Template-Induced Precursor Formation in Heterogeneous Nucleation: Controlling Polymorph Selection and Nucleation Efficiency

We present an atomistic study of heterogeneous nucleation in Ni employing transition path sampling, which reveals a template precursor-mediated mechanism of crystallization. Most notably, we find that the ability of tiny templates to modify the structural features of the liquid and promote the formation of precursor regions with enhanced bond-orientational order is key to determining their nucleation efficiency and the polymorphs that crystallize. Our results reveal an intrinsic link between structural liquid heterogeneity and the nucleating ability of templates, which significantly advances our understanding toward the control of nucleation efficiency and polymorph selection.

Figure 1

Left: ${\overline{q}}_4-{\overline{q}}_6$ values for preorder liquid particles in precritical clusters ($n_s=50$) of the TPE that reach critical size and crystallize (purple dots), and in the precritical clusters of smaller sizes that commit back to the liquid phase (pink dots). Left top: Schematic representation of preordered liquid clusters with higher bond-orientational order (purple) that crystallize and lower bond-orientational order (pink) that melt. Right: ${\overline{q}}_4-{\overline{q}}_6$ values of selected seeds with different structures.

Figure 2

(a) Frequency of the formation of preordered clusters at the seed in supercooled liquid. Shown is the number of atoms $n$ of the seed that belong to the largest preordered cluster that emerges during fluctuations in the supercooled liquid, at $\mathrm{\Delta }T/T_m=0.20$. (b) ${\overline{q}}_4-{\overline{q}}_6$ values for preordered liquid particles that belong to the largest precritical cluster that emerges within homogeneous liquid (pink) and at selected seeds (black) in the liquid. Additional distributions for all other seeds are shown in SM [47].

Figure 3

(a) Distributions of the number of seed atoms $n$ included in precritical clusters ($n_s=50$) that successfully grow and crystallize (effective precursors). The precritical configurations were obtained from the TPE. (b) ${\overline{q}}_4,\overline{q_6}$ distributions of the effective precursors in the presence of different seeds. The yellow circle indicates the maximum of the distribution. (c) Free energy profiles $\mathrm{\Delta }G(n_s)$ and nucleation barriers for crystallization in the presence of various seeds, in comparison to homogeneous nucleation in Ni. (d) Structural composition of the growing nucleus in the presence of various seeds.