Research Topics
Charge disproportionation and charge transfer in A-site ordered perovskites containing Fe
New A-site ordered perovskite structure oxides, CaCu3Fe4O12 and LaCu3Fe4O12, were synthesized under high-pressure and high-temperature conditions. The compounds contain unusually high valence states of iron: Fe4+ in CaCu3Fe4O12 and Fe3.75+ in LaCu3Fe4O12. Instabilities of the high oxidation states at low temperatures are resolved in CaCu3Fe4O12 by charge disproportionation from Fe4+ to Fe3+ and Fe5+ and in LaCu3Fe4O12 by charge transfer between A-site Cu and B-site Fe ions. The charge disproportionation and the charge transfer are accompanied by significant changes in structural, transport, and magnetic properties.
[ Papers ]
"A perovskite containing quadrivalent iron as a charge-disproportionated ferrimagnet", I. Yamada, et al., Angew. Chem. Int. Ed., (2008).
"Temperature-induced A-B intersite charge transfer in an A-site-ordered LaCu3Mn4O12 perovskite", Y.W. Long, et al., Nature, (2009).
Single-crystal epitaxial thin films of SrFeO2 with FeO2"infinite layers"
Selective reduction of layers at low temperature in artificial superlattice thin films
Single-crystal thin films of SrFeO2, which is an oxygen deficient perovskite with "infinite layers" of FeO2, 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 c-axis oriented SrFeO2.
[ Papers ]
"Selective reduction of layers at low temperature in artificial superlattice thin films", K. Matsumoto, et al., Scientific Reports, (2011).
"Anisotropic oxygen diffusion at low temperature in perovskite-structure iron oxides", S. Inoue, et al., Nature Chem., (2010).
"Single-crystal epitaxial thin films of SrFeO2 with FeO2 "infinite layers"", S. Inoue, et al., Appl. Phys. Lett., (2008).
A-site ordered perovskites with intriguing physical properties
Los of intriguing physical properties of new A-site-ordered perovskite structure oxides were recently found. High-pressure and high-temperature conditions stabilize the square-coordinated Jahn-Teller Cu2+ ions in the original 12-fold-coordinated positions of the perovskite structure. The special-ordered arrangement of the square-coordinated A'O4 units that align perpendicularly to each other sets a characteristic structural framework, and the presence of Cu2+ ions at the A' site and A'-B interactions play crucial roles for the diverse and intriguing physical properties.
[ Paper ]
"A-site ordered perovskites with intriguing physical properties", Y. Shimakawa, Inorg. Chem. , (2008).
"Half-metallic ferrimagnet BiCu3Mn4O12", K. Takata, et al., Phys. Rev. B, (2007).
"Ferromagnetic cuprates, CaCu3Ge4O12 and CaCu3Sn4O12, with A-site ordered perovskite structures", H. Shiraki, et al., Phys. Rev. B (Papid), (2007).
Blue-light emission from Ar+ -irradiated SrTiO3
SrTiO3 is of particular importance due to a wide variety of interests from fundamental solid-state physics and chemistry to technological applications.
We found blue-light emission at room temperature from the Ar-irradiated-milled SrTiO3.
The Ar-beam irradiation causes local oxygen defects in SrTiO3 leading to metallic property of the material.
The defects produce donor levels for both photo- and cathode-luminescence within a bandgap.
Measurments of reciprocal-lattice maps around the (114) STO Bragg reflection using synchrotron radiation revealed a slightly lattice-expanded surface layer with unit cells of axaxc due to oxygen vacancies. The lattice expansion reaches 0.67% at the topmost surface and that the oxygen vacancies are introduced to a depth of about 21 nm.
Small size patterned emission with the local oxygen-deficient region is also demonstrated.
[ Papers ]
"Structural characterization of Ar+-irradiated SrTiO3 showing room-temperature blue luminescence" D. Kan, et al.,
Jpn. J. App. Phys., (2007).
"Blue luminescence from electron-doped SrTiO3", D. Kan, et al.,
App. Phys. Lett., (2006).
"Blue-light emission at room temperature from Ar+ -Irradiated SrTiO3", D. Kan, et al.,
Nature Materials, (2005).
New multi-ferroelectric compound Bi2NiMnO6
Magnetic ferroelectrics, also called multiferroics, are attracting much attention because of their possible applications for data storage and sensor devices.
We found a new ferromagnetic ferroelectric compound Bi2NiMnO6 by means of both high-pressure synthesis and epitaxial thin-film growth techniques.
The crystal structure of the compound is a significantly distorted double perovskite with Ni2+ and Mn4+ ions ordered in a rock salt configuration.
The presence of 6s2 lone pairs of Bi3+ ions and the covalent Bi-O bonds give ferroelectric properties with ferroelectric Curie temperature of 485 K,
while -Ni2+-O-Mn4+-O-Ni2+- magnetic paths lead to a ferromagnetism (the Kanamori-Goodenough rules) with transition temperature of 140 K.
[ Papers ]
"A Designed New Ferromagnetic Ferroelectric Bi2NiMnO6", M. Azuma, et al.,
J. Am. Chem. Soc., (2005).
"Multiferroic thin film of Bi2NiMnO6 with ordered double-perovskite structure", M. Sakai, et al.,
App. Phys. Lett., (2007).
"Direct Observation of B-site Ordering in Multiferroic Bi2NiMnO6 Thin Film", Y. Shimakawa, et al.,
Jap. J. App. Phys. (Letter)., (2007).
New perovskite-type oxides with large ferroelectric polarizations
We have succeeded to prepare new perovskite-type oxides PbVO3 and BiCoO3, which are isotypic with ferroelectric PbTiO3, by means of high-pressure synthesis at 6 Gpa.
The material shows large tetragonal distortion along the c axis.
Existence of the 6s lone-pair electrons of Pb2+ (Bi3+) ions and orbital hybridization between the Pb(Bi)-6s and O-2p states play crucial roles for the distortion.
The large displacements of constituent atoms indicate that the materials are displcive-type ferroelectrics with polarizations more than 100 uC/cm2.
The finding of possible ferroelecrtic materials with large polarization attracts great concerns of memory device applications.
[ Paper ]
"Crystallographic Features and Tetragonal Phase Stability of PbVO3, a New Member of PbTiO3 Family",
A.A. Belik, et al., Chem. Mat., (2005).
BiNiO3
Charge ordering in oxides, as observed at the Verwey transition of Fe3O4, drives many important phenomena, and it often leads to exotic conducting phenomena near the insulator to metal boundary. BiNiO3 has the unusual charge distribution Bi3+0.5Bi5+0.5Ni2+O3 with ordering of Bi3+ and Bi5+ charges on the A sites of a highly distorted perovskite structure. An insulator to metal transition accompanied by a structural distortion was occurs at 4 GPa pressure, and the pressure-induced melting of the charge disproportionated state leads to a simultaneous charge transfer from Ni to Bi, so that the HP phase is metallic Bi3+Ni3+O3. This unprecedented charge transfer between A and B site cations may offer new possibilities for tuning electronic phenomena in perovskite-related oxides.
[ Paper ]
"Pressure Induced Intermetallic Valence Transition in BiNiO3", M. Azuma, et al.,
J. Amer. Chem. Soc., (2007).
