  We report the synthesis and characterization of metal deficient
(Ti$_{0.2}$Zr$_{0.2}$Hf$_{0.2}$Nb$_{0.2}$Ta$_{0.2}$)$_{1-\delta}$B$_{2}$
high-entropy diborides (HEBs). A single homogeneous AlB$_{2}$-type phase is
successfully obtained over the $\delta$ range of 0.03 $\leq$ $\delta$ $\leq$
0.18. With increasing $\delta$, the unit-cell volume exhibits a nonmonotonic
variation with a maximum at $\delta$ = 0.07. These metal-deficient HEBs possess
high Vickers hardness of 16.6-18.9 GPa at a load of 9.8 N and their phase
stability is attributed to the increased mixing entropy. Our results not only
present the first series of metal-deficient AlB$_{2}$-type HEBs, but also
suggest the existence of similar multicomponent diborides.
