Theoretical predictions--motivated by recent advances in epitaxial engineering--indicate a wealth of complex behaviour arising in superlattices of perovskite-type metal oxides. These include the enhancement of polarization by strain and the possibility of asymmetric properties in three-component superlattices. Here we fabricate superlattices consisting of barium titanate (BaTiO3), strontium titanate (SrTiO3) and calcium titanate (CaTiO3) with atomic-scale control by high-pressure pulsed laser deposition on conducting, atomically flat strontium ruthenate (SrRuO3) layers. The strain in BaTiO3 layers is fully maintained as long as the BaTiO3 thickness does not exceed the combined thicknesses of the CaTiO3 and SrTiO3 layers. By preserving full strain and combining heterointerfacial couplings, we find an overall 50% enhancement of the superlattice global polarization with respect to similarly grown pure BaTiO3, despite the fact that half the layers in the superlattice are nominally non-ferroelectric. We further show that even superlattices containing only single-unit-cell layers of BaTiO3 in a paraelectric matrix remain ferroelectric. Our data reveal that the specific interface structure and local asymmetries play an unexpected role in the polarization enhancement.

ABSTRACT:Strain is known to greatly influence low temperature oxygen electrocatalysis on noble metal films, leading to significant enhancements in bifunctional activity essential for fuel cells and metal-air batteries. However, its catalytic impact on transition metal oxide (TMO) thin films, such as perovskites, is not widely understood. Here, we epitaxially strain the conducting perovskite LaNiO 3 to systematically determine its influence on both the oxygen reduction (ORR) and oxygen evolution reaction (OER). Uniquely, we found that compressive strain could significantly enhance both reactions, yielding a bifunctional catalyst that surpasses the performance of noble metals such as Pt. We attribute the improved bifunctionality to straininduced splitting of the e g orbitals, which can customize orbital asymmetry at the surface. Analogous to straininduced shifts in the d-band center of noble metals relative to Fermi level, such splitting can dramatically affect catalytic activity in this perovskite and other potentially more active oxides. Advancements in energy storage are essential for driving the development of more sophisticated mobile technologies as well as continuing the trend towards a greener economy. At the forefront of this push are high energy density devices, such as regenerative fuel cells and metal-air batteries. 1 In these and related electrochemical systems, both the oxygen reduction and oxygen evolution reactions (ORR and OER, respectively) are crucial towards successful operation. 2 Traditionally, conductive catalysts incorporating noble metals (e.g., Pt and IrO 2 ) have been used to facilitate these reactions near room temperature. 3 To alleviate costs and poor stabilities during OER in alkaline solutions, significant efforts have focused on transition metal oxides (TMOs) with multivalent Ni, Fe, Co, and Mn, such as NiFeO x .4 Similar to alloying in noble metals, the majority of research into increasing oxygen activities of TMOs involves cationic doping, which often promotes either the ORR or OER but not bifunctionality. 5Here, we explore how another factor, i.e. strain, can influence bifunctionality in TMOs. Contemporary work on high-temperature (>500 C) oxide catalysis in an aprotic environment (e.g. ORR: O 2 + 4e -→ 2O 2-) has emphasized the importance of tensile strain for activating defects to improve catalytic reactions. 6 In an alkaline environment (e. . Among TMOs, the ABO 3 perovskites, in which A is traditionally from Groups I-III and B is a transition metal ion with six-fold octahedral coordination, also have electronic structures that are sensitive to strain. Due to strong hybridization between the O 2p and the transition metal d z 2 and d x 2 -y 2 lobes in the BO 6 octahedra, the σ* states near E F consist of e g orbitals. 10,10b Similar to the Jahn-Teller distortion, strain is known to lift the degeneracy in these symmetry-localized d z 2 and d x 2 -y 2 orbitals, yielding changes in the orbital occupancy, or polarization.7a, 11 By using strain to control the degree of this ...

