Molecular dynamics simulation of the six- to four-coordinate pressure-driven transition in MX nanocrystals: Mechanistic consequences of Σ3 grain boundaries in the high-pressure starting structure

Constant-pressure molecular dynamics has been used to simulate the six- to four-coordinate downstroke pressure-driven phase transition in B1 nanocrystals. The nanocrystals considered have previously been formed in upstroke B3→B1 simulations, giving them an amorphous surface region and interior Σ₃ grain boundaries. Nucleation occurs in the interior of the crystal, with multiple nucleation events observed along grain boundaries, in contrast to previous decompression simulations of single-domain nanocrystals with crystallographically well-defined surfaces. Competing mechanisms give rise to B3, B4, and d-BCT domains in the productstructures. Four distinct mechanisms are observed, including two B4↔B1 mechanisms. The B4↔IB1 mechanism is the same as seen in previous simulations of single-domain nanocrystalline and bulk systems, while the observed B3↔B1 and B4↔IIB1 display a [111]B3↔[100]B1 correspondence, in agreement with experimental observations of repeatedly transformed CdSe nanocrystals; both of which are different to those seen in previous upstroke simulations. The interaction between these competing mechanisms determines the domain structure of the product nanocrystals.