Comparing the Impacts of Strain Types on Oxygen-Vacancy Formation in a Perovskite Oxide via Nanometer-Scale Strain Fields.

The utilization of an in-plane lattice misfit in an oxide epitaxially grown on another oxide with a different lattice parameter is a well-known approach to induce strains in oxide materials. However, achieving a sufficiently large misfit strain in this heteroepitaxial configuration is usually challenging, unless the thickness of the grown oxide is kept well below a critical value to prevent the formation of misfit dislocations at the interface for relaxation. Instead of adhering to this conventional approach, here, we employ nanometer-scale large strain fields built around misfit dislocations to examine the effects of two distinct types of strains─tension and compression─on the generation of oxygen vacancies in heteroepitaxial LaCoO3 films. Our atomic-level observations, coupled with local electron-beam irradiation, clarify that the in-plane compression notably suppresses the creation of oxygen vacancies, whereas the formation of vacancies is facilitated under tensile strain. Demonstrating that the defect generation can considerably vary with the type of strain, our study highlights that the experimental approach adopted in this work is applicable to other oxide systems when investigating the strain effects on vacancy formation.

Surface triggered stabilization of metastable charge-ordered phase in SrTiO3.

Charge ordering (CO), characterized by a periodic modulation of electron density and lattice distortion, has been a fundamental topic in condensed matter physics, serving as a potential platform for inducing novel functional properties. The charge-ordered phase is known to occur in a doped system with high d-electron occupancy, rather than low occupancy. Here, we report the realization of the charge-ordered phase in electron-doped (100) SrTiO3 epitaxial thin films that have the lowest d-electron occupancy i.e., d1-d0. Theoretical calculation predicts the presence of a metastable CO state in the bulk state of electron-doped SrTiO3. Atomic scale analysis reveals that (100) surface distortion favors electron-lattice coupling for the charge-ordered state, and triggering the stabilization of the CO phase from a correlated metal state. This stabilization extends up to six unit cells from the top surface to the interior. Our approach offers an insight into the means of stabilizing a new phase of matter, extending CO phase to the lowest electron occupancy and encompassing a wide range of 3d transition metal oxides.

Configurable Crack Wall Conduction in a Complex Oxide.

Mobile defects in solid-state materials play a significant role in memristive switching and energy-efficient neuromorphic computation. Techniques for confining and manipulating point defects may have great promise for low-dimensional memories. Here, we report the spontaneous gathering of oxygen vacancies at strain-relaxed crack walls in SrTiO3 thin films grown on DyScO3 substrates as a result of flexoelectricity. We found that electronic conductance at the crack walls was enhanced compared to the crack-free region, by a factor of 104. A switchable asymmetric diode-like feature was also observed, and the mechanism is discussed, based on the electrical migration of oxygen vacancy donors in the background of Sr-deficient acceptors forming n+-n or n-n+ junctions. By tracing the temporal relaxations of surface potential and lattice expansion of a formed region, we determine the diffusivity of mobile defects in crack walls to be 1.4 × 10-16 cm2/s, which is consistent with oxygen vacancy kinetics.

Stabilizing hidden room-temperature ferroelectricity via a metastable atomic distortion pattern.

Nonequilibrium atomic structures can host exotic and technologically relevant properties in otherwise conventional materials. Oxygen octahedral rotation forms a fundamental atomic distortion in perovskite oxides, but only a few patterns are predominantly present at equilibrium. This has restricted the range of possible properties and functions of perovskite oxides, necessitating the utilization of nonequilibrium patterns of octahedral rotation. Here, we report that a designed metastable pattern of octahedral rotation leads to robust room-temperature ferroelectricity in CaTiO3, which is otherwise nonpolar down to 0 K. Guided by density-functional theory, we selectively stabilize the metastable pattern, distinct from the equilibrium pattern and cooperative with ferroelectricity, in heteroepitaxial films of CaTiO3. Atomic-scale imaging combined with deep neural network analysis confirms a close correlation between the metastable pattern and ferroelectricity. This work reveals a hidden but functional pattern of oxygen octahedral rotation and opens avenues for designing multifunctional materials.