The possibility of the new class ferroelectric materials of wurtzite structure simple chalcogenide was discussed using modern first-principles calculation technique. Ferroelectricity in the wurtzite structure (P6₃mc) can be understood by structure distortion from centrosymmetric P6₃/mmc by relative displacement of cation against anion along c-axis. Calculated potential surface of these compounds shows typical double well between two polar variants. The potential barriers for the ferroelectric polarization switching were estimated to be 0.25 eV/f.u. for ZnO. It is slightly higher energy to the common perovskite ferroelectric compound PbTiO₃. Epitaxial tensile strain on the ab-plane (0001) is effective to lower the potential barrier. The potential barrier decreased from 0.25 to 0.15 eV/f.u. by 5% ab-plane expansion in wurtzite structure ZnO. Epitaxial ZnO thin film with donor type defect reduction should be a possible candidate to confirm this ferroelectricity in wurtzite structure simple chalcogenide.

LiTi₂O₄ is a unique compound in that it is the only known spinel oxide superconductor. The lack of high quality single crystals has thus far prevented systematic investigations of its transport properties. Here we report a careful study of transport and tunnelling spectroscopy in epitaxial LiTi₂O₄ thin films. An unusual magnetoresistance is observed which changes from nearly isotropic negative to prominently anisotropic positive as the temperature is decreased. We present evidence that shows that the negative magnetoresistance likely stems from the suppression of local spin fluctuations or spin-orbit scattering centres. The positive magnetoresistance suggests the presence of an orbital-related state, also supported by the fact that the superconducting energy gap decreases as a quadratic function of magnetic field. These observations indicate that the spin-orbital fluctuations play an important role in LiTi₂O₄ in a manner similar to high-temperature superconductors.

An electric field–temperature (E-T) phase diagram for a lead-free 0.5 mol. % Mn-doped Bi(Na_0.1K_0.9)TiO₃ ceramics was investigated. The x-ray diffraction, dielectric and polarization measurements revealed relaxor behavior and were used to characterize the stability regions of the non-ergodic relaxor, ergodic relaxor and electric field induced ferroelectric states. As indicated by the polarization–current density profiles, transformation between two electric fields, induced ferroelectric states with opposite polarization direction arise via a two-step process through an intermediate relaxor state. Interplay between the ferroelectric state conversion and intermediate relaxor state is governed by the dynamics of polarization relaxation. The presented E-T phase diagram revealed the effects of the applied electric field and temperature on stability regions. This is of special interest since the Bi_0.5(Na_0.1K_0.9)_0.5TiO₃ ceramics were proposed as a potential piezoceramic material.

The properties of a potentially new class of ferroelectric materials based on wurtzite-structured ZnO thin films are examined using the first-principles calculations. Theoretical P-E hysteresis loops were calculated using the fixed-D method for both unstrained and (biaxially) strained single crystals. Ferroelectric polarization switching in ZnO (S.G. P6₃mc) is shown to occur via an intermediate non-polar structure with centrosymmetric P6₃/mmc symmetry by displacement of cations relative to anions in the long-axis direction. The calculated coercive electric field (E_c) for polarization switching was estimated to be 7.2 MV/cm for defect-free monocrystalline ZnO. During switching, the short- and long-axis lattice parameters expand and contract, respectively. The large structural distortion required for switching may explain why ferroelectricity in this compound has not been reported experimentally for pure ZnO. Applying an epitaxial tensile strain parallel to the basal plane is shown to be effective in lowering E_c during polarization, with a 5% biaxial expansion resulting in a decrease of E_c to 3.5 MV/cm. Comparison with calculated values for conventional ferroelectric materials suggests that the ferroelectric polarization switching of wurtzite-structured ZnO may be achievable by preparing high-quality ZnO thin films with suitable strain levels and low defect concentrations.

Thin films of Bi(Fe1−xScx)O3 (BFSO) system were fabricated on (111)Pt∕TiO2∕SiO2∕(100)Si substrates by chemical solution deposition to improve the electrical resistivity by substituting electrically unstable Fe3+ cations for stable Sc3+ cations. A single phase of perovskite was obtained in the range of x=0–0.30, in which selective replacement of Fe3+ and Sc3+ was confirmed in x=0–0.15 by using Raman measurement. The leakage current density of the BFSO films was reduced by increasing x. A well-saturated polarization–electric field hysteresis loop was obtained for BFSO films with x=0.15, showing remanent polarization of approximately 35μC∕cm2.