  The recent discovery of superconductivity up to 32 K in the pressurized MoB2
reignites the interests in exploring high-Tc superconductors in
transition-metal diborides. Inspired by that work, we turn our attention to the
5d transition-metal diborides. Here we systematically investigate the responses
of both structural and physical properties of WB2 and ReB2 to external
pressure, which possess different types of boron layers. Similar to MoB2, the
pressure-induced superconductivity was also observed in WB2 above 60 GPa with a
maximum Tc of 15 K at 100 GPa, while no superconductivity was detected in ReB2
in this pressure range. Interestingly, the structures at ambient pressure for
both WB2 and ReB2 persist to high pressure without structural phase
transitions. Theoretical calculations suggest that the ratio of flat boron
layers in this class of transition-metal diborides may be crucial for the
appearance of high Tc. The combined theoretical and experimental results
highlight the effect of geometry of boron layers on superconductivity and shed
light on the exploration of novel high-Tc superconductors in borides.
