Ongoing Projects
 
Summary The work in the Bobev research group addresses important issues related to the understanding of the relationships among the composition, structure, and electronic structure in complex intermetallic compounds and their properties. Our efforts are currently focused in three research directions, all of which cut across the traditional boundaries of the science disciplines – synthetic inorganic chemistry, theoretical chemistry, condensed-matter physics, X-ray crystallography, materials science, etc.  The first project (Structural relationships and magnetism in compounds of the lanthanide elements) is aimed at studying the fundamental chemistry and physics of new intermetallic compounds that can be dubbed for short “magnetic materials”. They have implications that stretch far beyond magnetism and into superconductivity, and although much research has already been done in this field, still surprisingly little is known about the basic principles, which make magnetic and superconductive materials behave as such. With this motivation in mind, our general long-term goal here is the discovery of classes of new lanthanide-based inter-metallic compounds with unusual magnetic and electronic properties. We seek to develop a rationale for finding new magnetic materials and for the effective optimization of existing ones that is not based empirical tenets, but instead, relies on understanding the principles governing the structures and properties. These activities are supported by a NSF CAREER award. The second project in the Bobev group is driven by two of the greatest challenges of our time – energy beyond fossil fuels and environment. We embarked on this new endeavor, Novel compounds for thermoelectric applications, recognizing that the technologies based on thermoelectricity are environmentally benign and have the potential for widespread applications; however, they are not yet part of the everyday life because of their low efficiency. This limitation, in essence, is a result of the unsatisfactory properties of almost all currently available materials. From the standpoint of synthetic chemists, we see both a societal need and a scientific opportunity to contribute to this area by synthesizing fundamentally new materials with higher thermoelectric efficiency. We believe a breakthrough can be achieved by bringing together (in one material) the desirable heat and charge transport properties of semiconductors and the magnetism and the correlated electron behavior, which are signatures of the d- or f-elements. Our approach builds upon rationally designed syntheses, coupled with thorough and systematic structural studies and property measurements. These research efforts are supported by an ACS-PRF grant. The last and most recent project in our laboratory (Crystal chemistry and physical properties of new Si, Ge and Sn clathrates) is funded by DOE. Herein, we seek to develop the fundamental chemistry and physics of new intermetallic clathrates (open framework materials) that can find applications in thermoelectric energy conversion. Not long ago, complex inorganic materials with open framework structures, such as the skutterudites and the clathrates have been suggested to be of great importance to this field. The basic hypothesis for the thermoelectric properties of such structures is that the conducting host-framework provides a high Seebeck coefficient (S) and electrical conductivity (σ), while low-lying optical modes, associated with the vibration of the guest-atoms, hybridize with the acoustic modes. The thermal conductivity (κ) is thus lowered significantly due to the resonant scattering of the heat carrying phonons. Together, this leads to high values of the thermoelectric figure of merit ZT=TS2σ/κ. Our efforts focus on the structural characterization, including description of the lattice dynamics of the system, which is essential for understanding the thermoelectric properties, and also for any rational design of new materials.