Science

I15-1 is the dedicated X-ray Pair Distribution Function (XPDF) beamline. XPDF gives local structure information for materials, allowing you to see correlations between atoms on length-scales all the way from single bonds up to tens of nanometers. It's very useful data that can give information on a wide range of samples. Very few researchers have access to XPDF in their home labs, but on I15-1 we can run a sample in about 10 minutes. We therefore can give many more people access to this technique by scheduling occasional days for Rapid Access experiments throughout the year. We often work with several user groups on each of those days.

We've been working hard to make it quick and easy for users to load their samples for use with the robot. It will greatly increase our capacity for Rapid Access experiments, both at room temperature and under variable temperature conditions.

While I was working as a post-doc at ISIS neutron and muon source, I was the main developer of the RMCProfile7 program, dedicated to large box Reverse Monte Carlo simulations and refinement of Pair Distribution Function (PDF) data. An update to the program contained several novel capabilities, and we needed some specific datasets to test it.

I applied for a Rapid Access to the Diamond I15-1 beamline, which is state-of-the-art for high-quality PDF data on powder samples. I was granted beamtime within a week, and the experiment was performed remotely by the beamline staff a month later. The beamline staff also helped analyse the data.

The new feature for RMCProfile is the "rigid body" constraint. It should allow quicker refinement for structures such as molecular crystals, where the structure is built of molecules which - by definition - are rigid bodies.”

The overall aim of our research project is to investigate the relationships between the structure and electrochemical properties of clathrate compounds in the context of their use as potential electrode materials in lithium-ion battery applications. These compounds have cage-like structures and there has been little understanding of how the features of the initial structure evolve during lithium insertion reactions, particularly how the cage-structure collapses when the material is stuffed with too much lithium.

Pair distribution analysis has emerged as a powerful tool in the battery community for studying phase transformations that take place in Li-ion battery electrodes, particularly for probing the local structure when amorphisation reactions take place. We used the I15-1 Rapid Access opportunity to obtain PDF data of silicon and germanium-based clathrate electrode materials with different amounts of electrochemically inserted lithium.

The results showed substantial changes in the local structure upon reaction with lithium, which motivated us to submit a proposal for on-site access to investigate more samples and obtain a holistic view of the phase transformations and amorphisation reactions that take place. With the aid of in situ heating of the samples during our on-site beamtime, we were able to assess the stability of the lithiated amorphous phases and their relationship to known crystalline phases. The end result is a better understanding of the lithiation pathway in clathrate materials and the structural origins of the observed electrochemical properties, which can aid the design of novel materials for improved battery performance.