Fast, reversible redox reactions in solids at low temperatures without thermomechanical degradation are a promising strategy for enhancing the overall performance and lifetime of many energy materials and devices. However, the robust nature of the cation's oxidation state and the high thermodynamic barrier have hindered the realization of fast catalysis and bulk diffusion at low temperatures. Here, we report a significant lowering of the redox temperature by epitaxial stabilization of strontium cobaltites (SrCoO(x)) grown directly as one of two distinct crystalline phases, either the perovskite SrCoO(3-δ) or the brownmillerite SrCoO(2.5). Importantly, these two phases can be reversibly switched at a remarkably reduced temperature (200-300 °C) in a considerably short time (< 1 min) without destroying the parent framework. The fast, low-temperature redox activity in SrCoO(3-δ) is attributed to a small Gibbs free-energy difference between two topotatic phases. Our findings thus provide useful information for developing highly sensitive electrochemical sensors and low-temperature cathode materials.

Oxygen stoichiometry is one of the most important elements in determining the physical properties of transition metal oxides (TMOs). A small change in the oxygen content results in the variation of valence state of the transition metal, drastically modifying the materials functionalities. The latter includes, for instance, (super-)conductivity, magnetism, ferroelectricity, bulk ionic conduction, and catalytic surface reactions. [1][2][3][4][5] In particular, among those applications, TMOs with mixed valence states have attracted attention for many environmental and renewable energy applications, including catalysts, hydrogen generation from water splitting, cathodes in rechargeable batteries and solid oxide fuel cells, and oxygen separation membranes. [6][7][8] For example, previous studies have shown that the ability to control the number of d-band electron population and detailed spin configuration in TMOs is critical for improved catalytic performance of TMOs. [9,10] In this context, SrCoO x (2.5 ≤ x ≤ 3.0) is an ideal class of materials to study the evolution of the physical properties by modifying the valence state in TMOs, due to the existence of two structurally distinct topotactic phases, i.e. the brownmillerite SrCoO 2.5 (BM-2 SCO) (see Figure 1a) and the perovskite SrCoO 3 . [11,12] Especially, BM-SCO has atomicallyordered one-dimensional vacancy channels (see Figure 1a), which can accommodate additional oxygen when the valence state of Co is changed. Moreover, SrCoO x exhibits a wide spectrum of physical properties from antiferromagnetic insulator to ferromagnetic metal depending on the oxygen stoichiometry. [11][12][13] Since SrCoO x has only a single control knob, i.e. the oxygen contentx, to modify the Co valence state without cation doping, it is straightforward to study the valence state (i.e. oxygen content) dependent physical properties. However, so far, the growth of high quality single crystalline materials has not been as successful due to difficulty in controlling the right oxidation state.In this work, we report on the epitaxial growth of high quality BM-SCO single crystalline films on SrTiO 3 (STO) substrates by pulsed laser epitaxy (PLE). In order to examine the topotactic phase transformation to the perovskite SrCoO 3- (P-SCO), some of the samples were subsequently in-situ annealed at various oxygen pressure (P(O 2 )) to fill the oxygen vacancies.While the direct growth of P-SCO films with x = 3.0 was an arduous task, we found that postannealing in high P(O 2 ) (> several hundreds of Torr) could fill sufficient amount of oxygen vacancies, yielding systematic evolution in electronic, magnetic, and thermoelectric properties. (Figure 1b) demonstrate that the films are of high quality. X-ray rocking curve ω-scans revealed a full width half maximum (FWHM) of < 0.04º, demonstrating the excellent crystallinity (cf., FWHM of the 002 STO peak was ~0.02º) of our films (data not shown).While we have shown the XRD data from a well-optimized, high quality thin film, it is worthwhile to mention tha...

Epitaxial strain imposed in complex oxide thin films by heteroepitaxy is recognized as a powerful tool for identifying new properties and exploring the vast potential of materials performance. A particular example is LaCoO(3), a zero spin, nonmagnetic material in the bulk, whose strong ferromagnetism in a thin film remains enigmatic despite a decade of intense research. Here, we use scanning transmission electron microscopy complemented by X-ray and optical spectroscopy to study LaCoO(3) epitaxial thin films under different strain states. We observed an unconventional strain relaxation behavior resulting in stripe-like, lattice modulated patterns, which did not involve uncontrolled misfit dislocations or other defects. The modulation entails the formation of ferromagnetically ordered sheets comprising intermediate or high spin Co(3+), thus offering an unambiguous description for the exotic magnetism found in epitaxially strained LaCoO(3) films. This observation provides a novel route to tailoring the electronic and magnetic properties of functional oxide heterostructures.