Advancements in the Search for Superhard Ultra-Incompressible Metal Borides

Dense transition metal borides have recently been identified as superhard materials that offer the possibility of ambient pressure synthesis compared to the conventional high pressure, high temperature approach. This feature article begins with a discussion of the relevant physical properties for this class of compounds, followed by a summary of the synthesis and properties of several transition metal borides. A strong emphasis is placed on correlating mechanical properties with electronic and atomic structure of these materials in an effort to better predict new superhard compounds. It concludes with a perspective of future research directions, highlighting some recent results and presenting several new ideas that remain to be tested. Recent work on the synthesis and characterization of hard and superhard transition metal borides containing, Ru, Os, W, and Re is summarized. Rhenium diboride (pictured in both powder and crystalline form) is discussed in detail, having received a great deal of interest for its exceptional hardness and incompressibility. The mechanical properties of several of these compounds are discussed and correlated with recent developments in superhard metal boride synthesis.