Modulating Hardness in Sc₂(Ru_5-xTM_x)B₄ Through Empirical Considerations and Computational Analysis

Ternary and higher-order borides remain an underexplored area in the search for hard materials. The difficulties associated with purely systematic experimental investigations have largely hindered the consideration of such complex phases. Here, traditional design rules are merged with computation-based methods in an effort to direct synthetic efforts, addressing this challenge. Ternary transition metal borides, like Sc₂Ru₅B₄, were selected to demonstrate this approach. The target phases were first prepared using arc melting, and the crystal structures were resolved with single crystal X-ray diffraction. Vickers microhardness indentation revealed that these compounds are hard materials, and the mechanical properties can be enhanced by substituting denser, isoelectronic transition metals into the original structures following an empirical understanding of hardness in borides. Analyzing the ensuing density of states (DOS) curves indicates that the Fermi levels nevertheless fall in unfavorable positions requiring a change in the electron count to optimize the electronic structures. The subsequent synthesis of the transition metal substituted compounds alters the electron count moving the Fermi levels from the peaks. At the same time, Vickers indentation measurements, combined with DFT-level stress-strain calculations and a bonding analysis, show shifting the Fermi level reduces the occupation of antibonding interactions, which also increases the hardness. These data suggest that electron density, Fermi level position, and chemical bonding are all essential markers when developing high hardness materials.