Polymorphism, which describes the occurrence of different lattice structures in a crystalline material, is a critical phenomenon in materials science and condensed matter physics. Recently, configuration disorder was compositionally engineered into single lattices, leading to the discovery of high-entropy alloys and high-entropy oxides. For these novel entropy-stabilized forms of crystalline matter with extremely high structural stability, is polymorphism still possible? Here by employing in situ high-pressure synchrotron radiation X-ray diffraction, we reveal a polymorphic transition from face-centred-cubic (fcc) structure to hexagonal-close-packing (hcp) structure in the prototype CoCrFeMnNi high-entropy alloy. The transition is irreversible, and our in situ high-temperature synchrotron radiation X-ray diffraction experiments at different pressures of the retained hcp high-entropy alloy reveal that the fcc phase is a stable polymorph at high temperatures, while the hcp structure is more thermodynamically favourable at lower temperatures. As pressure is increased, the critical temperature for the hcp-to-fcc transformation also rises.

Stress is a long standing challenge for the applications of silicon (Si) anodes in lithium (Li) ion batteries. Nanostructured Si are important materials to address mechanical stress issues in batteries although their stress were only calculated and no experimental data are available. Using in situ Raman microscopy to monitor the shift of the first-order Raman peak of © 2016. This manuscript version is made available under the Elsevier user license http://www.elsevier.com/open-access/userlicense/1.0/ Si, we were able to measure for the first time the lithiation-induced stress in Si nanoparticles.The shift of Raman peak of Si under hydrostatic stress was calibrated via an in situ high pressure Raman experiment. We observed a transition in the stress in Si core of nanoparticles during lithiation, from tensile to compressive. At the beginning of lithiation, the reduction of the native oxide surface layer of the Si particle results in a tensile stress of approximately 0.2 GPa in Si.During the formation of amorphous Li x Si in the outer layer of the nanoparticles, an increasing compressive stress up to 0.3 GPa is introduced to the Si core. This evolving stress explains the cracks that developed in the amorphous Li x Si layer during lithiation of the Si nanoparticles, and is also consistent with modeling results. These results improve our understanding of lithiationinduced stress in nanostructured Si anodes, and provide valuable information for their theoretical study and future design.