Polarization of ionic and electronic defects in response to high electric fields plays an essential role in determining properties of materials in applications such as memristive devices. However, isolating the polarization response of individual defects has been challenging for both models and measurements. Here the authors quantify the nonlinear dielectric response of neutral oxygen vacancies, comprised of strongly localized electrons at an oxygen vacancy site, in perovskite oxides of the form
Oxygen Deficiency and Migration-Mediated Electric Polarization in Magnetic Fe,Co-Substituted SrTiO3−δ
We use density functional theory (DFT) calculations to show that oxygen vacancies (vO) and mobility induce noncentrosymmetric polar structures in SrTi1−x−yFexCoyO3−δ (STFC, x=y=0.125) with δ={0.125,0.25}, enhance the saturation magnetization, and give rise to large changes in the electric polarization |ΔP|. We present an intuitive set of rules to describe the properties of STFC, which are based on the interplay between (Co/Fe)-vO defects, magnetic cation coordination, and topological vacancy disorder. STFC structures consist of layered crystals with sheets of linearly organized O4,5,6-coordinated Fe–Co pairs, sandwiched with layers of O5-coordinated Ti. (Co/Fe)-vO defects are the source of crystal distortions, cation off-centering and bending of the oxygen octahedra which, considering the charge redistribution mediated by vO and the cations’ electronegativity and valence states, triggers an effective electric polarization. Oxygen migration for δ=0.125 leads to |ΔP|>∼10 µC/cm2 due to quantum-of-polarization differences between δ=0.125 structures. Increasing the oxygen deficiency to δ=0.25 yields |ΔP|, the O migration of which resolved polarization for δ=0.25 is >∼3 µC/cm2. Magnetism is dominated by the Fe,Co spin states for δ=0.125, and there is a contribution from Ti magnetic moments (∼1 μB) for δ=0.25. Magnetic and electric order parameters change for variations of δ or oxygen migration for a given oxygen deficiency. Our results capture characteristics observed in the end members of the series SrTi(Co,Fe)O3, and suggest the existence of a broader set of rules for oxygen-deficient multiferroic oxides.
more »
« less
- Award ID(s):
- 2132623
- PAR ID:
- 10392547
- Date Published:
- Journal Name:
- Magnetochemistry
- Volume:
- 8
- Issue:
- 11
- ISSN:
- 2312-7481
- Page Range / eLocation ID:
- 144
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
more » « less
Abstract AB O3. Their approach implements a computationally efficient local HubbardU correction in density functional theory simulations. These calculations indicate that the electric dipole moment of this defect is correlated positively with the lattice volume, which they varied by elastic strain and by A‐site cation species. In addition, the dipole of the neutral oxygen vacancy under electric field increases with increasing reducibility of the B‐site cation. The predicted relationship among point defect polarization, mechanical strain, and transition metal chemistry provides insights for the properties of memristive materials and devices under high electric fields. -
Sr(Ti 1−x Fe x )O 3−δ (STF) has recently been explored as an oxygen electrode for solid oxide electrochemical cells (SOCs). Model thin film electrode studies show oxygen surface exchange rates that generally improve with increasing Fe content when x < 0.5, and are comparable to the best Co-containing perovskite electrode materials. Recent results on porous electrodes with the specific composition Sr(Ti 0.3 Fe 0.7 )O 3−δ show excellent electrode performance and stability, but other compositions have not been tested. Here we report results for porous electrodes with a range of compositions from x = 0.5 to 0.9. The polarization resistance decreases with increasing Fe content up to x = 0.7, but increases for further increases in x . This results from the interaction of two effects – the oxygen solid state diffusion coefficient increases with increasing x , but the electrode surface area and surface oxygen exchange rate decrease due to increased sinterability and Sr surface segregation for the Fe-rich compositions. Symmetric cells showed no degradation during 1000 h life tests at 700 °C even at a current density of 1.5 A cm −2 , showing that all the STF electrode compositions worked stably in both fuel cell mode and electrolysis modes. The excellent stability may be explained by X-ray Photoelectron Spectroscopy (XPS) results showing that the amount of surface segregated Sr did not change during the long-term testing, and by relatively low polarization resistances that help avoid electrode delamination.more » « less
