Uncovering Design Principles of Vacancy-Ordered Structures in Oxygen-Deficient Perovskites and the Impact of Vacancy-Order on Functional Properties

Non-technical summary Innovations in materials are at the center of recent technological advances. The ability to modify and control material structures, and understand their structure-property relationships can lead to exciting scientific discoveries and new technologies. In addition to pushing the boundaries of science, such advances lead to the improved prosperity of society. An important family of oxide materials are perovskite oxides. The structure and functional properties of these materials can be modified by different synthesis methods. With support from the Solid State and Materials Chemistry program in NSF’s Division of Materials Research and the Office of Strategic Initiatives, Prof. Ramezanipour at the University of Louisville, investigates the incorporation of oxygen-vacancies into perovskite oxides, where some of the positions that would typically be occupied by oxygen atoms are vacant. The distribution of these vacant sites in the material structure can be either random or ordered. In this project, several hypotheses are evaluated in an effort to understand how the ordering of oxygen-vacancies can be induced and controlled. Several chemical principles are used, such as the effect of electronic properties and geometric stability. Based on these criteria, a wide range of oxygen-deficient perovskites with different vacancy-arrangements and ordering patterns are synthesized, helping to establish guidelines for rational design of vacancy-ordered materials. Broader impacts of this project include training student researchers, enhancing undergraduate students’ laboratory skills, and outreach activities aimed at motivating high school students to pursue higher education, particularly in STEM fields. Technical summary This project, supported by the Solid State and Materials Chemistry program in NSF’s Division of Materials Research and the Office of Strategic Initiatives, establishes guidelines for rational design of vacancy-ordered structures in oxygen-deficient perovskites and their derivatives. These studies are motivated by remarkable properties of this family of materials, which are different from those of stoichiometric systems. The research is built around several hypotheses, based on fundamental chemistry considerations, to explore the formation and arrangement of oxygen-vacancies. Methods of tuning the concentration of oxygen-vacancies are studied to obtain perovskite-based oxides with different degrees of vacancies. Several types of ordering can be realized in oxygen-deficient perovskites, each leading to different types of coordination, such as tetrahedral, square-pyramidal, and octahedral, for metal cations within the material lattice. In this project, these different ordering-types are investigated, and the principal investigator and his research group explore several factors as potential parameters that affect the ordering of oxygen-vacancies. A series of experimental techniques are used to obtain the target materials and investigated by an array of state-of-the art characterization methods. Overall, the researchers use a combination of compositional and synthetic considerations to develop design principles for the realization of each type of ordering in oxygen-deficient perovskites, which might enable new materials for various technological applications, thereby increasing the economic competitiveness of the U. S.. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.