Atomic displacements in High-Entropy Alloys examined

High-entropy alloys of 3d metals have intriguing properties that are interesting for applications in the energy sector. An international team at BESSY II has now investigated the local order on an atomic scale in a so-called high-entropy Cantor alloy of chromium, manganese, iron, cobalt and nickel. The results from combined spectroscopic studies and statistical simulations expand the understanding of this group of materials.

High-entropy alloys are under discussion for very different applications: Some materials from this group are suitable for hydrogen storage, others for noble metal-free electrocatalysis, radiation shielding or as supercapacitors.

The microscopic structure of high-entropy alloys is very diverse and changeable; in particular, the local ordering and the presence of different secondary phases affect significantly the macroscopic properties such as hardness, corrosion resistance and also magnetism. The so-called Cantor alloy, which consists of the elements chromium, manganese, iron, cobalt and nickel mixed in an equimolar proportion, can be considered as a suitable model system for the whole class of these materials.

Local structure studied at BESSY II

Scientists from the Federal Institute for Materials Research (BAM, Berlin), the University of Latvia in Riga, Latvia, the Ruhr University in Bochum and the HZB have now studied the local structure of this model system in detail. Using X-ray absorption spectroscopy (EXAFS) at BESSY II, they were able to precisely track each individual element and their displacements from the ideal lattice positions for this system in the most unbiased manner with the help of statistical calculations and the reverse Monte Carlo method.

Read more on the HZB website

Image: The supercell is randomly filled with the five elements on the fcc-lattice positions; In the starting configuration, all layers are precisely on top of each other. The displacements of all elements in the final configuration have been revealed by a simultaneous fit of the independent experimental spectra with a use of Reverse Monte Carlo simulations.

Credit: © A.Kuzmin / University of Latvia and A. Smekhova / HZ

In situ spectroscopy as a probe of electrocatalyst performance

Hydrogen fuel cells generally require expensive and scarce platinum catalysts in order to function. Researchers have created highly reactive platinum-nickel nanowires with the potential to reduce the amount of platinum required in fuel cells. Research at PIPOXS examines the atomic-level mechanisms of this catalyst, forming a foundation for the development and commercialization of more efficient fuel cell technology.

What is the new discovery?


The oxygen reduction reaction (ORR) is an important and often limiting component of hydrogen fuel cell operation.  To facilitate this reaction, platinum-based catalysts are often used to increase its rate, though the expense and limited availability of Pt present challenges to its widespread use.  In this work, researchers selectively replaced a portion of the nickel atoms of nickel nanowires with platinum to create platinum-nickel nanowires (PtNi-NWs) as high surface area catalysts that reduced the total amount of platinum required.  These PtNi-NWs were found to be highly active, and so operando x-ray absorption spectroscopy and extended x-ray absorption fine structure (EXAFS) experiments were conducted at the PIPOXS beamline to assess the electronic and geometric changes occurring in these catalysts during their use.   These data enabled the researchers to determine that the Pt formed an alloy with the Ni in the NW and that its interaction with oxygen remained constant regardless of the external potential applied.  

Read more on the CHESS website

Image: Schematic showing the electrochemical cell used for the operando measurements, and how the EXAFS data can be used to deduce the chemistry happening during this reaction.

Visualising shared-ligand intermediates of metal exchange

Visualized by Rapid Freeze Quench and Selenium EXAFS of Se-Labeled Metallochaperones. A Paradigm for Studying Copper-Mediated Host-Pathogen Interactions.

Mammalian hosts defend against invading pathogens via the import of toxic concentrations of copper into the phagolysosome. To combat this host-defense strategy, gram negative pathogens respond via sophisticated copper export systems which are able to neutralize the copper onslaught2. Chemical mechanisms of metal exchange between protein components of metal exporters are thus important factors in understanding pathogenic virulence and are believed to occur via formation of intermediates in which the metal is coordinated by ligands derived from each partner.  However, since these ligand sets are often similar (or even identical), following the kinetics of transfer has been challenging, and has required the development of sophisticated spectroscopic approaches.

>Read more on the SSRL website

Image: Middle: Se EXAFS Fourier transforms at increasing time points for the reaction of SeM-labeled apo-CusF with unlabeled Cu(I)-loaded CusB.  Left and right: in silico models of the proposed protein-protein interface and shared-ligand intermediate.