  We have studied the pressure-induced structural transition of ZnO
nanocrystals using constant pressure molecular dynamics simulations for finite
system. We have observed the transition from the fourfold coordination wurtzite
to the sixfold coordination rocksalt structure, and the process of transition
is strongly dependent on the morphology of the nanocrystals. It is found that
the perfect faceted ZnO nanocrystals undergo wurtzite to rocksalt transition
with a perfect fivefold h-MgO structure as the intermediate status. But for the
faceted ones without perfect surface structure, as the number of the atoms
removed from the (001) and (00-1) surface edge increases, the local morphology
will become more similar to spherical. The nanocrystal will receive equal
stress from every direction and it will be more difficult to compress the
structure along only c axis as the perfect faceted ZnO nanocrystal. In this
situation, only partial structure experiences intermediate fivefold
coordination structure or even no intermediate fivefold coordination structure
exists dependent on the surface disorder level.
