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.
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Effects of local compositional and structural disorder on vacancy formation in entropy-stabilized oxides from first-principles
Entropic stabilization has evolved into a strategy to create new oxide materials and realize novel functional properties engineered through the alloy composition. Achieving an atomistic understanding of these properties to enable their design, however, has been challenging due to the local compositional and structural disorder that underlies their fundamental structure-property relationships. Here, we combine high-throughput atomistic calculations and linear regression algorithms to investigate the role of local configurational and structural disorder on the thermodynamics of vacancy formation in (MgCoNiCuZn)O-based entropy-stabilized oxides (ESOs) and their influence on the electrical properties. We find that the cation-vacancy formation energies decrease with increasing local tensile strain caused by the deviation of the bond lengths in ESOs from the equilibrium bond length in the binary oxides. The oxygen-vacancy formation strongly depends on structural distortions associated with the local configuration of chemical species. Vacancies in ESOs exhibit deep thermodynamic transition levels that inhibit electrical conduction. By applying the charge-neutrality condition, we determine that the equilibrium concentrations of both oxygen and cation vacancies increase with increasing Cu mole fraction. Our results demonstrate that tuning the local chemistry and associated structural distortions by varying alloy composition acts an engineering principle that enables controlled defect formation in multi-component alloys.
more » « less- NSF-PAR ID:
- 10366613
- Publisher / Repository:
- Nature Publishing Group
- Date Published:
- Journal Name:
- npj Computational Materials
- Volume:
- 8
- Issue:
- 1
- ISSN:
- 2057-3960
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract 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
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. -
Despite the well-known tendency for many alloys to undergo ordering transformations, the microscopic mechanism of ordering and its dependence on alloy composition remains largely unknown. Using the example of Pt 85 Fe 15 and Pt 65 Fe 35 alloy nanoparticles (NPs), herein we demonstrate the composition-dependent ordering processes on the single-particle level, where the nanoscale size effect allows for close interplay between surface and bulk in controlling the phase evolution. Using in situ electron microscopy observations, we show that the ordering transformation in Pt 85 Fe 15 NPs during vacuum annealing occurs via the surface nucleation and growth of L1 2 -ordered Pt 3 Fe domains that propagate into the bulk, followed by the self-sacrifice transformation of the surface region of the L1 2 Pt 3 Fe into a Pt skin. By contrast, the ordering in Pt 65 Fe 35 NPs proceeds via an interface mechanism by which the rapid formation of an L1 0 PtFe skin occurs on the NPs and the transformation boundary moves inward along with outward Pt diffusion. Although both the “nucleation and growth” and the “interface” mechanisms result in a core–shell configuration with a thin Pt-rich skin, Pt 85 Fe 15 NPs have an L1 2 Pt 3 Fe core, whereas Pt 65 Fe 35 NPs are composed of an L1 0 PtFe core. Using atomistic modeling, we identify the composition-dependent vacancy-assisted counterdiffusion of Pt and Fe atoms between the surface and core regions in controlling the ordering transformation pathway. This vacancy-assisted diffusion is further demonstrated by oxygen annealing, for which the selective oxidation of Fe results in a large number of Fe vacancies and thereby greatly accelerates the transformation kinetics.more » « less
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Abstract Mn‐redox‐based oxides and oxyfluorides are considered the most promising earth‐abundant high‐energy cathode materials for next‐generation lithium‐ion batteries. While high capacities are obtained in high‐Mn content cathodes such as Li‐ and Mn‐rich layered and spinel‐type materials, local structure changes and structural distortions ( often lead to voltage fade, capacity decay, and impedance rise, resulting in unacceptable electrochemical performance upon cycling. In the present study, structural transformations that exploit the high capacity of Mn‐rich oxyfluorides while enabling stable cycling, in stark contrast to commonly observed structural changes that result in rapid performance degradation, are reported. It is shown that upon cycling of a cation‐disordered rocksalt (DRX) cathode (Li1.1Mn0.8Ti0.1O1.9F0.1, an ultrahigh capacity of ≈320 mAh g−1(energy density of ≈900 Wh kg−1) can be obtained through dynamic structural rearrangements upon cycling , along with a unique voltage profile evolution and capacity rise. At high voltage, the presence of Mn4+and Li+vacancies promotes local cation ordering, leading to the formation of domains of a “
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