  Rare-earth (RE) based honeycomb-lattice materials with strong spin-orbit
coupled Jeff=1/2 moments have attracted great interest as a platform to realize
Kitaev quantum spin liquid (QSL) state. Herein, we report the discovery of a
new family of RE based honeycomb-lattice magnets Ba9RE2(SiO4)6(RE=Ho-Yb), which
crystallize into the rhombohedral structure with space group R-3. In these
serial compounds, magnetic RE3+ ions are arranged on a perfect honeycomb
lattice within the ab-plane and stacked in the ABCABC-type fashion along the
c-axis. All Ba9RE2(SiO4)6(RE=Ho-Yb) polycrystals exhibit the dominant
antiferromagnetic interactions and absence of magnetic order down to 2 K. In
combination with the magnetization and electron spin resonance (ESR) results,
distinct anisotropic magnetic behaviors are proposed for compounds with
different RE ions. Moreover, the synthesized Ba9Yb2Si6O24 single crystals show
large magnetic frustration and no long-range magnetic ordering down to 0.15 K,
being a possible QSL candidate state. These serial compounds are attractive for
exploring the exotic magnetic phases of Kitaev materials with 4f electrons.
