Giant transformation strain and rotation in Ni-Pt nanoparticles caused by atomic ordering

We fabricated single crystalline faceted nanoparticles of near-stoichiometric NiPt alloy employing the solid state dewetting of Ni-Pt bilayers deposited on a sapphire substrate. The particles were annealed within the stability range of the ordered L1₀ phase. We uncovered recurrent changes of the fraction of disordered (100)-oriented particles, characterized by intermittent disorder-order transformations coupled with particle reorientation. The nucleation and expansion of ordered domains within the disordered (111)-oriented particles resulted in an increase of internal stresses and concomitant nucleation of twinning dislocations. Subsequent out-of-plane rotation via twinning has resulted in intermittent nanoparticles disordering due to the slip geometry in the ordered L1₀ phase. Notably, the re-orienting particles exhibited a macroscopic linear transformation strain reaching a value of 23 %. The discovery of ordering-induced particle rotation and reorientation in our study introduces a novel approach for engineering the functional properties of supported metal nanoparticles.