This project is designed to develop a long-term international collaborative research and education effort among research teams at Illinois Institute of Technology (IIT), Indian Association for the Cultivation of Science (IACS) and S.N. Bose National Center for Basic Sciences, in Kolkota, India. The research focuses on investigating the influence of local structure on the physical properties of complex oxides. This class of materials exhibit many interesting phenomena, for example, coupled structural transitions and collective electronic effects such as metal-insulator transitions, phase separation at low temperatures, and multiferroic behavior. This research collaboration intends to address the important issue of sample quality, which often leads to contradictory results; controlling sample quality is important to understand the manifestly different physical behavior. The specific objectives of the project include: synthesis and characterization of high quality samples of these complex oxides in both the US and Indian laboratories; determination of the electronic properties using photoelectron spectroscopies; measurement of local structure using x-ray diffraction and absorption spectroscopy with synchrotron radiation at the Advanced Photon Source at Argonne National Laboratory; and generation of high impact scientific and technological results along with an understanding of many-body interactions using theoretical tools. The results of these studies are expected to resolve several outstanding questions in this field and contribute to a better understanding of the fundamental properties of these materials, which are promising candidates for potential applications in data manipulation and storage. Furthermore, the complementary expertise of the collaborating research groups provides an ideal opportunity to train graduate students and junior researchers through extended visits to the collaborating laboratories. The students and junior researchers who participate in these exchanges will gain technical expertise and will develop international collaborations, which will benefit their careers. Training gained by junior Indian researchers at the Materials Research Collaborative Access Team beamline of the Advanced Photon Source, will prepare them to take leadership roles in the development and use of the synchrotron source in India. Inclusion of summer IIT undergraduate student interns in the project is designed to provide hands-on scientific research experience in an international setting and expose them to career opportunities through graduate studies. This award is jointly funded by the Division of Materials Research in the Mathematical and Physical Sciences Directorate and the Office of International Science and Engineering ?Africa, Near East & South Asia Program.
The function of a material is substantially governed by its atomic structure and x-ray methods are fundamental tools for determining these structures. However, laboratory x-ray sources are only able to measure spatial and time-averaged structures which is insufficient for solid solutions and changing inhomogeneous mixtures of crystalline phases. The use of synchrotron radiation techniques such as x-ray absorption spectroscopy (XAS) and time-resolved x-ray diffraction (t-XRD) can provide complementary information and lead to a more complete structural description of technologically valuable materials. Over the course of this project, the PI has disseminated information about the utility of synchrotron radiation experiments and XAS in particular through an online graduate course on synchrotron radiation offered on a regular basis by Illinois Institute of Technology (IIT) not only for its own students but also online at a reduced cost for international students. Students taking this course have later written General User Proposals to the Advanced Photon Source (APS) and have come to the PI's laboratory on extended scientific visits to collect data and perform analysis. In addition the PI has developed a mini-course on XAS analysis which has been offered to students and researchers who have come to collect data at the APS. The project has supported collaborations with research groups in India (D.D. Sarma - Indian Institute of Science) and Italy (M. Strumendo - University of Padova). In all, 4 students from these two laboratories have come to IIT and to perform experiments at the APS and analyze the data under the PIs supervision and 2 IIT students have made extended visits to the IISC in India These students have developed long-lasting collaborations with their peers in Italy and India and there is a continuing collaboration among the laboratories which will be supported by other funding. At the time of report submission, the project has produced 3 published journal articles (in print or online), has had two additional manuscripts accepted for publication and 2 Ph.D. dissertations (one from IIT and the other at Padova) submitted. As examples of the kind of insight that has been obtained by using synchrotron methods, the attached figures present two cases where XAS can provide useful complementary information. Figure 1 is data from the Cd1-xZnxS system where we show that while the x-ray diffraction patterns exhibit a linear change (Vegard's Law) as a function of composition, the metal-sulfur bonds are independent of concentration and dependent on the ionic radii of the participants in the bond. The metal-metal distances are already less differentiated and by the time we look at the third coordinatoin shell, Vegard's law is recovered. Using theoretical modeling in addition to the EXAFS data, we have shown that the local strain in these structures is relieved by rotation of the rigid MS4 tetrahedra. Figure 2 shows the monoclinic ordered vacancy structure of TiO1-x. When measured by EXAFS, however, the mnominal monoclinic structure is clearly not a good model for the experimental data. Allowing the Ti-Ti distances to shift by small amounts improves the fit significantly, indicating that the vacancies have a distorting effect on the local structure not observable by x-ray diffraction.
