Chemical bonding in complex metallic alloys
Final Report Abstract
Complex metallic alloys (CMA) have emerged as a new research field in materials science. Understanding of the relationship between the atomic interactions and the bonding-related physical properties of these intermetallic phases with complex crystal structures is the aim of this project. Chemical bonding situations in crystalline solids can be investigated in real space by applying topological analysis techniques to the electron density and electron localizability indicator. Two groups of compounds were considered: Ba8TxGe46-x clathrate-I phases and Al-rich Al-Co binary compounds. In the former, T is either a main group element, Li, Mg, Al, Ga, Ge, or a late transition metal element from groups 9 through 12. The complexity in this group stems from the mixed occupancy of a host site due to T and Ge atoms and presence of vacancies. Since the nature of atomic interactions is essentially local in real space, the fully ordered models with composition Ba8T6Ge40 can be employed. Likewise, the Al-Co binary compounds Al5Co2, Al13Co4 and Al9Co2 are approximants of the quasicrystalline decagonal phases with rather small unit cells but having local environments similar to those in decagonal quasicrystals. The T atoms in the Ba8T6Ge40 type-I clathrates form two-center bonds with their Ge near neighbors. The polarity of these bonds is found to increase with increasing electronegativity difference for main group elements. However, for the case of late transition metals, bond polarity-electronegativity difference relationship is not as expected. Bond polarity does not change for the 3d elements considered, and it decreases as electronegativity difference increases for the 4d and 5d cases. Another surprising finding is the presence of highly polar two-center T-Ba2 bonds for T = Ag, Cd, Pt and Au. The implications of these interactions for physical properties such as lattice thermal conductivity require further studies. The nature of chemical bonding situation changes as we go through Al5Co2, Al13Co4 and Al9Co2 with increasing valence electron concentrations. Two-center bonds are regarded as indicating covalent interactions while multi-center bonds imply metallic interactions. Al5Co2 has two- and threedimensional substructures each bonded by effectively twocenter interactions. These substructures and an isolated Al type interact through multi-center bonds. In Al13Co4 multicenter bonds are dominant, but the two-center bonds found in the nearly linear Co-Al-Co groups seem to play an important role in various physical properties. In the electron richest Al9Co2 most of the valence electrons participate in multicenter bonds.
Publications
- “Chemical Bonding in Al5Co2: The Electron Localizability – Electron Density Approach“, Israel Jour. Chem., 51 (2011) 1349-1354
A. Ormeci, Yu. Grin
- “Complex Metallic Phases in Catalysis”, in Complex Metallic Alloys: Fundamentals and Applications, Wiley-VCH, Weinheim, (2011), 385-400
M. Armbrüster, K, Kovnir, Yu. Grin, R. Schlögl
- “Solid State Chemistry of Clathrate Phases: Crystal Structure, Chemical Bonding and Preparation Routes”, in The Physics and Chemistry of Inorganic Clathrates, Springer Dordrecht, Heidelberg, New York, London, (2014), 35-64
M. Baitinger, B. Böhme, A. Ormeci, Yu. Grin
