  Six high-entropy rare earth tetraborides of the tetragonal UB4-prototyped
structure have been successfully synthesized for the first time. The specimens
are prepared from elemental precursors via high-energy ball mill and in-situ
reactive spark plasma sintering. The sintered specimens are >98% in relative
densities without detectable oxide impurities (albeit the presence of minor
hexaborides in some compositions). No detectable secondary phase is observed in
the composition (Y$_{0.2}$Nd$_{0.2}$Sm$_{0.2}$Gd$_{0.2}$Tb$_{0.2}$)B$_{4}$,
which is proven homogeneous at both microscale and nanoscale. The Vickers
microhardness are determined to be ~13-15 GPa at a standard indentation load of
9.8 N. A scientifically interesting observation is represented by the
anisotropic lattice distortion from the rule-of-mixture averages. This work
expands the family of high-entropy ceramics via fabricating a new class of
high-entropy borides with a unique tetragonal quasi-layered crystal structure.
