Atomistic description for temperature-driven phase transitions in BaTiO3

Barium titanate (BaTiO3) is a prototypical ferroelectric perovskite that undergoes the rhombohedral-orthorhombic-tetragonal-cubic phase transitions as the temperature increases. In this paper, we develop a classical interatomic potential for BaTiO3 within the framework of the bond-valence theory. The force field is parametrized from first-principles results, enabling accurate large-scale molecular dynamics (MD) simulations at finite temperatures. Our model potential for BaTiO3 reproduces the temperature-driven phase transitions in isobaric-isothermal ensemble (NPT) MD simulations. This potential allows for the analysis of BaTiO3 structures with atomic resolution. By analyzing the local displacements of Ti atoms, we demonstrate that the phase transitions of BaTiO3 exhibit a mix of order-disorder and displacive characters. Besides, from a detailed observation of structural dynamics during phase transition, we discover that the global phase transition is associated with changes in the equilibrium value and fluctuations of each polarization component, including the ones already averaging to zero, Contrary to the conventional understanding that temperature increase generally causes bond-softening transition, the x-polarization component (the one which is polar in both the orthorhombic and the tetragonal phases) exhibits a bond-hardening character during the orthorhombic-to-tetragonal transition. These results provide further insight about the temperature-driven phase transitions in BaTiO3.