  We investigate the atomistic mechanism of homogeneous nucleation during
solidification in molybdenum employing transition path sampling. The mechanism
is characterized by the formation of a pre-structured region of high
bond-orientational order in the supercooled liquid followed by the nucleation
of the crystalline bulk phase within the center of the growing solid cluster.
This precursor plays a crucial role in the process, as it provides a diffusive
interface between the liquid and crystalline core, which lowers the interfacial
free energy and facilitates the nucleation of the bulk phase. Furthermore, the
structural features of the pre-ordered regions are distinct from the liquid and
solid phases, and preselect the specific polymorph that nucleates. The
similarity in the nucleation mechanism of Mo with that of metals that exhibit
different crystalline bulk phases indicates that the formation of a precursor
is a general feature observed in these materials. The strong influence of the
structural characteristics of the precursors on the final crystalline bulk
phase demonstrates that for the investigated system polymorph selection takes
place in the very early stages of nucleation.
