Tadashi Ogitsu

Abstract:

Due to the unique bonding properties, sometime described by the location in the periodic table, elemental boron and boron compound exhibit unique structural, electronic and optical properties. Although their unique properties could be a great advantage in the development of novel functional materials, the complexity of boron chemistry has rather been a challenge for such attempt. Nevertheless, recent advancement in both computational approaches and experimental characterization techniques has contributed significantly to our understanding of these materials. At the presentation, we will first review the progresses in understanding the boron allotropy where theoretical studies using high performance computing have provided very important insights as to how unique boron chemistry lead to the peculiar enthalpic stabilization via introduction of macroscopic amount of disordered defects in I²-rhombohedral boron. These recent theoretical studies also provided explanations for many unusual behaviors of boron allotropes reported over several decades, leading to more comprehensive understanding of elemental boron. For example, it is now suggested that another allotrope boron, T-phase, might also be stabilized by defects, without which, observation of T-50 phase could be rationalized as a thermodynamically stable phase. Boron compounds are also known to exhibit unique and complex properties inheriting the complex boron chemistry. For example, it has been known that boron carbide, known as a lightweight superhard material, exhibit unique mechanical properties such as too low Hugoniot elastic limit compared to the predicted value from its hardness. We will discuss the thermodynamic stability of boron compounds and possible role of kinetics that might be relevant in precise understanding of mechanical properties of boron carbide. We hope that further development of accurate and comprehensive picture on the properties of boron and its compound will lead to rational design of novel functional materials that serve our future. The work performed under the auspices of the U.S. DOE by LLNL under Contract No. DE-AC52-07NA27344