Yuichi Michiue

A ferroelectric-like structural transition in a metal

Metals cannot exhibit ferroelectricity because static internal electric fields are screened by conduction electrons, but in 1965, Anderson and Blount predicted the possibility of a ferroelectric metal, in which a ferroelectric-like structural transition occurs in the metallic state. Up to now, no clear example of such a material has been identified. Here we report on a centrosymmetric (R3c) to non-centrosymmetric (R3c) transition in metallic LiOsO3 that is structurally equivalent to the ferroelectric transition of LiNbO3 (ref. 3). The transition involves a continuous shift in the mean position of Li(+) ions on cooling below 140 K. Its discovery realizes the scenario described in ref. 2, and establishes a new class of materials whose properties may differ from those of normal metals.

Unconventional Colossal Magnetoresistance in Sodium Chromium Oxide with a Mixed‐Valence State

ions. This material demonstrates an unusual CMReffect that is closely related to its unconventional magneticstructure caused by spin frustration. The CMR of thiscompound is unique from several aspects. First, it is observedin a chromium oxide, not in a manganese oxide. Second, it isfound in a single-phase material having an insulating groundstate. Third, the CMR is not limited to the vicinity of themagnetic phase transition but becomes progressively moreprominent with decreasing temperature down to 0 K. Thediscovery of the NaCr

Boron carbide, B13-xC2-y (x = 0.12, y = 0.01)

Boron carbide phases exist over a widely varying compositional range B12+xC3-x (0.06 < x < 1.7). One idealized structure corresponds to the B13C2 composition (space group R-3m) and contains one icosahedral B12 unit and one linear C—B—C chain. The B12 units are composed of crystallographically distinct B atoms BP (polar, B1) and BEq (equatorial, B2). Boron icosahedra are interconnected by C atoms via their BEq atoms, forming layers parallel to (001), while the B12 units of the adjacent layers are linked through intericosahedral BP—BP bonds. The unique B atom (BC) connects the two C atoms of adjacent layers, forming a C—B—C chain along [001]. Depending on the carbon concentration, the carbon and BP sites exhibit mixed B/C occupancies to varying degrees; besides, the BC site shows partial occupancy. The decrease in carbon content was reported to be realized via an increasing number of chainless unit cells. On the basis of X-ray single-crystal refinement, we have concluded that the unit cell of the given boron-rich crystal contains following structural units: [B12] and [B11C] icosahedra (about 96 and 4%, respectively) and C—B—C chains (87%). Besides, there is a fraction of unit cells (13%) with the B atom located against the triangular face of a neighboring icosahedron formed by BEq (B2) thus rendering the formula B0.87(B0.98C0.02)12(B0.13C0.87)2 for the current boron carbide crystal.

Structure of Ga2O3(ZnO)6: a member of the homologous series Ga2O3(ZnO)m

The structure of Ga(2)O(3)(ZnO)(6) was determined using single-crystal X-ray diffraction techniques in the space group Cmcm. The metal ion sublattice resembles some of the Zn ions in the wurtzite ZnO structure. The oxygen ion sublattice in Ga(2)O(3)(ZnO)(6) also resembles some of the O ions in ZnO. Structural relationships between Ga(2)O(3)(ZnO)(6) and ZnO are discussed, illustrating the process for obtaining the centrosymmetric Ga(2)O(3)(ZnO)(6) structure from the noncentrosymmetric ZnO. Structures of phases in the homologous series Ga(2)O(3)(ZnO)(m) are predicted on the basis of the structural data for Ga(2)O(3)(ZnO)(6). The structures of even m are constructed by simply extending the structure units seen in Ga(2)O(3)(ZnO)(6), while those of odd m consist of structure units which are of different types from those used for even m.

β-Vesignieite BaCu3V2O8(OH)2: a structurally perfect S = 1/2 kagomé antiferromagnet

A novel copper mineral β-vesignieite BaCu3V2O8(OH)2 with trigonal symmetry has been synthesized for the first time using a hydrothermal technique. β-Vesignieite is an ideal spin-1/2 kagome antiferromagnet. Antiferromagnetic long-range order is found at 9 K, probably due to a finite Dzyaloshinski–Moriya interaction caused by the kagome symmetry.

