The energetics and dynamics of Ca2+ and Mn2+ substituents were investigated ab initio. It was found that Ca replaced both K and Ta ions. The O vacancies were bound to isolated Ca ions which resided at Ta sites; thus causing off-center Ca displacements and forming large dipoles. There was also evidence that oppositely charged defects could cluster together. The calculations predicted that the activation energy for dipole reorientation, via O-vacancy hopping within the first-neighbor shell of Ta-substituting Ca or Mn, was greater than 2eV. On the other hand, Mn2+ which was substituted at K sites was displaced off-center along the (100) direction; forming a dipole. This dipole could reorient via Mn hopping, with an activation energy of about 0.18eV. This figure was in reasonable agreement with experimental data. It was suggested that metal-ion hopping at the A site, rather than O-vacancy hopping, was responsible for the small observed activation energies.

First-Principles Study of Ca2 and Mn2 Substituents in KTaO3. K.Leung: Physical Review B, 2001, 63[13], 134415