Chemical substitutions that increase the compositional complexity of the perovskite structure are known to give rise to properties that cannot be realized in simpler materials. However, as the compositional complexity increases detailed knowledge of the bonding interactions that drive atoms to assemble into homogeneous single phase materials is needed to rationally approach the synthesis and design of such materials. The proposed research involves synthesis of new perovskites and the use of advanced structural characterization tools, and property measurements to study these materials. The specific research objectives involve: (1) Synthesis and characterization of new perovskites with useful dielectric, magnetic and ionic conduction properties by simultaneously ordering A- and B-site cation arrays. (2) Studies of perovskites where octahedral tilting disrupts the corner sharing connectivity of the octahedral framework. (3) Studies of phase transitions in oxyfluoride double perovskite, coupled with use of cation bonding preferences to control the orientational ordering of anionic oxyfluoride polyhedra. Variable temperature Raman spectroscopy studies will be used to follow changes in local and long range structure that occur in response to changes in temperature or composition. New materials with attractive dielectric, magnetic, nonlinear optical and electrical properties are anticipated.
NON-TECHNICAL SUMMARY:
Perovskite materials play a central role in many important technologies including: dielectric materials for electronic applications, ion conductors for batteries and fuel cells, superconductivity and magnetoresistance to name a few. This grant will advance our understanding of how to control atomic scale ordering of elements to produce new materials for these important technologies. The research involves collaborations with internationally renowned electron microscopy centers in Australia and Spain. To disseminate this knowledge Web based resources will be developed that will positively impact teaching and research across several disciplines. An upper level laboratory experiment that teaches the principles of X-ray powder diffraction and crystal packing forces will be created, implemented and disseminated. Finally the project will provide important opportunities for graduate and undergraduate students to learn skills that are needed for modern materials chemistry research. Students will create important contacts through national and international collaborations supported by this project.
Through research supported by this grant we have advanced our ability to synthesize new functional materials and using advanced characterization methods we have been able to understand the forces that lead to unexpected complexity in the crystal structures of these materials. These findings will advance our ability to design new polar materials for use in applications such as capacitors, piezoelectrics, and second harmonic generation crystals for doubling the frequency of laser light. Specifically, we have been able to 1. Show how octahedral tilting can be used to manipulate the formation of stripe and checkerboard nanoscale modulations in AA'MM'O6 perovskites. 2. We have solved the structures of perovskites distorted by non-cooperative octahedral tilting, such as Sr3WO6 and K3AlF6. The complexity of these structures and prevalence of twinning in single crystal samples have long frustrated attempts to structurally characterize these phases. We have been able to solve these structures by combining electron microscopy, synchrotron and neutron powder diffraction methods. This demonstration should motivate advances in other challenging crystallographic problems. As an example we have been able to build on this knowledge to determine the crystal structures of ferroelectric oxyfluorides K3MoO3F3 and Rb3MoO3F3 for the first time. This is an important breakthrough in understanding this large family of ferroelectric materials. 3. Our studies of Na1.5Ag1.5MoO3F3 and Na1.5Ag1.5WO3F3 are the first examples of oxyfluoride derivatives of the LiNbO3 structure. Crystallographic studies show that although Na+ and Ag+ are similar in size, the polar MO3F33– (M = Mo, W) units orient themselves so that the "hard" Na+ cation is surrounded exclusively by fluoride ions while the "soft" Ag+ cation is coordinated exclusively by oxide ions. This is an important result because it provides a strategy for design of new polar solids in the oxyfluoride family, based on the use of hard and soft cations to control orientation of polar oxyfluoride building units. 4. In the oxynitride perovskite family we have shown that octahedral tilting can be used to control the position of the conduction band edge, while the oxygen-to-nitrogen ratio can be used to control the position of the valance band edge. These are critically important findings for understanding how to designing new photocatalytic materials. This grant has supported the research of two students to complete their PhDs in chemistry and another two students to complete their MS degrees in chemistry. All are now making positive contributions to society through the skills and knowledge that were enabled by this grant. One undergraduate student was supported who has now entered graduate school. Two laboratory experiments for use in teaching 1st year general chemistry courses were developed and have been implemented at Ohio State University. These labs expose students to concepts in materials chemistry and crystallography that they would not otherwise see in general chemistry. Last year alone over 1000 students went through the new labs.
