Pressure-Driven Phase Transitions in Crystalline Nanoparticles: Surface Effects on Hysteresis

Abstract

Single domain nanocrystals in a dense rock salt-structured (B1) polymorph of a material that adopts the wurtzite (B4) structure under ambient conditions may be prepared by pressurizing a suspension of wurtzite nanoparticles. Here the way the particle surface affects the reverse transition (B1 → B4) on depressurization is examined in molecular dynamics simulations: it is shown to affect the degree of hysteresis in the phase transition and the possibility of recovering metastable single domain nanocrystals of the dense polymorph at low pressures. B1-structured nanocrystals with a hexagonal prismatic shape, which have been formed by previous pressurization simulations of B4 → B1 transitions, show a single surface nucleation event and subsequent ripening to form single domain products with a retention of the crystallographic orientation. Cubic B1-nanocrystals with only {100}B1 surfaces exposed, which represent well-annealed particles of the high-pressure phase, transform in a two-step process with multiple surface nucleation events and delayed growth into the particle interior to form complex multigrain products. The local atomic rearrangement mechanism and the domain growth process are described in detail, and how these interact with the morphology of the starting crystal to determine the internal geometry and morphology of the product low-pressure particles is examined.