Melting of Oxygen Vacancy Order at Oxide-Heterostructure Interface.

Modifications in oxygen coordination environments in heterostructures consisting of dissimilar oxides often emerge and lead to unusual properties of the constituent materials. Although lots of attention has been paid to slight modifications in the rigid oxygen octahedra of perovskite-based heterointerfaces, revealing the modification behaviors of the oxygen coordination environments in the heterostructures containing oxides with oxygen vacancies have been challenging. Here, we show that a significant modification in the oxygen coordination environments-melting of oxygen vacancy order-is induced at the heterointerface between SrFeO2.5 (SFO) and DyScO3 (DSO). When an oxygen-deficient perovskite (brownmillerite structure) SrFeO2.5 film grows epitaxially on a perovskite DyScO3 substrate, both FeO6 octahedra and FeO4 tetrahedra in the (101)-oriented SrFeO2.5 thin film connect to ScO6 octahedra in DyScO3. As a consequence of accommodating a structural mismatch, the alternately ordered arrangement of oxygen vacancies is significantly disturbed and reconstructed in the 2 nm thick heterointerface region. The stabilized heterointerface structure consists of Fe3+ octahedra with an oxygen vacancy disorder. The melting of the oxygen vacancy order, which in bulk SrFeO2.5 occurs at 1103 K, is induced at the present heterointerface at ambient temperatures.

Two Charge Ordering Patterns in the Topochemically Synthesized Layer-Structured Perovskite LaCa2Fe3O9 with Unusually High Valence Fe3.67.

A-site-ordered layer-structured perovskite LaCa2Fe3O9 with unusually high valence Fe3.67+ was obtained by low-temperature topochemical oxidation of the A-site layer-ordered LaCa2Fe3O8. The unusually high valence Fe3.67+ in LaCa2Fe3O9 shows charge disproportionation of Fe3+ and Fe5+ first along the layer-stacking ⟨010⟩ direction below 230 K. Fe3+ is located between the La3+ and Ca2+ layers, while Fe5+ is between the Ca2+ layers. The two-dimensional electrostatic potential due to the A-site layered arrangement results in the quasi-stable ⟨010⟩ charge ordering pattern. Below 170 K, the charge ordering pattern changes, and the 2:1 charge-disproportionated Fe3+ and Fe5+ ions are ordered along the ⟨111⟩ direction. The ground-state charge ordering pattern is stabilized primarily by the electrostatic lattice energy, and the Fe5+ ions are arranged to make the distances between the nearest neighboring Fe5+ as large as possible.

Successive Charge Transitions of Unusually High-Valence Fe3.5+ : Charge Disproportionation and Intermetallic Charge Transfer.

A perovskite-structure oxide containing unusually high-valence Fe3.5+ was obtained by high-pressure synthesis. Instability of the Fe3.5+ in Ca0.5 Bi0.5 FeO3 is relieved first by charge disproportionation at 250 K and then by intermetallic charge transfer between A-site Bi and B-site Fe at 200 K. These previously unobserved successive charge transitions are due to competing intermetallic and disproportionation charge instabilities. Both transitions change magnetic and structural properties significantly, indicating strong coupling of charge, spin, and lattice in the present system.

Crystal Structures at Atomic Resolution of the Perovskite-Related GdBaMnFeO5 and Its Oxidized GdBaMnFeO6.

Perovskite-related GdBaMnFeO5 and the corresponding oxidized phase GdBaMnFeO6, with long-range layered-type ordering of the Ba and Gd atoms have been synthesized. Oxidation retains the cation ordering but drives a modulation of the crystal structure associated with the incorporation of the oxygen atoms between the Gd layers. Oxidation of GdBaMnFeO5 increases the oxidation state of Mn from 2+ to 4+, while the oxidation state of Fe remains 3+. Determination of the crystal structure of both GdBaMnFeO5 and GdBaMnFeO6 is carried out at atomic resolution by means of a combination of advanced transmission electron microscopy techniques. Crystal structure refinements from synchrotron X-ray diffraction data support the structural models proposed from the TEM data. The oxidation states of the Mn and Fe atoms are evaluated by means of EELS and Mössbauer spectroscopy, which also reveals the different magnetic behavior of these oxides.

