Since the discovery of high-transition-temperature (high-T(c)) superconductivity in layered copper oxides, many researchers have searched for similar behaviour in other layered metal oxides involving 3d-transition metals, such as cobalt and nickel. Such attempts have so far failed, with the result that the copper oxide layer is thought to be essential for superconductivity. Here we report that Na(x)CoO2*yH2O (x approximately 0.35, y approximately 1.3) is a superconductor with a T(c) of about 5 K. This compound consists of two-dimensional CoO2 layers separated by a thick insulating layer of Na+ ions and H2O molecules. There is a marked resemblance in superconducting properties between the present material and high-T(c) copper oxides, suggesting that the two systems have similar underlying physics.

The crystal and magnetic structures of polycrystalline BiCoO 3 have been determined by the Rietveld method from neutron diffraction data measured at temperatures from 5 to 520 K. BiCoO 3 (space group P4mm; Z ) 1; a ) 3.72937(7) Å and c ) 4.72382(15) Å at room temperature; tetragonality c/a ) 1.267) is isotypic with BaTiO 3 and PbTiO 3 in the whole temperature range. BiCoO 3 is an insulator with a Ne ´el temperature of 470 K. A possible model for antiferromagnetic order is proposed with a propagation vector of k ) ( 1 / 2 , 1 / 2 , 0). In this model, magnetic moments of Co 3+ ions are parallel to the c direction and align antiferromagnetically in the ab plane. The antiferromagnetic ab layers stack ferromagnetically along the c axis, forming a C-type antiferromagnetic structure. Refined magnetic moments at 5 and 300 K are 3.24(2)µ B and 2.93(2)µ B , respectively. The structure refinements revealed no deviation from stoichiometry in BiCoO 3 . BiCoO 3 decomposed in air above 720 K to give Co 3 O 4 and sillenite-like Bi 25 -CoO 39 .

Structural properties of polycrystalline single-phased BiMnO3 samples prepared at 6 GPa and 1383 K have been studied by selected area electron diffraction (SAED), convergent beam electron diffraction (CBED), and the Rietveld method using neutron diffraction data measured at 300 and 550 K. The SAED and CBED data showed that BiMnO3 crystallizes in the centrosymmetric space group C2/c at 300 K. The crystallographic data are a = 9.5415(2) A, b = 5.61263(8) A, c = 9.8632(2) A, beta = 110.6584(12) degrees at 300 K and a = 9.5866(3) A, b = 5.59903(15) A, c = 9.7427(3) A, beta = 108.601(2) degrees at 550 K, Z = 8, space group C2/c. The analysis of Mn-O bond lengths suggested that the orbital order present in BiMnO3 at 300 K melts above TOO = 474 K. The phase transition at 474 K is of the first order and accompanied by a jump of magnetization and small changes of the effective magnetic moment and Weiss temperature, mueff = 4.69 microB and theta = 138.0 K at 300-450 K and mueff = 4.79 microB and theta = 132.6 K at 480-600 K.

The (1x)BiFeO 3 -xLaFeO 3 system has been investigated and characterized by room-temperature and high-temperature laboratory and synchrotron powder X-ray diffraction, electron diffraction, high-resolution transmission electron microscopy, differential scanning calorimetry, and magnetization measurements. At room temperature, the ferroelectric R3c phase is observed for 0.0 e x e 0.10. The PbZrO 3 -related ffiffi ffi p 2a p  2 ffiffi ffi p 2a p  4a p superstructure (where a p is the parameter of the cubic perovskite subcell) is observed for Bi 0.82 La 0.18 FeO 3 , while an incommensurately modulated phase is formed for 0.19 e x e 0.30 with the ffiffi ffi p 2a p  2a p  ffiffi ffi p 2a p basic unit cell. The GdFeO 3 -type phase with space group Pnma ( ffiffi ffi p 2a p  2a p  ffiffi ffi p 2a p ) is stable at 0.50 e x e 1. Bi 0.82 La 0.18 FeO 3 has no detectable homogeneity range (space group Pnam, a=5.6004(1) A ˚, b=11.2493(3) A ˚, c=15.6179(3) A ˚).The incommensurately modulated Bi 0.75 La 0.25 FeO 3 structure was solved from synchrotron X-ray powder diffraction data (Imma(00γ)s00 superspace group, a = 5.5956(1) A ˚, b = 7.8171(1) A ˚, c = 5.62055(8) A ˚, q=0.4855(4)c*, R P =0.023, R wP =0.033). In this structure, cooperative displacements of the Bi and O atoms occur, which order within the (AO) (where A=Bi, La) layers, resulting in an antipolar structure. Local fluctuations of the intralayer antipolar ordering are compensated by an interaction with the neighboring (AO) layers. A coupling of the antipolar displacements with the cooperative tilting distortion of the perovskite octahedral framework is proposed as the origin of the incommensurability. All the phases transform to the GdFeO 3 -type structure at high temperatures. Bi 0.82 La 0.18 FeO 3 shows an intermediate PbZrO 3 -type phase with ffiffi ffi p 2a p  2 ffiffi ffi p 2a p  2a p (space group Pbam; a = 5.6154(2) A ˚, b = 11.2710(4) A ˚, and c = 7.8248(2) A ˚at 570 K). The compounds in the compositional range of 0.18 e x e 0.95 are canted antiferromagnets.

This review shows the highlights of a 4-year-long research project supported by the Japanese Government to explore new superconducting materials and relevant functional materials. The project found several tens of new superconductors by examining ∼1000 materials, each of which was chosen by Japanese experts with a background in solid state chemistry. This review summarizes the major achievements of the project in newly found superconducting materials, and the fabrication wires and tapes of iron-based superconductors; it incorporates a list of ∼700 unsuccessful materials examined for superconductivity in the project. In addition, described are new functional materials and functionalities discovered during the project.

The structural and electronic properties of BiCoO(3) under high pressure have been investigated. Synchrotron X-ray and neutron powder diffraction studies show that the structure changes from a polar PbTiO(3) type to a centrosymmetric GdFeO(3) type above 3 GPa with a large volume decrease of 13% at room temperature revealing a spin-state change. The first-order transition is accompanied by a drop of electrical resistivity. Structural results show that Co(3+) is present in the low spin state at high pressures, but X-ray emission spectra suggest that the intermediate spin state is present. The pressure-temperature phase diagram of BiCoO(3) has been constructed enabling the transition temperature at ambient pressure to be estimated as 800-900 K.

The ferroelectric BaTiO(3) is a band-gap insulator. Itinerant electrons can be introduced in this material by doping, for example, with oxygen vacancies. Above a critical electron concentration of n(c) approximately 1 x 10(20) cm(-3), BaTiO(3-delta) becomes metallic. This immediately raises a question: Does metallic BaTiO(3-delta) still retain ferroelectricity? One may expect itinerant electrons to destroy ferroelectricity as they screen the long-range Coulomb interactions. We followed the phase transitions in BaTiO(3-delta) as a function of n far into metallic phase. Although their stability range decreases with n, the low-symmetry phases in metallic BaTiO(3-delta) are still retained up to an estimated concentration of n* approximately 1.9 x 10(21) cm(-3). Moreover, it appears that the itinerant electrons partially stabilize the ferroelectric phases in metallic BaTiO(3-delta) by screening strong crystal field perturbations caused by oxygen vacancies.

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