Non-deterministic Dynamics in η-to-θ Phase Transition of Alumina Nanoparticles

Phase diagrams and crystallography are standard tools for studying structural phase transitions while acquiring kinetic information at the atomistic level has been considered essential but challenging. Here, we report a rare example of a statistical kinetics study, showing that the η-to-θ phase transition of alumina, unidirectional in bulk and retaining the crystal lattice orientation, occurs non-deterministically via an ergodic equilibrium via molten state, as observed for nanoparticles on bulk Al(OH)3 surface. The memory of the lattice orientation is lost. Statistical kinetic analysis of individual nanoparticles revealed that the crystalline and amorphous states are similar in enthalpy. Thus, the interconversion is an entropy-controlled thermal process, and this non-deterministic kinetics should be a key feature of crystal nucleation in high surface energy regions of bulk crystals.