Determination of Elemental Ratio in an Atomic Column by Electron Energy Loss Spectroscopy.

Atomic-resolution quantification of the elemental ratio of Fe to Mn at the octahedral and tetrahedral sites in brownmillerite Ca2Fe1.07Mn0.93O5 was determined using electron energy-loss spectroscopy combined with aberration-corrected scanning transmission electron microscopy. The combined techniques revealed that oversampling of the spectral imaging data yielded a spatially resolved area that very nearly reflects atomic resolution (∼1.2 Šradius). The average experimental ratios of Fe to Mn within this region were 17.5:82.5 for the octahedral sites and 81.6:18.4 for the tetrahedral sites. The elemental ratio in an octahedral atomic column was successfully extracted by estimating the mixing of signals from nearest neighbor columns. The results indicated that the ratio of Fe to Mn was 13:87 at the octahedral site, which is in good agreement with the results of neutron diffraction analysis. In addition, the uncertainty of experimental results obtained by using an average 1.2 Šradius was less than 10% at octahedral sites, depending on the sample thickness. In contrast, the experimental error due to dechanneling of incident electrons was larger at the tetrahedral sites. This experimental procedure has wide application for determining the spatially resolved composition ratio of elements in perovskite-like compounds.

Two-dimensional charge disproportionation of the unusual high valence state Fe(4+) in a layered double perovskite.

The crystal and magnetic structures of charge-disproportionated Ca2FeMnO6 were analyzed by neutron powder diffraction. Ca2FeMnO6 is a layered double perovskite oxide with a two-dimensional arrangement of Mn(4+) and unusual high valence Fe(4+) at room temperature. When cooled, the compound shows charge disproportionation followed by magnetic transition. Around 200 K, the Fe(4+) shows the charge disproportionation to Fe(3+) and Fe(5+), which are ordered in a checkerboard pattern in the two-dimensional FeO6 octahedral layers. The magnetic transition occurs at 95 K, which is much lower than the charge disproportionation temperature. The magnetic structure is commensurate but noncollinear, and the antiferromagnetic coupling of Fe(3+) and Fe(5+) spins in the FeO6 octahedral layers gives the ferrimagnetic moments. The unique magnetic structure is described as a result of two-dimensional localization of the ligand holes with effective spins.

Strain-induced significant increase in metal-insulator transition temperature in oxygen-deficient Fe oxide epitaxial thin films.

Oxygen coordination of transition metals is a key for functional properties of transition-metal oxides, because hybridization of transition-metal d and oxygen p orbitals determines correlations between charges, spins and lattices. Strain often modifies the oxygen coordination environment and affects such correlations in the oxides, resulting in the emergence of unusual properties and, in some cases, fascinating behaviors. While these strain effects have been studied in many of the fully-oxygenated oxides, such as ABO3 perovskites, those in oxygen-deficient oxides consisting of various oxygen coordination environments like tetrahedra and pyramids as well as octahedra remain unexplored. Here we report on the discovery of a strain-induced significant increase, by 550 K, in the metal-insulator transition temperature of an oxygen-deficient Fe oxide epitaxial thin film. The observed transition at 620 K is ascribed to charge disproportionation of Fe(3.66+) into Fe(4+) and Fe(3+), associated with oxygen-vacancy ordering. The significant increase in the metal-insulator transition temperature, from 70 K in the bulk material, demonstrates that epitaxial growth of oxygen-deficient oxides under substrate-induced strain is a promising route for exploring novel functionality.

Low-temperature reduction of brownmillerite CaFeO2.5 in LaAlO3/CaFeO2.5 heterostructures made on SrTiO3.