-
The significant role of perovskite defect chemistry through A-site doping of strontium titanate with lanthanum for CO 2 electrolysis properties is demonstrated. Here we present a dual strategy of A-site deficiency and promoting adsorption/activation by making use of redox active dopants such as Mn/Cr linked to oxygen vacancies to facilitate CO 2 reduction at perovskite titanate cathode surfaces. Solid oxide electrolysers based on oxygen-excess La 0.2 Sr 0.8 Ti 0.9 Mn(Cr) 0.1 O 3+δ , A-site deficient (La 0.2 Sr 0.8 ) 0.9 Ti 0.9 Mn(Cr) 0.1 O 3−δ and undoped La 0.2 Sr 0.8 Ti 1.0 O 3+δ cathodes are evaluated. In situ infrared spectroscopy reveals that the adsorbed and activated CO 2 adopts an intermediate chemical state between a carbon dioxide molecule and a carbonate ion. The double strategy leads to optimal performance being observed after 100 h of high-temperature operation and 3 redox cycles, suggesting a promising cathode material for CO 2 electrolysis.more » « less
-
more » « less
Abstract Oxygen coordination and vacancy ordering play an important role in dictating the functionality of complex oxides. In this work, an unconventional layering of oxygen ions in a mixed conductor SrCo1‐xFexO3‐δ(SCFO) thin film grown epitaxially on SrTiO3(STO) is reported. Scanning transmission electron microscopy (STEM) reveals alternating layers of oxygen deficiency along the growth direction, with the oxygen‐rich layer correlated with the neighboring Co,Fe‐site intensity, and contraction of the Sr–Sr distance. Density functional theory (DFT) calculations and STEM image simulations support the emergence of periodic (Co,Fe)O6and (Co,Fe)O4/(Co,Fe)O5layers, an ordering that is also sensitive to the Co:Fe ratio.
-
Both aliovalent doping and the charge state of multivalent lattice ions determine the oxygen non-stoichiometry ( δ ) of mixed ionic and electronic conductors (MIECs). Unfortunately, it has been challenging for both modeling and experiments to determine the multivalent ion charge states in MIECs. Here, the Fe charge state distribution was determined for various compositions and phases of the MIEC La 1−x Sr x FeO 3−δ (LSF) using the spin-polarized density functional theory (DFT)-predicted magnetic moments on Fe. It was found that electron occupancy and crystal-field-splitting-induced differences between the Fe 3d-orbitals of the square pyramidally coordinated, oxygen-vacancy-adjacent Fe atoms and the octahedrally-coordinated, oxygen-vacancy-distant-Fe atoms determined whether the excess electrons produced during oxygen vacancy formation remained localized at the first nearest neighbor Fe atoms (resulting in small oxygen vacancy polarons, as in LaFeO 3 ) or were distributed to the second-nearest-neighbor Fe atoms (resulting in large oxygen vacancy polarons, as in SrFeO 3 ). The progressively larger polaron size and anisotropic shape changes with increasing Sr resulted in increasing oxygen vacancy interactions, as indicated by an increase in the oxygen vacancy formation energy above a critical δ threshold. This was consistent with experimental results showing that Sr-rich LSF and highly oxygen deficient compositions are prone to oxygen-vacancy-ordering-induced phase transformations, while Sr-poor and oxygen-rich LSF compositions are not. Since oxygen vacancy induced phase transformations cause a decrease in the mobile oxygen vacancy site fraction ( X ), both δ and X were predicted as a function of temperature and oxygen partial pressure, for multiple LSF compositions and phases using a combined thermodynamics and DFT approach.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.3390/magnetochemistry8110144
