Dynamics of lattice disorder in perovskite materials, polarization nanoclusters and ferroelectric domain wall structures

The nexus between classic ferroelectricity and the structure of perovskite materials hinges on the concept of lattice disorder. Although the ordered perovskites display short-range displacements of the central cations around their equilibrium points, the lattice disorder dynamically unfolds to generate a myriad of distorted rhombohedral lattices characterized by the hopping of the central cations across <111> directions. It is discovered that the lattice disorder correlates with the emergence of minimum configuration energy <100> pathways for the central cations, resulting in spatially modulated ultrafast polarization nanocluster arrangements that are stabilized by the electric charge defects in the material. Through high-resolution phonon dispersion analyses encompassing molecular dynamics (MD) and density functional theory (DFT) simulations, we provide unequivocal evidence linking the hopping of central cations to the development of diffuse soft phonon modes observed throughout the phase transitions of the perovskite. Through massive MD simulations, we unveil the impact of lattice disorder on the structures of domain walls at finite-temperature vis-à-vis collective activation and deactivation of <100> pathways. Furthermore, our simulations demonstrate the development of hierarchical morphotropic phase boundary (MPB) nanostructures under the combined influence of externally applied pressure and stress relaxation, characterized by sudden emergence of zig-zagged monoclinic arrangements that involve dual <111> shifts of the central cations. These findings have implications for tailoring MPBs in thin-film structures and for the light-induced mobilization of DWs. Avenues are finally uncovered to the exploration of lattice disorder through gradual shear strain application.

Jan Očenášek, Ján Minár, Jorge Alcalá