Structure and Dynamics in Oxide Conducting Catalytic Ceramics

• Grant number: 24/04662-2
• Support Opportunities: Regular Research Grants
• Start date: October 01, 2024
• End date: September 30, 2028
• Field of knowledge: Physical Sciences and Mathematics - Physics - Condensed Matter Physics
• Principal Investigator: Hellmut Eckert
• Grantee: Hellmut Eckert
• Principal researcher abroad: Tzonka Ivanova Mineva
• Institution abroad: Centre National de la Recherche Scientifique, France
• Host Institution: Instituto de Física de São Carlos (IFSC). Universidade de São Paulo (USP). São Carlos , SP, Brazil
• Associated researchers: Claudio José Magon ; Marcos de Oliveira Junior

Abstract

Oxides with Perovskite structure are a fascinating class of materials with a huge variety of technological applications, e.g. ion conductors for energy storage and transformation, electrocatalyst, solar cells, magnetism, etc. While the undistorted ABO3 structure shows a simple cubic structure, it turned out recently that many of these compounds present a more complex structure on a local level, essentially based on micro-twinning and oxygen vacancy ordering. Such phases show a very particular domain structure with mesoscopic periodicities, consisting of Brownmillerite Building Blocks interconnected by an interface zone. The latter also serves to switch domain orientations, forming anti-phase twin domains, thus promoting the creation of possible polar twin boundaries, even in case of a non-polar ferroelastic ground state of the underlying structure. The polarizing character of such interface structure becomes especially interesting to promote catalytic reactions at the gas-solid interface, wherever molecules showing a dipole moment are involved. In this regard we have evidenced recently, in Brownmillerite frameworks with the formula A2BB'O5, that octahedral B or tetrahedral B' sites can be selectively substituted on either site. This is, e.g., the case for Sr2ScGaO5, where Sc and Ga are maintaining their respective octahedral and tetrahedral coordination while substituting one of these sites by 3d transition metal atoms, e.g. Mn, Fe or Co. We could further evidence, by combining neutron-pdf analysis and RIXS spectroscopy, that the local coordination for B and B' is preserved both for the Brownmillerite and its cubic-Perovskite polymorph. This opens new ways of structural and microstructural fine tuning, allowing to create distributions of catalytic active sites tailored to the requirement of more selective reactions. The project aims to explore up to which extent such kind of microstructural engineering can be conceptually applied to materials synthesis aspects. The generated "structural ordering", together with a controlled domain structure, will be evaluated in promoting chemical reactivity in selected catalytic reactions as well as in fostering solid state diffusion mechanisms of oxygen and protons at low temperatures, down to room temperature. The domain engineering approach will answer the question why oxygen as well as proton conductivity in these single crystalline materials exceed those of polycrystalline ceramics by several orders of magnitude. Indeed, neutron-pdf and single crystal synchrotron diffraction have revealed a complex nano-domain structure, consisting of regular blocks with the Brownmillerite structure-type comprising characteristic 1D oxygen vacancy channels. We will apply the control of the domain size by thermal annealing to the investigation of low-T oxygen and proton conductivity as a function of particle size, from oriented single crystals to nanostructured materials. This will allow, besides an unprecedented correlation of microstructure and ion mobility, a distinction between bulk and interface ion conductivity. Domain, charge, and oxygen vacancy ordering, together with dynamic aspects, will be explored by diffraction/spectroscopy in the laboratory but also at large scale facilities (synchrotron/neutron). In addition, emphasis is put on the development of 17O/45Sc/71Ga/1H/2H solid-state NMR methodology for characterizing both the local structures and the dynamic aspects of mobile proton species, which will be correlated with the results from ab-initio molecular dynamics simulations. These studies will be complemented by 16O/18O isotope exchange and catalytic reactions. (AU)