When LaAlO3/CaFeO2.5 thin-film heterostructures made on SrTiO3 were annealed with CaH2 at low temperatures below 300 °C, the brownmillerite CaFeO2.5 layer was reduced to CaFeO2 with an infinite-layer structure while both the LaAlO3 capping layer and the SrTiO3 substrate remained intact. The reduction behaviour strongly depends on the lattice matching of LaAlO3 to CaFeO2.5, suggesting that oxygen ions migrate through the coherently grown LaAlO3 layer of the heterostructure predominantly in the out-of-plane direction. The structural defects near the interface in the relaxed-structure LaAlO3 capping layer prevent the oxygen ions from migrating.

Reduction and oxidation of SrCoO2.5 thin films at low temperatures.

Brownmillerite SrCoO(2.5) (010) thin films synthesized by pulsed laser deposition became amorphous when reduced at low temperatures by CaH(2), indicating that the infinite-layer structure with the square planar Co(2+)O(4) configuration is unstable. Ferromagnetic and conducting perovskite SrCoO(3) epitaxial thin films, on the other hand, were obtained topotactically at room temperature by oxidation with NaClO.

Selective reduction of layers at low temperature in artificial superlattice thin films.

Reduction and oxidation in transition-metal oxides are keys to develop technologies related to energy and the environment. Here we report the selective topochemical reduction observed when artificial superlattices with transition-metal oxides are treated at a temperature below 300 °C with CaH(2). [CaFeO(2)](m)/[SrTiO(3)](n) infinite-layer/perovskite artificial superlattice thin films were obtained by low-temperature reduction of [CaFeO(2.5)](m)/[SrTiO(3)](n) brownmillerite/perovskite artificial superlattice thin films. By the reduction only the CaFeO(2.5) layers in the artificial superlattices were reduced to the CaFeO(2) infinite layers whereas the SrTiO(3) layers were unchanged. The observed low-temperature reduction behaviors strongly suggest that the oxygen ion diffusion in the artificial superlattices is confined within the two-dimensional brownmillerite layers. The reduced artificial superlattice could be reoxidized, and thus, the selective reduction and oxidation of the constituent layers in the perovskite-structure framework occur reversibly.

Multiferroic Compounds with Double-Perovskite Structures.

New multiferroic compounds with double-perovskite structures were synthesized. Bi2NiMnO6 was synthesized in bulk form by high-pressure synthesis and also in a thin-film form by epitaxial growth. The material showed both ferromagnetic and ferroelectric properties, i.e., the multiferroic property at low temperature. Bi2FeCrO6 was also fabricated in a (1 1 1) oriented BiFeO3/BiCrO3 artificial superlattice, with a 1/1 stacking period. The superlattice film showed ferromagnetic behavior and polarization switching at room temperature. In the compounds, Bi3+ ion, located at the A site in the perovskite structure, caused ferroelectric structural distortion, and the B-site ordering of the Ni2+ and Mn4+ ions (Fe3+ and Cr3+ ions) in a rock-salt configuration led to ferromagnetism according to the Kanamori-Goodenough rule.

Anisotropic oxygen diffusion at low temperature in perovskite-structure iron oxides.

Oxygen-ion conduction in transition-metal oxides is exploited in, for example, electrolytes in solid-oxide fuel cells and oxygen-separation membranes, which currently work at high temperatures. Conduction at low temperature is a key to developing further utilization, and an understanding of the structures that enable conduction is also important to gain insight into oxygen-diffusion pathways. Here we report the structural changes observed when single-crystalline, epitaxial CaFeO2.5 thin films were changed into CaFeO2 by low-temperature reductions with CaH2. During the reduction process from the brownmillerite CaFeO2.5 into the infinite-layer structure of CaFeO2, some of the oxygen atoms are released from and others are rearranged within the perovskite-structure framework. We evaluated these changes and the reaction time they required, and found two oxygen diffusion pathways and the related kinetics at low temperature. The results demonstrate that oxygen diffusion in the brownmillerite is highly anisotropic, significantly higher along the lateral direction of the tetrahedral and octahedral layers.