With the potential to surpass diamond in utility as cutting tools, ultra hard metal borides have attracted much synthetic and theoretical attention. To highlight the comparison, diamond cannot be easily machined and is expensive, whereas metal-borides can be quickly machined with electric discharge and can be made of inexpensive materials. Theoretically, these materials pose a challenge. One would like to have a set of bonding principles guiding synthetic efforts towards harder materials. However, hardness is difficult to define microscopically. It is defined by an experiment, and at its core is a response to distortion. Thus, hardness should not be fully understandable with static bonding theories. How a bond changes upon mechanic deformation becomes just as important as its equilibrium properties. This necessitates bring dynamical effects into our description of bonds. We start our description of metal-borides with small molecular clusters containing key interactions from the bulk. Insights from these “minimal clusters” are taken in conjunction with experiment and periodic calculations and mapped onto the bulk. We start by examining two phases of WB and demonstrating that, contrary to previous theories, metal-boron bonds are critical for understanding the materials anisotropic strength. We then take this mindset and revisit three well-known diborides: ReB2 OsB2 and TiB2. All are layered structures with corrugated or flat boron sheets sandwiching the metal. Using our minimal cluster approach we show why only Re leads to ultra-hardness. It interacts strongly enough with boron to form a fortifying backbond, but not strongly enough as to allow for electrons to escape the boron upon deformation. We further apply these bonding principles to address questions of mixed valency and defect formation.

Dynamical bonding in metal borides: Ultra-hardness and mixed valency

256th National Meeting (2018)

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