Role of pre-ordered liquid in the selection mechanism of crystal polymorphs during nucleation

TL;DR

This study reveals that pre-structured regions in supercooled molybdenum liquid guide the nucleation process and influence the specific crystal polymorph formed, highlighting early-stage polymorph selection during nucleation.

Contribution

It uncovers the atomistic mechanism of polymorph selection in molybdenum nucleation, emphasizing the role of pre-structured regions as precursors that determine the final crystal form.

Findings

Pre-structured regions facilitate nucleation by lowering interfacial free energy.

Precursor structures are distinct from liquid and solid phases.

Polymorph selection occurs early in the nucleation process.

Abstract

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.