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Abstract
Defect chemistry, strain, and structural, magnetic and electronic degrees of freedom constitute a rich space for the design of functional properties in transition metal oxides. Here, we show that it is possible to engineer polarity and ferroelectricity in non-polar perovskite oxides via polar defect pairs formed by anion vacancies coupled to substitutional cations. We use a self-consistent site-dependent DFT+$U$ approach that accounts for local structural and chemical changes upon defect creation and which is crucial to reconcile predictions with the available experimental data. Our results for Fe-doped oxygen-deficient SrMnO$_3$ show that substitutional Fe and oxygen vacancies can promote polarity due to an off-center displacement of the defect charge resulting in a net electric dipole moment, which polarizes the lattice in the defect neighborhood. The formation of these defects and the resulting polarization can be tuned by epitaxial strain, resulting in enhanced polarization also for strain values lower than the ones necessary to induce a polar phase transition in undoped SrMnO$_3$. For high enough defect concentrations, these defect dipoles couple in a parallel fashion, thus enabling defect- and strain-based engineering of ferroelectricity in SrMnO$_3$.
Highlights
The interplay between electric polarization, magnetism, strain, and the defect chemistry constitutes a rich phase diagram for the design and control of novel functional properties in transition-metal perovskites [1,2,3,4,5,6,7,8,9,10,11]
Our results suggest that defect engineering, through controlling the concentration and distribution of polar defect pairs formed by anion vacancies coupled to substitutional cations, constitutes a parameter to design multiferroic materials
For VOOP, the Mn sites lying at nearest-neighbor positions relative to the defect [NN, in purple in Fig. 2(a)] or far away from it [next-next-nearest-neighbor (NNNN) positions, in green in Fig. 2(a)] are equivalent by symmetry, while two different groups of Mn ions can be distinguished for substitution sites in next-nearest-neighbor positions relative to the vacancy [NNN, in orange and brown in Fig. 2(a)] for a total of four symmetry-inequivalent FeMn-VOOP defect pairs
Summary
The interplay between electric polarization, magnetism, strain, and the defect chemistry constitutes a rich phase diagram for the design and control of novel functional properties in transition-metal perovskites [1,2,3,4,5,6,7,8,9,10,11]. Strain imposed, for example, by lattice matching with the substrate during coherent epitaxial growth of thin films is an established route to engineer polarity and ferroelectricity in nonpolar complex oxides [5,7,8,9,10,12]. Substitutional defects coupled to oxygen vacancies (VO), such as FeTi-VO defect pairs, were shown to align with the direction of the lattice polarization in ferroelectric PbTiO3 [5] or to promote ferroelectricity in paraelectric SrTiO3 [13]. Strain, and stoichiometry can couple or compete in determining the material properties as shown for oxygen-deficient SrMnO3 (SMO) thin films [9], the material we use as a model system in the present study. It was predicted from theory that biaxial epitaxial strain induces a polar distortion in SMO, mainly associated with Mn ions displacing from their
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