Abstract
A first-principles approach was used to investigate the roles of bond topology and
covalency in the phase stability and elastic strength of a series of 5d transition-metal
diborides, focusing on the three mid-row elements (W, Re, Os) with the compressibilities
among the lowest for metals. Among all of the phases studied, the c-axis of the ReB2-
type structure is shown to exhibit the largest incompressibility, comparable to that of
diamond. The ReB2-type structure is found to be the ground structure for WB2 and ReB2
but a pressure-induced phase (above 2.5 GPa) for OsB2. The origin of the ultraincompressibility
correlates not only with strong covalency, but also with the zigzag
topology of interconnected covalent bonds. Interestingly, the Vickers hardness of WB2 is
estimated to be similar to that of superhard ReB2.