Electronic and Structural Origin of Ultraincompressibility of 5d Transition-Metal Diborides MB2 (M=W, Re, Os)

First-principles theory was used to investigate the roles of bond topology and covalency in the phase stability and elastic strength of 5d transition-metal diborides, focusing on elements (M=W, Re, Os) that have among the lowest compressibilities of all metals. Among the phases studied, the ReB₂-type structure exhibits the largest incompressibility (c axis), comparable to that of diamond. This ReB₂ structure is predicted to be the ground-state phase for WB₂ and a pressure-induced phase (above 2.5 GPa) for OsB₂. Both strong covalency and a zigzag topology of interconnected bonds underlie these ultraincompressibilities. Interestingly, the Vickers hardness of WB₂ is estimated to be similar to that of superhard ReB₂.