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

Activation of order-disorder dynamics in crystalline Buckybowls

Dibenzo[ghi,mno]Fluoranthene, akacorannulene (C20H10), is a peculiar bowl-shaped molecule displaying unusual pentagonal symmetry and building block of the most celebrated Buckminster Fullerene – C60

Its nanostructured arrangement together with the eminent dipole moment of 2.1 Debye and a high electron affinity, make this system largely appealing for its use in energy-related applications, such as in hydrogen storage, ion-batteries, or super-capacitors. Additionally, this molecule has been suggested to be a component of interstellar dusts.   
In this work, published in Carbon, an international group including researchers from Italy, United Kingdom, Spain, and China has brought to the fore the unexpectedly rich thermophysical behaviour of this system in the thermal range 200 – 600 K, not anticipated on the basis of previous studies.

Combining state-of-the-art synchrotron (MCX beamline, Elettra) and neutron (IRIS beamline, ISIS) scattering techniques, together with differential scanning calorimetry (DSC), for the first time a well defined pre-melting transition has been clearly identified starting at about 382 K, well below the melting point of 540 K, resulting in the progressive suppression of molecular and supramolecular order and associated to the emergence of rotor-like states, as highlighted by the decrease in the elastic intensity and the sizeable increase in the quasi-elastic scattering (see Fig 1b– showing a marked separation in temperature between the two regimes).  

Read more on the Elettra website

Image: Figure 1.  (a) Synchrotron powder diffraction and Rietveld refinement of the room temperature average structure of Corannulene. (b) Temperature dependent quasi-elastic (IQE) and elastic (Iel) fractions, highlighting the transition to the dynamic state.