Atomic structural mechanism for ferroelectric-antiferroelectric transformation in perovskite NaNbO3

Sodium niobate (NaNbO3 or NN) is described as "the most complex perovskite system,"which exhibits transitions between, as well as coexistence of, several ferroelectrics (FE) and antiferroelectric (AFE) phases at different temperatures. Recently, solid solutions of NN with stabilized AFE phases(s) have gained attention for energy-related applications, such as high-density energy storage and electrocaloric cooling. A better understanding of the atomic mechanisms responsible for AFE/FE phase transitions in NaNbO3 can enable a more rational design of its solid-solution systems with tunable functional properties. Here, we have investigated changes in the average and local atomic structure of NN using a combination of x-ray/neutron diffraction and neutron pair-distribution function (PDF) analyses. The Rietveld refinement of the x-ray/neutron-diffraction patterns indicates a coexistence of the FE Q (P21ma) and AFE P (Pbma) phases in the temperature range of 300K≤T≤615K, while PDF analysis indicated that the local structure (r<8Å) is better described by a P21ma symmetry. Above 615 K, the average structure transitions to an AFE R phase (Pmmn or Pnma), while PDF analysis shows an increased disordering of the octahedral distortions and Na displacements at the local scale. These results indicate that the average P/Q/R phase transitions in NN can be described as a result of complex ordering of distorted octahedral tilts at the nanoscale and off-centered displacements of the Na atoms

From theory to experiment: BaFe0.125Co0.125Zr0.75O3-delta, a highly promising cathode for intermediate temperature SOFCs

In a recent theoretical study [Jacobs et al., Adv. Energy Mater., 2018, 8, 1702708], BaFe0.125Co0.125Zr0.75O3-delta was predicted to be a stable phase with outstanding performance as an auspicious cathode for intermediate-temperature solid oxide fuel cells (IT-SOFCs). It is shown here that the theoretical predictions are valid. The material can be synthesized by the citrate method as a single cubic Pm3m phase with a significant amount of oxygen vacancies, randomly distributed in the anionic sublattice facilitating oxygen vacancy conduction. A thermal expansion coefficient of 8.1 x 10(-6) K-1 suggests acceptable compatibility with common electrolytes. Electrochemical impedance spectroscopy of symmetrical cells gives an area-specific resistance of 0.33 Omega cm(2) at 700 degrees C and 0.13 Omega cm(2) at 800 degrees C. These values are reduced to 0.13 Omega cm(2) at 700 degrees C and 0.05 Omega cm(2) at 800 degrees C when the material is mixed with 30 wt% Ce0.9Gd0.1O2-delta