In this paper, we introduced a method to measure grain rotation of nanomaterials under external stress using a high pressure diamond anvil cell and the Laue microdiffraction technique at a synchrotron facility. We used tungsten carbide marker crystals to investigate grain rotation activities of 3 and 500 nm nickel media. Our results show that the grain rotation of 3 and 500 nm nickel nanocrystals increase with pressure and finally rotation of 500 nm nickel tends to stop at a lower pressure/stress level than 3 nm nickel. 3 nm nickel nanocrystals show a higher rotation magnitude than 500 nm nickel nanocrystals. Our measurements show an effective method to study the grain rotation of nanomaterials especially in ultrafine nanocrystals.

Jahn-Teller distortions are often observed in CuII-containing complex oxides resulting in remarkable changes of physical properties. Tuning the Jahn-Teller distortions with pressure has attracted considerable interest but is less studied in CuII-containing high-entropy oxides (HEOs). We found that pressure can suppress the structural distortions originating from the Jahn-Teller effect of CuO6 octahedra in the rock salt-type (Mg0.2Ni0.2Co0.2Zn0.2Cu0.2)O HEO and significantly modify its electronic properties. Synchrotron x-ray diffraction shows that the structural distortions arising from the Jahn-Teller effect of CuO6 octahedra are progressively suppressed, and the distorted structure evolves into a nearly ideal form under compression. Alternating current impedance and ultraviolet-visible absorption measurements reveal a dramatic resistance drop by more than three orders of magnitude and an obvious bandgap decrease in ∼0.1 eV, respectively, accompanied by the pressure-induced suppression of structural distortions. Our study presents a promising route for tuning the structural distortions and the electronic structures of CuII-containing HEOs for optimizing materials functionalities.

The conventional belief, based on the Read-Shockley model for the grain rotation mechanism, has been that smaller grains rotate more under stress due to the motion of grain boundary dislocations. However, in our high-pressure synchrotron Laue x-ray microdiffraction experiments, 70 nm nickel particles are found to rotate more than any other grain size. We infer that the reversal in the size dependence of the grain rotation arises from the crossover between the grain boundary dislocation-mediated and grain interior dislocation-mediated deformation mechanisms. The dislocation activities in the grain interiors are evidenced by the deformation texture of nickel nanocrystals. This new finding reshapes our view on the mechanism of grain rotation and helps us to better understand the plastic deformation of nanomaterials, particularly of the competing effects of grain boundary and grain interior dislocations.