The effect of a variety of intrinsic defects and defect clusters in bulk and thin films of ${\mathrm{SrTiO}}_3$ on ferroelectric polarization and switching mechanisms is investigated by means of density-functional-theory based calculations and the Berry phase approach. Our results show that both the titanium ${\mathrm{Ti}}_{\mathrm{Sr}}^{••}$ and strontium ${\mathrm{Sr}}_{\mathrm{Ti}}^{\prime \prime }$ antisite defects induce ferroelectric polarization in ${\mathrm{SrTiO}}_3$, with the ${\mathrm{Ti}}_{\mathrm{Sr}}^{••}$ defect causing a more pronounced spontaneous polarization and higher activation barriers of polarization reversal than ${\mathrm{Sr}}_{\mathrm{Ti}}^{\prime \prime }$. The presence of oxygen vacancies bound to the antisite defects can either enhance or diminish polarization depending on the configuration of the defect pair, but it always leads to larger activation barriers of polarization switching as compared to the antisite defects with no oxygen vacancies. We also show that the magnitude of spontaneous polarization in ${\mathrm{SrTiO}}_3$ can be tuned by controlling the degree of Sr/Ti nonstroichiometry. Other intrinsic point defects such as Frenkel defect pairs and electron small polarons also contribute to the emergence of ferroelectric polarization in ${\mathrm{SrTiO}}_3$.

Single-phase multiferroic materials that allow the coexistence of ferroelectric and magnetic ordering above room temperature are highly desirable, motivating an ongoing search for mechanisms for unconventional ferroelectricity in magnetic oxides. Here, we report an antisite defect mechanism for room temperature ferroelectricity in epitaxial thin films of yttrium orthoferrite, YFeO₃, a perovskite-structured canted antiferromagnet. A combination of piezoresponse force microscopy, atomically resolved elemental mapping with aberration corrected scanning transmission electron microscopy and density functional theory calculations reveals that the presence of Y_Fe antisite defects facilitates a non-centrosymmetric distortion promoting ferroelectricity. This mechanism is predicted to work analogously for other rare earth orthoferrites, with a dependence of the polarization on the radius of the rare earth cation. Our work uncovers the distinctive role of antisite defects in providing a mechanism for ferroelectricity in a range of magnetic orthoferrites and further augments the functionality of this family of complex oxides for multiferroic applications.

Recent experimental results have demonstrated ferroelectricity in thin films of ${\mathrm{SrTiO}}_3$ induced by antisite ${\mathrm{Ti}}_{\mathrm{Sr}}$ defects. This opens a possibility to use ${\mathrm{SrTiO}}_3$ as a barrier layer in ferroelectric tunnel junctions (FTJs)—emerging electronic devices promising for applications in nanoelectronics. Here using density functional theory combined with quantum-transport calculations applied to a prototypical $\mathrm{Pt}/\mathrm{SrTi}{\mathrm{O}}_3/\mathrm{Pt}$ FTJ, we demonstrate that the localized in-gap energy states produced by the antisite ${\mathrm{Ti}}_{\mathrm{Sr}}$ defects are responsible for the enhanced electron tunneling conductance which can be controlled by ferroelectric polarization. Our tight-binding modeling, which takes into account multiple defects, shows that the predicted defect-assisted tunneling electroresistance effect is greatly amplified when the defect energy levels are brought to the Fermi energy by one of the polarization states. Our results have implications for FTJs based on conventional ferroelectric barriers with defects and can be employed for the design of new types of FTJs with enhanced performance.