La0.3(Ba0.5Sr0.5)0.7Co0.8Fe0.2O3−δ is a promising bifunctional perovskite catalyst for the oxygen reduction reaction and the oxygen evolution reaction. This catalyst has circa 10 nm-scale rhombohedral LaCoO3 cobaltite particles distributed on the surface. The dynamic microstructure phenomena are attributed to the charge imbalance from the replacement of A-site cations with La3+ and local stress on Co-site sub-lattice with the cubic perovskite structure.

A Bifunctional Perovskite Catalyst for Oxygen Reduction and Evolution

Using neutron pair distribution function analysis over the temperature range from 1000 to 15 K, we demonstrate the existence of local polarization and the formation of medium-range, polar nanoregions (PNRs) with local rhombohedral order in a prototypical relaxor ferroelectric Pb(Mg(1/3)Nb(2/3))O3. We estimate the volume fraction of the PNRs as a function of temperature and show that this fraction steadily increases from 0% to a maximum of approximately 30% as the temperature decreases from 650 to 15 K. Below T approximately 200 K the volume fraction of the PNRs becomes significant, and PNRs freeze into the spin-glass-like state.

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Direct observation of the formation of polar nanoregions in Pb(Mg1/3Nb2/3)O3 using neutron pair distribution function analysis.

The phase diagram of (1 − x)(Bi0.5Na0.5)TiO3–xSrTiO3 was completed and investigations on polarization and strain in this system were carried out. (1 − x)(Bi0.5Na0.5)TiO3–xSrTiO3-ceramics were prepared by conventional mixed oxide processing. The depolarization temperature (Td), the temperature of the rhombohedral–tetragonal phase transition (Tr–t) and the Curie temperature (Tm) were determined by measuring the temperature dependence of the relative permittivity. All solid solutions of (1 − x)(Bi0.5Na0.5)TiO3–xSrTiO3 show relaxor behavior (A-site relaxor). From XRD-measurements a broad maximum of the lattice parameter can be observed around x = 0.5 but no structural evidence for a morphotropic phase boundary was found. SEM-analysis revealed a decrease of the grain size for increasing SrTiO3-content. At room temperature a maximum of strain of about 0.29% was found at x = 0.25 which coincides with a transition from a ferroelectric to an antiferroelectric phase. The temperature dependence of the displacement indicates an additional contribution from a structural transition (rhombohedral–tetragonal), which would be of certain relevance for the existence of a morphotropic phase boundary.

Piezoelectric properties and phase transition temperatures of the solid solution of (1−x)(Bi0.5Na0.5)TiO3–xSrTiO3

Extensive studies that have been conducted on the Pb(Mg1/3Nb2/3)O3 (PMN) family of relaxor ferroelectrics have led to the establishment and acceptance of the“space-charge” model as a basis for explaining their structures and dielectric properties. In this model, the arrangement of the metal cations on the octahedral sites of the perovskite structure is interpreted in terms of the formation of nega-tively charged ordered nanodomains that are dispersed in a positively charged disordered matrix. The primary experi-mental support for this interpretation has come from the apparent absence of any growth of the domains or change in the degree of ordering as the heat treatment is extended. Here, we report on experiments that have been conducted on the tantalate relaxor, Pb(Mg1/3Ta2/3)O3(PMT), and its solid solutions with PbZrO3, in which the size of the do-mains and the degree of cation ordering have been in-creased by two orders of magnitude through annealing that has been conducted at a temperature of 1325°C. Moreover, fully ordered ceramics that are comprised of large domains retain relaxor behavior. These results cannot be explained by the space-charge model and support a charge-balanced, “random-site” model for the ordering of the metal cations

Domain Growth in Pb(Mg1/3Ta2/3)O3 Perovskite Relaxor Ferroelectric Oxides

https://www.seas.upenn.edu/~davies/papers/pmnjpcs.pdf

Li Ping

Papers

Structural origin of relaxor perovskites