Hexagonal manganites, h-RMnO3 (R=Sc, Y, Ho–Lu), have been intensively studied for their multiferroic properties, magnetoelectric coupling, topological defects and electrically conducting domain walls. Although point defects strongly affect the conductivity of transition metal oxides, the defect chemistry of h-RMnO3 has received little attention. We use a combination of experiments and first principles electronic structure calculations to elucidate the effect of interstitial oxygen anions, Oi, on the electrical and structural properties of h-YMnO3. Enthalpy stabilized interstitial oxygen anions are shown to be the main source of p-type electronic conductivity, without reducing the spontaneous ferroelectric polarization. A low energy barrier interstitialcy mechanism is inferred from Density Functional Theory calculations to be the microscopic migration path of Oi. Since the Oi content governs the concentration of charge carrier holes, controlling the thermal and atmospheric history provides a simple and fully reversible way of tuning the electrical properties of h-RMnO3.

Ferroelectric BiFeO 3 has attractive properties such as high strain and polarization, but a wide range of applications of bulk BiFeO 3 are hindered due to high leakage currents and a high coercive electric field.Here, we report on the thermal behaviour of the electrical conductivity and thermopower of BiFeO 3 substituted with 10 and 20 mol% Bi 0.5 K 0.5 TiO 3 . A change from p-type to n-type conductivity in these semi-conducting materials was demonstrated by the change in the sign of the Seebeck coefficient and the change in the slope of the isothermal conductivity versus partial pressure of O. A minimum in the isothermal conductivity was observed at B10 À2 bar O 2 partial pressure for both solid solutions. The strong dependence of the conductivity on the partial pressure of O 2 was rationalized by a point defect model describing qualitatively the conductivity involving oxidation/reduction of Fe 3+ , the dominating oxidation state of Fe in stoichiometric BiFeO 3 . The ferroelectric to paraelectric phase transition of 80 and 90 mol% BiFeO 3 was observed at 648 AE 15 and 723 AE 15 1C respectively by differential thermal analysis and confirmed by dielectric spectroscopy and high temperature powder X-ray diffraction.

Conventional solid‐state synthesis was used to synthesize dense and phase pure ceramics in the (1−x) Bi0.5K0.5TiO3–xBi0.5Na0.5ZrO3 (BKT–BNZ) system. Structural characterization was done using X‐ray diffraction at both room temperature and elevated temperatures, identifying a transition from tetragonal xBi0.5Na0.5ZrO3 (xBNZ, x = 0–0.10) to pseudo cubic xBNZ for x = 0.15–0.80. Dielectric properties were investigated with respect to both temperature (RT = 600°C) and frequency (1–106 Hz). Relaxor‐like behavior was retained for all the materials investigated, evident by the broadening of the relative dielectric permittivity peaks at transition temperatures as well as frequency dispersion at their maximum. The maximum dielectric constant at elevated temperature was found for 0.15 BNZ. Electric field‐induced strain and polarization response were also investigated for several compositions at RT and the largest field‐induced strain was observed for the 0.10 BNZ ceramics. The composition range with best performance coincides with the transition from tetragonal to cubic crystal structure.

The influence of uniaxial compressive stress on the small signal direct piezoelectric coefficient of hard and soft Pb(Zr,Ti)O3 at the morphotropic phase boundary was investigated as a function of temperature from 25 °C to 450 °C. The stress--and temperature--dependent piezoelectric data indicate that stress is capable of either directly or indirectly modifying the orientation of polar defects in the crystal lattice. At higher temperatures, the mobility of polar defects was found to increase, corresponding to a two--step decrease in the direct piezoelectric coefficient and a decrease in the frequency dispersion. Quenching experiments were used to elucidate the role of the internal bias field on the stress--dependent piezoelectric response.

La0.2Sr0.8Fe0.8Ta0.2O3-(LSFT) is a mixed ionic electronic conductor (MIEC) at elevated temperatures and as such a candidate material for applications both in syn-gas synthesis and as electrodes in solid oxide fuel cells (SOFC). This study addresses the variation in oxygen permeation rates for LSFT symmetric-and asymmetric-membranes at temperatures between 800 and 1000 o C with and without surface modification. The surface was strutured in two different scales, macro (porous LSFT-layer) and micro (acid etching). The asymmetric membranes showed a significant variation in permeation rate with surface treatment with increasing rate in the sequence from non-treated to macro-structured and finally micro structured, corresponding to oxygen permeation being controlled by surface exchange and gas diffusion. It was found that the permeation rate was sensitive to the gas sweep rate when H2-mixtures was introduced on the permeate side, which was rationalized by adsorption of H2O-molucules on the surface hampering the exchange of oxygen.