  Understanding the underlying mechanism of crystal nucleation during
solidification is a fundamental aspect in the prediction and control of
materials properties. Classical nucleation theory (CNT) assumes that
homogeneous nucleation occurs via random fluctuations within the supercooled
liquid, that the structure of the growing clusters resembles the most stable
bulk phase, and that the nucleus size is the sole reaction coordinate (RC) of
the process. Many materials are, however, known to exhibit multiple steps
during crystallization, forming different polymorphs. As a consequence, more
complex RCs are often required to capture all relevant information about the
process. In this work, we employ transition path sampling together with a
maximum likelihood analysis of candidate order parameters to identify suitable
reaction coordinates for the nucleation mechanism during solidification in Ni.
In contrast to CNT, the analysis of the reweighted path ensemble shows that a
pre-structured liquid region that surrounds the crystal cluster is a relevant
order parameter that enhances the RC and therefore plays a key role in the
description of the growing nucleus and the interfacial free energy. We
demonstrate that pre-structured liquid clusters that emerge within the liquid
act as precursors of the crystallization in a non-classical two-step mechanism
which predetermines the coordination of the polymorphs that are being selected.
