Layer-by-layer phase transformation in Ti3O5 revealed by machine-learning molecular dynamics simulations

Abstract

Reconstructive phase transitions involving breaking and reconstruction of primary chemical bonds are ubiquitous and important for many technological applications. In contrast to displacive phase transitions, the dynamics of reconstructive phase transitions are usually slow due to the large energy barrier. Nevertheless, the reconstructive phase transformation from β- to λ-Ti3O5 exhibits an ultrafast and reversible behavior. Despite extensive studies, the underlying microscopic mechanism remains unclear. Here, we discover a kinetically favorable in-plane nucleated layer-by-layer transformation mechanism through metadynamics and large-scale molecular dynamics simulations. This is enabled by developing an efficient machine learning potential with near first-principles accuracy through an on-the-fly active learning method and an advanced sampling technique. Our results reveal that the β−λ phase transformation initiates with the formation of two-dimensional nuclei in the ab-plane and then proceeds layer-by-layer through a multistep barrier-lowering kinetic process via intermediate metastable phases. Our work not only provides important insight into the ultrafast and reversible nature of the β−λ transition, but also presents useful strategies and methods for tackling other complex structural phase transitions.

Effect of Surface Free Energies and Particle Diameter on the Ti3O5 β → λ Phase Transition Temperature: A Theoretical Study

“…This overestimates the experimental value, 470 K, 2 by 55 K. More recently, a similar phase transition temperature of T p = 535 K was calculated by means of well-tempered metadynamics simulations employing the PBE/520 eV method. 28 These simulations were also based on bulk models of Ti 3 O 5 . Our goal is to improve the theoretical model ansatz by incorporating surface effects.…”

“…The β → λ phase transition temperature T p , which was earlier calculated with bulk models, 24,28,33 was recalculated by taking surface effects into account (Figure 5(c)).…”