Anisotropic Lattice Vibration, Motion of Atoms, and Local Structure in Pure Zirconia Crystals with Vacancies

In this study, we performed classical molecular dynamics computer simulations at a constant pressure and temperature for a pure zirconia (ZrO2) crystal containing zirconium vacancies and oxygen vacancies by employing the Buckingham potential. We examined the temporal evolution of positions of atoms in the steady state and investigated the anisotropic motion of atoms and the local structure of a pure ZrO2 crystal. The results indicated anisotropic lattice vibrations of oxygen and zirconium atoms in tetragonal ZrO2. The component of the lattice vibration of zirconium atoms parallel to the c-axis exceeded that parallel to the a-axis in tetragonal phase. On the other hand, the component of the lattice vibration of oxygen atoms parallel to the a-axis exceeded that parallel to the c-axis in the tetragonal phase. This was because oxygen atoms in the tetragonal phase were alternately shifted up and down from the corners of a rectangular parallelepiped parallel to the c-axis. The results indicated anisotropic behavior in the large displacements of atoms in both phases. In the tetragonal phase, the component of large displacements parallel to the c-axis of oxygen atoms was suppressed owing to the geometrical constraint of a tetragonal phase. Furthermore, the anisotropic direction and length of the large displacements of oxygen atoms in the cubic phase changed with time owing to thermal fluctuations although the crystal structure was isotropic.