Masanori Kawai

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.

Single-crystal epitaxial thin films of SrFeO2 with FeO2 “infinite layers”

Single-crystal thin films of SrFeO2, which is an oxygen-deficient perovskite with “infinite layers” of Fe2+O2, were prepared by using CaH2 for low-temperature reduction of epitaxial SrFeO2.5 single-crystal films deposited on KTaO3 substrates. This reduction process, removing oxygen ions from the perovskite structure framework and causing rearrangements of oxygen ions, topotactically transforms the brownmillerite SrFeO2.5 to the c-axis oriented SrFeO2.

Thermally formed conducting filaments in a single-crystalline NiO thin film

Effects of thermal annealing on resistance switching properties of a single-crystalline NiO thin film grown on a Pt0.8Ir0.2 bottom electrode were investigated. Annealing the NiO thin film above 430 °C produced conducting filaments in the single-crystalline NiO and the resulting low-resistance state changed to a high-resistance state at an electrical reset bias voltage. Rearrangements of crystalline defects such as Ni vacancies play a role in forming the conducting filaments. This “thermal forming” is essentially the same as “electrical forming” but does not need large bias voltages to make the initially ON state in unipolar switching devices.

Reversible changes of epitaxial thin films from perovskite LaNiO3 to infinite-layer structure LaNiO2

An infinite-layer-structure epitaxial thin film of LaNiO2 was prepared by low-temperature reduction with CaH2 from a LaNiO3 epitaxial thin film grown on a SrTiO3(100) substrate. The oxygen content changed reversibly from the perovskite LaNiO3 to the infinite-layer LaNiO2 without losing the structural framework and the topotactic relationship. Consequently, the oxidation state of Ni ions in the film changed from trivalent to divalent to monovalent.

Crystal Structures and Electric Properties of (1-x)BiFeO3–xBiCoO3 Thin Films Prepared by Chemical Solution Deposition

The crystal structures and electric properties of epitaxial (1-x)BiFeO3–xBiCoO3 thin films with x = 0–0.30 prepared by chemical solution deposition on SrTiO3(001) and SrRuO3(001)/SrTiO3(001) substrates were investigated. The crystal structures changed from rhombohedral to tetragonal at x = 0.15–0.2. Saturated ferroelectric polarizations were observed for films with x = 0–0.10, and films with x = 0.15–0.20 did not show polarization reversal. The piezoelectric constant d33 initially increased with increasing Co content and showed a maximum value of 80 pm/V at x = 0.05.

Enhanced Piezoelectric Constant of (1-x)BiFeO3?xBiCoO3 Thin Films Grown on LaAlO3 Substrate

Thin films of a rhombohedral-tetragonal solid solution (1-x)BiFeO3–xBiCoO3 with x = 0–0.30 were prepared by chemical solution deposition on LaAlO3(001) and (La0.5,Sr0.5)CoO3(001)/LaAlO3(001) substrates. A BiCoO3-type tetragonal structure with a large c/a of 1.23 was grown adjacent to the substrate whereas a BiFeO3-type rhombohedral one was grown on the tetragonal phase. Co substitution in the rhombohedral phase led to enhancement of piezoelectric constant and d33 reached 100 pm/V, almost double that of BiFeO3.

Critical thickness control by deposition rate for epitaxial BaTiO3 thin films grown on SrTiO3(001)

BaTiO3∕SrTiO3(001) epitaxial thin films were prepared at various growth rates by pulsed laser deposition, and their heterostructures were evaluated by synchrotron x-ray diffraction measurements and cross-sectional scanning transmission electron microscopy observations. In a film grown at a low deposition rate (0.01nm∕s), misfit dislocations are found near the interface and a fully relaxed BaTiO3 thin film grows epitaxially on the substrate. On the other hand, a film grown at a high deposition rate (0.04nm∕s) consists of strained and relaxed BaTiO3 lattices. Our results showed that the critical thickness of BaTiO3∕SrTiO3(001) epitaxial thin films can be controlled by the deposition rate and that the critical thickness increases with increasing deposition rate, and by adjusting the deposition rate, we were able to prepare epitaxial thin films consisting of fully strained BaTiO3, partially strained BaTiO3, or fully relaxed BaTiO3.

Resistance switching in a single-crystalline NiO thin film grown on a Pt0.8Ir0.2 electrode

A single-crystalline NiO thin film was grown epitaxially on an atomically flat Pt0.8Ir0.2 bottom electrode layer grown epitaxially on a SrTiO3(100) substrate. The memory cells of the single-crystalline NiO thin film with Pt top electrodes showed unipolar resistance switching behaviors. The result demonstrates that a unipolar resistance switching is not a characteristic phenomenon in the polycrystalline NiO but it can also occur in the single-crystalline NiO.

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 CaH2. [CaFeO2]m/[SrTiO3]n infinite-layer/perovskite artificial superlattice thin films were obtained by low-temperature reduction of [CaFeO2.5]m/[SrTiO3]n brownmillerite/perovskite artificial superlattice thin films. By the reduction only the CaFeO2.5 layers in the artificial superlattices were reduced to the CaFeO2 infinite layers whereas the SrTiO3 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.

Local analysis of the edge dislocation core in BaTiO3 thin film by STEM-EELS

The a <100> edge dislocation core formed in an epitaxial BaTiO3 (BTO) thin film grown on a substrate was investigated by scanning transmission electron microscopy combined with electron energy‐loss spectroscopy. Elemental analysis using core‐loss spectrum indicates that the atomic ratios of O/Ti and Ba/Ti are decreased at the dislocation core. The near‐edge fine structure of the oxygen K‐edge recorded from the dislocation core differs slightly from that of relaxed BTO region, which suggests that Ba‐O bonding is decreased at the dislocation core. The structure of the dislocation core is discussed using a high‐angle annular dark‐field image and the electron energy‐loss spectroscopy results.