Amorphization-Mediated Si-I to Si-V Phase Transition and Reversible Amorphous Si-V Phase Memory in Silicon Nanoparticles

Abstract An experiment has shown that ∼10 nm Si nanoparticles undergo a Si–I (diamond cubic) to Si–V (simple hexagonal) phase transition under compression, in contrast to the Si–I to Si–II (tetragonal) transition observed in bulk silicon. However, the atomistic mechanism underlying this size-dependent transition pathway remains unclear. Here, we employ molecular dynamics simulations with a machine learning interatomic potential to reveal a stress triaxiality-driven, two-step Si–I to Si–V transition pathway in a spherical Si nanoparticle subjected to an idealized triaxial contact loading model. An intermediate amorphous phase nucleates at the nanoparticle surface and propagates inward around the Si–I core in regions of low stress triaxiality dominated by shear. Within this amorphous shell, Si–V recrystallizes at locations with increased stress triaxiality and hydrostatic pressure. Upon unloading, the Si–V structure transforms into an amorphous state. A subsequent loading–unloading cycle applied to this amorphous nanoparticle reveals a reversible amorphous-to-Si–V transformation, demonstrating a nanoscale phase memory effect.