Effects of vanadium doping on the ferroelectric properties of Bi4Ti3O12 were investigated using dense ceramics. The incorporation of vanadium resulted in a large remanent polarization (2Pr) of over 40 μC/cm2 without sacrificing other physical properties, and the polarization characteristics were shown to be superior to SrBi2Ta2O9 and Sr0.8Bi2.2Ta2O9. In addition, dense ceramics of vanadium-doped Bi4Ti3O12 could be obtained by sintering at temperatures 100–200 °C lower than those for the SrBi2Ta2O9 system.

The effects of concentration and distribution of defects controlled by quenching and doping of higher-valent cations on the ferroelectric properties of dense Bi4Ti3O12 ceramics were investigated. The remanent polarization (P r) of non-doped ceramics quenched from 800°C (above the Curie temperature) was twice as large as those of samples subjected to slow cooling to 25°C and quenched from 600°C (below the Curie temperature). These results imply that domain pinning by defects dominates the polarization properties. The incorporation of vanadium and tungsten into Ti site significantly reduced the influence of domain pinning, resulting in a very large 2P r of over 40 µC/cm2.

Leakage current properties of K0.5Na0.5NbO3 (KNN) single crystals grown by a flux method have been investigated to establish a guiding principle of defect control for high-performance lead-free piezoelectric devices. The substitution of Mn at the Nb site and the following annealing under moderate oxidation condition was effective for suppressing leakage current of KNN crystals. Electron spin resonance measurements demonstrate that oxidation of Mn during annealing plays an essential role in low leakage current in the KNN system. Mn-doped KNN crystals exhibited a low leakage current density (∼10−8A∕cm2) and relatively large remanent polarization of 40μC∕cm2 at 25°C.

Ferroelectric properties along the a(b) axis and c axis in single crystals of various bismuth layer-structured ferroelectrics (BLSFs) were investigated. By measuring saturated P-E hysteresis curves of BLSFs, values of the saturated remanent polarization and the coercive electric field were found to be related to the Curie temperature and the number of BO6 octahedra (m) between bismuth layers, respectively, along the a(b) axis. The saturated remanent polarization was larger in the BLSF with a higher Curie temperature. This is attributed to a large atomic displacement accompanied by a high Curie temperature. In contrast, the saturated coercive electric field was smaller in the BLSF with a larger number of m. This phenomenon is assumed to be caused by the decrease in the strain energy of the octahedra from the bismuth layer, which leads to an easy movement of the octahedral cations in the direction of an applied external electric field. Along the c axis, the BLSFs with an odd number of m had the same relationship, that is, the saturated remanent polarization was larger with the high Curie temperature, and the saturated coercive field was smaller with the large m number. However, no remanent polarization was confirmed in the BLSFs with an even number of m.

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V 2 O 5 /carbon composites were prepared from a homogeneous suspension in which vanadium pentoxide sol and acetylene black powder were mixed with acetone acting as a surfactant. The composite was loaded on indium-tin oxide ͑ITO͒-coated glass, and then the magnesium intercalation property and the high-rate charge-discharge performance were evaluated in an electrolyte of Mg(ClO 4 ) 2 /acetonitrile at room temperature. In cyclic voltammetry, two main peaks of a reversible redox reaction were observed. It was determined that 1.84 mol of Mg per mol of V 2 O 5 were inserted in the first cycle at a relatively slow sweep rate of 0.1 mV s Ϫ1 , which corresponds to a specific capacity of 540 mAh (g-V 2 O 5 ) Ϫ1 . Galvanostatic charge-discharge tests at various current densities showed a high capacity of about 600 mAh (g-V 2 O 5 ) Ϫ1 at a current density of 1.0 A (g-V 2 O 5 ) Ϫ1 , and about 300 mAh (g-V 2 O 5 ) Ϫ1 was maintained even at 20 A (g-V 2 O 5 ) Ϫ1 .

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Photoferroelectrics offer unique opportunities to explore light energy conversion based on their polarization-driven carrier separation and above-bandgap voltages. The problem associated with the wide bandgap of ferroelectric oxides, i.e., the vanishingly small photoresponse under visible light, has been overcome partly by bandgap tuning, but the narrowing of the bandgap is, in principle, accompanied by a substantial loss of ferroelectric polarization. In this article, we report an approach, ‘gap-state’ engineering, to produce photoferroelectrics, in which defect states within the bandgap act as a scaffold for photogeneration. Our first-principles calculations and single-domain thin-film experiments of BiFeO3 demonstrate that gap states half-filled with electrons can enhance not only photocurrents but also photovoltages over a broad photon-energy range that is different from intermediate bands in present semiconductor-based solar cells. Our approach opens a promising route to the material design of visible-light-active ferroelectrics without sacrificing spontaneous polarization.