3. A beam time (I-20160164 EC) at the beamline P02.1@PETRA III has been used to collect the data necessary to obtain the structure factors for all alkali borates investigated by aXPCS plus those of some high-entropy alloys. Prior to this beam time the measurements on a laboratory powder diffraction machine have proven to be inadequate in terms of signal to noise ratio. Further due to the hygroscopicity of the samples solid samples are preferable over powder samples due to the small surface to volume ratio. 

XPCS data already measured in earlier beam times and thus now available for data evaluation includes sodium-, potassium- and rubidium borate glasses at different concentrations, while the low and high Z alkalis, i.e. lithium and caesium, are projected for future beam times. Further tasks currently undertaken include computer modelling of the structure factors. Therefore, for a Born-Mayer-Huggins-type potential for alkali borates will be used for Metropolis Monte Carlo simulations. Originally this potential was developed in order to fit the vibrational spectra of alkali borates, but it fails to reproduce the structure factors. It is attempted to refine the potential parameters in order to fit the experimental structure factors. On success, this wild yield insight in to a highly probable real space configuration of the investigated alkali borates and thus allow to compute quantities like angle distributions.

Results

Work is currently in progress and we expect the first results to be published in early 2017.