Crystal structure and electric conductivity of K+-β-ferrite with ideal composition KFe11O17

Abstract Fe2+ in K1.22Fe11O17 single crystals has been oxidized by iodine to obtain an ideal composition of KFe11O17. The crystal structure was analysed based on its β-alumina structure and a final RF value of 0.034 was obtained. The atom positions in the spinel block were unchanged by the oxidation. On the other hand, while the amount of K+ in BR sites was comparable to that in K1.22Fe11O17, that in the aBR sites was largely decreased. The ionic conductivity of β-ferrite with the ideal composition was measured at 300 °C and found to be 3.53 · 10 −3 Ω −1 cm −1 in the (001) plane. The activation energy was Ea = 0.19 eV. This ionic conductivity was twenty times lower than that of K1.22Fe11O17. The activation energy was also lowered. This is because the amount of K+ in aBR sites was relatively small. The electronic conductivity was also measured and found to be 2.05 · 10 −5 Ω −1 cm −1 at 300 °C, which is two orders of magnitude smaller than that of K1.22Fe11O17. However, the activation energy (0.45 eV) was unchanged. The hopping pairs of Fe2+ and Fe3+ would be largely decreased by the oxidation.

Superspace description of the homologous series Ga2O3(ZnO)m

A unified description for the structures of the homologous series Ga(2)O(3)(ZnO)(m), gallium zinc oxide, is presented using the superspace formalism. The structures were treated as a compositely modulated structure consisting of two subsystems. One is constructed with metal ions and the other with O ions. The ideal model is given, in which the displacive modulations of ions are well described by the zigzag function with large amplitudes. Alternative settings are also proposed which are analogous to the so-called modular structures. The validity of the model has been confirmed by refinements for phases with m = 6 and m = 9 in the homologous series. A few complex phenomena in real structures are taken into account by modifying the ideal model.

Electrical, optical, and thermoelectric properties of Ga2O3(ZnO)9

The physical properties of the sintered sample of Ga2O3(ZnO)9, a member of the homologous series Ga2O3(ZnO)m and recently found to have new structures, were investigated. The material was found to be a new transparent conducting oxide, and is composed of relatively abundant and inexpensive elements compared to indium. The electrical conductivity of a sintered low 57% density sample, 13 S cm−1 as fired at 1723 K, could be varied by postheating in air or a reducing gas (H2 3% : Ar 97%) flow. The changes in conductivity were associated with the variations of absorbance in the visible light range, while the optical band gap, or the absorption edge, was almost unchanged. The thermoelectric properties showed n-type behavior. It was relatively easy to vary the thermoelectric properties through redox treatments, and reversibility was also observed. The maximum figure of merit Z reaches a value of close to 10−4 K−1 at 660 K for a sample with density of only 73%. The potential of Ga2O3(ZnO)9 as a thermoelectric material appears to be similar to or even greater than the related In2O3(ZnO)m system, and since Ga2O3(ZnO)9 has an advantage in the abundance of constituent elements, it is revealed to be a promising system for further investigations.

Modulated structure of the pseudohexagonal InFe1−x−4δTix+3δO3+x/2 (x = 0.61) composite crystal

The structure of pseudohexagonal-type InFe1−x−4δTix+3δO3+x/2 (x = 0.61, δ = 0.04), indium iron titanium oxide, was refined on the basis of a four-dimensional superspace group. The crystal has a compositely modulated structure consisting of two orthorhombic subsystems mutually incommensurate in b. The first subsystem InFe1−x−4δTix+3δO2 has a delafossite structure with lattice parameters a = 5.835 (3), b1 = 3.349 (1) and c = 12.082 (7) A. The second subsystem with b2 = 2.568 (6) A consists of O atoms. The superspace group of the overall structure is Ccmm(1, 1.305, 0)s00, which can be converted to Amam(0, 0, 0.305)0s0 (No. 63.8). Refinement on 1105 unique reflections converged to R = 0.0303 and wR = 0.0325 with 63 structural parameters. The structure of the first subsystem is the alternate stacking of an edge-shared InO6 octahedral layer and an Fe/Ti triangle-lattice plane along c. A sheet of O atoms in the second subsystem is also extending on the Fe/Ti plane, where displacive modulation of atoms is prominent.

Orthorhombic InFe0.33Ti0.67O3.33

The title compound, indium iron titanium oxide, is closely related to InFeO 3 , having a hexagonal structure which consists of alternating layers of InO 6 octahedra and FeO 5 trigonal bipyramids. According to substitution of Ti 4+ for Fe 3+ , excess O atoms are introduced into the Fe-O trigonal lattice plane of InFeO 3 . The inplane arrangement of O atoms can be described as partial occupation on a honeycomb lattice, although large displacement parameters indicate local shifts of 0 atoms due to repulsive interactions between them.