Atomistic In Situ Investigation of the Morphogenesis of Grains during Pressure‐Induced Phase Transitions: Molecular Dynamics Simulations of the B1–B2 Transformation of RbCl

Abstract

The mechanistic details of the pressure-induced B1-B2 phase transition of rubidium chloride are investigated in a series of transition path sampling molecular dynamics simulations. The B2→B1 transformation proceeds by nucleation and growth involving several, initially separated, nucleation centers. We show how independent and partially correlated nucleation events can function within a global mechanism and explore the evolution of phase domains during the transition. From this, the mechanisms of grain boundary formation are elaborated. The atomic structure of the domain-domain interfaces fully support the concept of Bernal polyhedra. Indeed, the manifold of different grain morphologies obtained from our simulations may be rationalized on the basis of essentially only two different kinds of Bernal polyhedra. The latter also play a crucial role for the B1→B2 transformation and specific grain boundary motifs are identified as preferred nucleation centers for this transition.

Polymorphic phase transitions: Macroscopic theory and molecular simulation

“…In contrast to this, first order transitions occur via nucleation and phase growth, and molecular organization occurring at the reactive interface for these transitions is unlikely to follow the structural changes proposed by Buerger-type analysis (see below in Section 5). This is also reflected by the final state found after a first order phase transition, as multiple nucleation events generally yield polycrystalline or domain structures with single crystal to single crystal transitions being exceptions and typically limited to small or nanocrystals [28,60].…”

“…In other terms, such reconstructive transformations provide comparably low strain within the phase domains at the cost of relatively high interface energy. For instance, reconstructive transitions were observed for the pressure-induced B1B2 transformation in alkali halides[60,133] (Figure 15). For these compounds transition-path sampling molecular dynamics simulations revealed that nucleation starts with the displacement of a single ion, followed by shifting of an entire column.…”

“…Thus, sufficiently large simulation models can capture coexisting nuclei and allow the investigation of phase domain coalescence, competing growth or merging by grain boundary formation. For a 14×14×14 supercell model of RbBr, the different senses of column-wise shuffling along the (initially) equivalent a, b and c direction of the parent lattice lead to 'mismatching' nuclei that, upon contact of the phase fronts, yielded a poly-crystalline structure comprising grains, even though the original structure was a coherent single crystal[60].…”

Collective displacements in a molecular crystal polymorphic transformation

“…Indeed, within the last decade, TPS simulations have revolutionized the investigation of solid-solid transformations by offering unbiased in situ investigations of phase nucleation and growth at the atomic level of detail. [22][23][24][25][26][27] Earlier, we reported molecular dynamics simulations of the b to a phase transformation in bulk crystals of DL-norleucine (2-aminohexanoic acid, C 6 H 13 NO 2 ) at a series of temperatures up to 390 K. 28,29 Both the a and b forms of DL-norleucine contain a 1 : 1 ratio of the R and S enantiomers in their respective lattices (Fig. 1) and display a similar molecular packing in the form of bilayers connected by tight hydrogen bonding between the -COO-and -NH 3 + groups.…”