  For the first time, a group of CaB6-typed cubic rare earth high-entropy
hexaborides have been successfully fabricated into dense bulk pellets (>98.5%
in relative densities). The specimens are prepared from elemental precursors
via in-situ metal-boron reactive spark plasma sintering. The sintered bulk
pellets are determined to be single-phase without any detectable oxides or
other secondary phases. The homogenous elemental distributions have been
confirmed at both microscale and nanoscale. The Vickers microhardness are
measured to be 16-18 GPa at a standard indentation load of 9.8 N. The
nanoindentation hardness and Young's moduli have been measured to be 19-22 GPa
and 190-250 GPa, respectively, by nanoindentation test using a maximum load of
500 mN. The material work functions are determined to be 3.7-4.0 eV by
ultraviolet photoelectron spectroscopy characterizations, which are
significantly higher than that of LaB6.
