Solid State Chemistry - Crystallography - Chemical Bonding Theory - Materials
The focus of our research is the elucidation of the chemical principles underlying the structures of the solid state compounds that form uponing alloying metals together: intermetallic compounds. In 1923, Linus Pauling's X-ray diffraction examinations of NaCd2 revealed intermetallics to be a realm of incredible complexity. While Na(s) and Cd(s) both form in crystal structures typical of metals (body-centered cubic and hexagonal close-packed lattices, respectively, each with just 2 atoms per unit cell), mixing them in a simple 1:2 ratio yields a giant 31 " cubic unit cell, containing more than 1000 atoms (top, right). Extensive families of intermetallic compounds with rivalling complexity have since been discovered, and as X-ray diffraction technology and crystallographic methods continue to advance, we continue to find new layers of intricacy in the structures of intermetallics. Examples include the Nowotny chimney ladder phases, a family of compounds formed from the threading of helices of Sn, Ga, or other main group elements through the insides of transition metal helices; and the icosahedral quasicrystals, such as YbCd5.7, whose structures are perhaps most easily comprehended via models in 6D space.
Our aim is to reveal the chemical origins of these beautiful structures, with the ultimate goal of gaining some degree of synthetic control of this structural diversity. With this knowledge in hand, we hope to use the atomic structures of these phases as parameters for the optimization of a variety materials properties important for energy technology, including superconductivity, thermoelectricity, and catalysis.
In our research we combine quantum mechanical calculations with solid state synthesis and advanced crystallographic methods. Students working in our group can adjust the balance between these theoretical and experimental components to best suit their interests and goals. Below you may find each of these aspects of our work described in more detail.
Theoretical investigation of intermetallics
Empirical observations and earlier quantum mechanical calculations have drawn intriguing connections between intermetallic phases and molecular chemistry. Electron-counting rules, atomic size effects and electronegativity differences all appear to be at work in these compounds. We are exploring these connections, using electronic structure calculations--ranging from the orbital-based extended H"ckel method to density functional calculations--to build theoretical schemes for understanding the chemical driving forces behind the structures of intermetallics. To this end, we're seeking new ways of extracting chemical stories from the vast arrays of numbers resulting from electronic structures calculations.
Our theoretical efforts are currently directed toward exploring a common theme that has emerging over the course of calculations of several families of structures: complex structures are built at the electronic level from fragments or slabs of simpler ones fused together in new ways. The NaCd2 structure is a shimmering example of this. Model calculations reveal it to consist of nanometer-sized blocks matching the much simpler structure of MgCu2. NaCd2 is, in essense, a crystal of nanometer-sized crystallites of the MgCu2 structure (below). We're working now to developing this theme of complexity as a perturbation on simple periodicity into a predictive theoretical framework.
Structure solution and description of complex alloy phases
The synthesis and structure determination of new intermetallic structures provides both input for our theoretical calculations and mode of expression of our new conceptual understanding of their bonding. Synthesizing these beautiful structures follows standard routes of solid state synthesis, often requiring little more than cooling of a molten mixture of metals.
Their structure determinations, on the other hand, pose exciting crystallographic problems: Superstructures, incommensurate modulations, twinning, and quasicrystalline order are just some of the issues we face in revealing the atomic arrangements in these compounds. To face these challenges, we employ state-of-the-art crystallographic methods, such as superspace modeling, in which complex structures are viewed as cross-sections of simpler structures in 4 or more dimensions. One project in this area is the structure determination of dodecagonal quasicrystals. A hint of the geometrical features within these structures is illustrated below with the approximant structure solved previously by Shoemaker and Shoemaker.
Optimization of materials properties through structural variation
In addtion to their often breath-taking structures, intermetallics are attractive for their materials properties. Complex magnetic ordering, superconductivity, thermoelectricity, and hydrogen storage are examples of the properties observed in this family of compounds. Unfortunately, the discovery of new materials exhibiting one of these phenomena is largely a matter of serendipity. This means that once one has exhausted the
possiblities for optimizing an existing material through elemental substitions and additives, one must start from scratch in the search for materials with improved properties. One long-term goal of our work is to examine how structural variations in intermetallics may supplement compositional tuning. In mastering the synthetic control of geometry, we hope to introduce an new variable--the structure--in the optimization of materials properties. One of our first ventures into this will be attempts to introduce structural variations into the superconductor Nb3Ge.