  We present an atomistic description of the {\it fcc}--to--{\it hcp}
transformation mechanism in solid argon (Ar) obtained from transition path
sampling molecular dynamics simulation. The phase transition pathways collected
during the sampling for an 8000--particle system reveal three transition types
according to the lattice deformation and relaxation details. In all three
transition types, we see a critical accumulation of defects and uniform growth
of a less ordered transition state, followed by a homogeneous growth of an
ordered phase. Stacking disorder is discussed to describe the transition
process and the cooperative motions of atoms in \{111\} planes. We investigate
the nucleation with larger system. In a system of 18000--particles, the
collective movements of atoms required for this transition are facilitated by
the formation and growth of stacking faults. However the enthalpy barrier is
still far beyond the thermal fluctuation. The high barrier explains previous
experimental observations of the inaccessibility of the bulk transition at low
pressure and its sluggishness even at extremely high pressure. The transition
mechanism in bulk Ar is different from Ar nanoclusters as the orthorhombic
intermediate structure proposed for the latter is not observed in any of our
simulations.
