Off-centre motion in doped cubic oxides: A general view on the instability J.A. Aramburu, P. Garcia-Fernandez ⇑ , M. Moreno Departamento de Ciencias de la Tierra y Física de la Materia Condensada, Universidad de Cantabria, Avenida de los Castros s/n, 39005 Santander, Spain article info Article history: Available online xxxx Keywords: Off-centre Pseudo Jahn–Teller Ferroelectricity Impurity Oxides 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 Ni 2+ impurity can happen in SrO, in agreement with experimental data, while not for smaller Cr 3+ or Ti 4+ 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 Mn 2+ in SrO. Our results indicate that an off-centre motion in MgO:Ni 2+ is rather unlikely and that the local contributions to the instability in pure BaTiO 3 are not enough to induce ferroelectricity if just the movement of Ti 4+ ion is considered. Ó 2015 Elsevier B.V. All rights reserved. 1. Introduction When an insulating material is slightly doped, the impurities replace a small fraction of host cations. Nevertheless, the position occupied by an impurity in the lattice is not necessarily that of the replaced host ion as, in some cases, the impurity actually moves from the ideal substitutional site [1,2]. In cubic insulating lattices, this off-centre motion gives rise to a local symmetry low- ering which, in turn, can induce deep changes into the physical and chemical properties associated with the impurity. As an example, the off-centre motion of Fe + impurities in SrCl 2 [3–5] or KTaO 3 [6,7] induces a huge magnetic anisotropy associated to the spin of the impurity [8,9]. Aside from this reason, the study of this kind of structural instability in slightly doped materials is also attractive due to its partial relationship [10–12] with the ferroelectric distor- tions in pure insulating materials like BaTiO 3 or PbTiO 3 , where, apart from a local component which could favour the off-centre motion, a cooperative component may also be present. Thus, as indicated above, determining whether a transition metal impurity remains on-centre or experiences an off-centre dis- tortion is a key element to understand its properties. In cubic lat- tices, partial information on this important issue can be obtained experimentally either by studying the symmetry of the spin- Hamiltonian associated with Electron Paramagnetic Resonance (EPR) data or by finding the temperature dependence of the oscillator strength of d–d transitions [2,13,14]. However, in many cases these techniques provide very little insight on whether the off-centre motion is spontaneous or induced by the presence of another defect in the vicinity of the impurity, such as a vacancy or an unwanted ion, which triggers the symmetry-breaking distor- tion. An efficient way to answer this question is provided by mod- ern first-principles calculations. For example, Density Functional Theory (DFT) calculations undoubtedly prove that the symmetry lowering observed by EPR in SrCl 2 :M (M = Fe + , Cu 2+ ) [3,5,15], SrF 2 :Cu 2+ [16] or CaF 2 :Ni + [17] obeys to a spontaneous off-centre motion of impurities without the need of any associated defect [4,18,19]. The present work explores, by means of first principles DFT cal- culations, the possible off-centre instabilities taking place in MO cubic oxides (M = Mg, Ca and Sr) doped with various ions like Na + , Ni 2+ , Mn 2+ , Cr 3+ or Ti 4+ . This research is partially motivated by some Extended X-ray Absorption Fine Structure (EXAFS) and optical works suggesting that Ni 2+ impurities in MgO move off- centre [20–22] in the same way as in SrO:Ni 2+ [23,24]. In the latter case EPR experiments [24] reveal the existence of a very large zero- field splitting constant, D = 20 cm 1 , ascribed to a motion of Ni 2+ along a h111i direction. Nevertheless, that conclusion on Ni 2+ in MgO contrasts with other EXAFS and EPR works indicating that Ni 2+ remains on-centre in this lattice [25–28]. While shedding some light on this particular set of experimental results will be the focus of the first part of our work, our broader aim is to provide some insight in the trends favouring or preventing an off-centre motion in an isolated impurity. At the same time we will discuss http://dx.doi.org/10.1016/j.chemphys.2015.05.018 0301-0104/Ó 2015 Elsevier B.V. All rights reserved. ⇑ Corresponding author. Tel.: +34 942202069; fax: +34 942201402. E-mail address: garciapa@unican.es (P. Garcia-Fernandez). Chemical Physics xxx (2015) xxx–xxx Contents lists available at ScienceDirect Chemical Physics journal homepage: www.elsevier.com/locate/chemphys Please cite this article in press as: J.A. Aramburu et al., Chem. Phys. (2015), http://dx.doi.org/10.1016/j.chemphys.2015.05.018