Off-centre motion in doped cubic oxides: A general view on the instability

Highlights: • Recent experiment showing an off-centre motion in MgO:Ni²⁺ is unlikely to be correct. • Correspondence between small ionic radius and off-centre motion does not exist. • Small impurity charge or small coupling to the lattice facilitate off-centre motion. - Abstract: The existence of an off-centre distortion for impurities in cubic oxides is explored by Density Functional Theory calculations. We find that models based on ionic radii are inadequate to explain the off-centre instability. By contrast, increase of the nominal impurity charge or decrease of the host lattice constant act against the off-centre distortion. This explains why the motion of just the Ni²⁺ impurity can happen in SrO, in agreement with experimental data, while not for smaller Cr³⁺ or Ti⁴⁺ ions in the same lattice. Moreover, the different electronic structure and coupling to the host lattice are shown to be behind a small but positive force constant for Na⁺ and Mn²⁺ in SrO. Our results indicate that an off-centre motion in MgO:Ni²⁺ is rather unlikely and that the local contributions to the instability in pure BaTiO₃ are not enough to induce ferroelectricity if just the movement of Ti⁴⁺ ion is considered.