  A new medium-entropy diboride (MEDB) (W1/3Re1/3Ru1/3)B2 has been synthesized
by spark plasma sintering of elemental powders at 1600 oC. Despite the
dissimilar structures of WB2, ReB2 and RuB2, the sintered MEDB consists of a
single hexagonal ReB2-type phase (space group P63/mmc) with a relative density
of 94.2% and an average grain size of 6.8 $\pm$ 2.2 um. Structural refinement
and electron microscopy measurements show that the W, Re, and Ru atoms occupy
the same crystallographic site and are distributed uniformly in the lattice.
The (W1/3Re1/3Ru1/3)B2 MEDB has Vickers hardnesses of 30.7 GPa at a load of
0.49 N and 20.5 GPa at a load of 9.8 N, which are comparable or higher than
those reported for individual binary counterparts.
