The electronic structure of complex oxides is important for understanding their functional properties. Here, we report the results of investigating multiferroic BiFeO3 using various x-ray spectroscopy techniques. Zn- and Mn-codoped and Mn-doped BiFeO3 samples were prepared with the aim of improving ferroelectric properties of BiFeO3. The valences of the doped Mn and host Fe were investigated. When oxygen vacancies exist in the sample, Mn acts as a hole acceptor. Furthermore, Mn and Fe stabilize the perovskite unit by changing their ionic radius. As a result, Mn and Fe atoms exhibit various valence states in the BiFeO3 system. Evidence of the electronic structure for Fe 3d-O 2p-Bi 6s hybridization is also presented.

(Bi_1/2Na_1/2)TiO₃ (BNT) is one of the candidates for lead-free piezoelectric ceramics. Recently, we found that the depolarization temperature T _d of BNT could be increased by approximately 40 °C–80 °C with a quenching treatment after the sintering. To elucidate the mechanism, high-energy synchrotron X-ray powder diffraction data were collected in SPring-8 to analyze their crystal structures in detail. In the case of a quenched BNT-based sample, Bi ions displaced more to the off-center position of the A-site in a perovskite structure with a more ordered structure. The increase in rhombohedral distortion then affects the T _d elevation in the quenched sample.

Bond length distributions of nearest- and higher-neighbor distances in KNbO ₃ , which is the parent compound of lead-free piezoelectric materials, have been obtained by X-ray pair-distribution function (PDF) analysis. The Rietveld analysis of high-energy powder X-ray diffraction data as a function of temperature from 15 to 1073 K over four phases shows the variation in structure from rhombohedral to cubic structures. The refinements on the local structure by the PDF method indicate that only the rhombohedral structure can describe the observed bond distributions within the unit cell. The rhombohedral distortion is maintained locally in all four phases.

We investigate A-site cation ordering in the ferroelectric perovskite Bi _0.5 Na _0.5 TiO ₃ (BNT) by synchrotron X-ray total scattering. Although BNT has a problem of low depolarization temperature, it is a promising lead-free piezoelectric material. Since the depolarization temperature is presumed to correspond to a relaxor-like gradual order–disorder phase transition, local structural analysis is necessary to understand the structure of the disorder phase. Using this approach, we reveal the elusive connection between chemical and structural heterogeneity. The short-range order structure largely deviates from the rhombohedral average structure due to the off-center shift of Ti with large randomness. Consequently, the Ti off-center shift is averaged out beyond the unit cell and the structure becomes identical to the average structure beyond the unit cell. We further locate nanoscale regions of monoclinic distortion due to the heterogeneity of the A-site ion composition. The change in the short-range-order structure with distance can determine the BNT depolarization temperature. The results show necessary models to develop next generation novel piezoelectric materials.

The electronic structure of BiFeO₃ (BFO), BiFeO₃-PbTiO(3) solid solution (BFO-PT), and Mn-doped BFO-PT (BFM-PT) films fabricated by chemical solution deposition was investigated by x-ray absorption fine structure (XAFS). The BiFeO₃ shows a large leakage current owing to the mixed valance state of Fe(2 +) and Fe(3 +). The BFO film has a blunt absorption edge jump indicating the charge fluctuated state of the iron ions. The BFO-PT and BFM-PT films have sharp absorption edges, and the absorption energy of these films shifted to opposite energy. The valence fluctuation of the iron ions was closely connected with the leakage current properties. The charge fluctuated BFO film showed a leaky feature, and the charge unfluctuated BFO-PT and BFM-PT films had improved leakage current properties. The valence fluctuation of the iron ions can be controlled by Mn substitution and by making solid solutions.