Eutectoid decompositions in Ce-containing ABO3 perovskites: Part I, the case of cooperative growth in CeAlO3.

Oxidation of perovskite CeAlO3 results in the eutectoid transformation to CeO2 and Al2O3. This phase transformation was recorded using thermal gravimetric analysis, X-ray diffraction, and scanning transmission electron microscopy. Lamellar features in the resultant microstructure indicates cooperative growth. Processing conditions dictate the lamellae sizes, which can be as small as a few nanometers, and coarsen into large domains with additional high temperature annealing.

Eutectoid decompositions in Ce-containing ABO3 perovskites: Part II, the case of divorced growth in CeCrO3.

A eutectoid phase transformation was exhibited by the perovskite CeCrO3 when heated in air. The kinetics of the reaction, microstructure of the product, and mechanisms of the transformation were studied using thermogravimetric analysis, X-ray and Raman spectroscopy, X-ray diffraction, and electron microscopy. Fluorite CeO2 and corundum Cr2O3 were formed from the decomposition of CeCrO3. The CeO2 particles were porous and polycrystalline, a consequence of nucleation, growth, and impingent of CeO2 domains on the CeCrO3 particles. Anisotropic growth is indicated by the morphology of the CeO2 grains, while the Cr2O3 particles were single crystals without any crystallographic orientation relationship to the parent phase. Unlike the CeAlO3 eutectoid, the microstructure comprised of CeO2 and Cr2O3 show no characteristics of a microstructure formed by cooperative growth. The disparity between the eutectoid reactions in CeCrO3 and CeAlO3 is attributed to a difference in interfacial energy between the fluorite and sesquioxide phases (i.e., CeO2/Al2O3 versus CeO2/Cr2O3).

Adlayer formation on C-plane (0001) and R-plane ( 1 1 ‒ 0 2 ) Al2O3 surfaces.

Adlayers on C-plane (0001) and R-plane ( 1 1 ‒ 02 ) terminated surfaces of corundum phase aluminum oxide were synthesized by annealing mixtures of two oxide powders, aluminum oxide with an additive. Using high-angle annular dark field scanning transmission electron microscopy, the adsorbed layers were characterized, and image simulations aided interpretation of the results. The adlayers were pseudomorphic, one atomic layer thick and with a fractional site occupancy. Atomic positions of the adlayer atoms relaxed and changed relative to the bulk structure, where there is evidence that the magnitude of the relaxation is sensitive to the ionic radius of the adsorbate. The pseudomorphic adlayer structure formed for different elements including, but not limited to, the lanthanides (i.e., Ge, Ba and Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm).

Approaches for the quantitative analysis of oxidation state in cerium oxide nanomaterials.

Cerium oxide nanomaterials (nanoceria, CNMs) are receiving increased attention from the research community due to their unique chemical properties, most prominent of which is their ability to alternate between the Ce3+ and Ce4+ oxidation states. While many analytical techniques and methods have been employed to characterize the amounts of Ce3+ and Ce4+ present (Ce3+/Ce4+ ratio) within nanoceria materials, to-date no studies have used multiple complementary analytical tools (orthogonal analysis) with technique-independent oxidation state controls for quantitative determinations of the Ce3+/Ce4+ ratio. Here, we describe the development of analytical methods measuring the oxidation states of nanoceria analytes using technique-independent Ce3+ (CeAlO3:Ge) and Ce4+ (CeO2) control materials, with a particular focus on x-ray photoelectron spectroscopy (XPS) and electron energy loss spectroscopy (EELS) approaches. The developed methods were demonstrated in characterizing a suite of commercial nanoceria products, where the two techniques (XPS and EELS) were found to be in good agreement with respect to Ce3+/Ce4+ ratio. Potential sources of artifacts and discrepancies in the measurement results were also identified and discussed, alongside suggestions for interpreting oxidation state results using the different analytical techniques. The results should be applicable towards producing more consistent and reproducible oxidation state analyses of nanoceria materials.

Investigation of Electric Field-Induced Structural Changes at Fe-Doped SrTiO3 Anode Interfaces by Second Harmonic Generation.

We report on the detection of electric field-induced second harmonic generation (EFISHG) from the anode interfaces of reduced and oxidized Fe-doped SrTiO3 (Fe:STO) single crystals. For the reduced crystal, we observe steady enhancements of the susceptibility components as the imposed dc-voltage increases. The enhancements are attributed to a field-stabilized electrostriction, leading to Fe:Ti-O bond stretching and bending in Fe:Ti-O6 octahedra. For the oxidized crystal, no obvious structural changes are observed below 16 kV/cm. Above 16 kV/cm, a sharp enhancement of the susceptibilities occurs due to local electrostrictive deformations in response to oxygen vacancy migrations away from the anode. Differences between the reduced and oxidized crystals are explained by their relative oxygen vacancy and free carrier concentrations which alter internal electric fields present at the Pt/Fe:STO interfaces. Our results show that the optical SHG technique is a powerful tool for detecting structural changes near perovskite-based oxide interfaces due to field-driven oxygen vacancy migration.