Ba 9 RE 2 (SiO 4 ) 6 (RE = Ho-Yb): A Family of Rare-Earth-Based Honeycomb-Lattice Magnets.

Rare-earth (RE)-based honeycomb-lattice materials with strong spin-orbit coupled J eff = 1/2 moments have attracted great interest as a platform to realize the Kitaev quantum spin liquid (QSL) state. Herein, we report the discovery of a family of RE-based honeycomb-lattice magnets Ba 9 RE 2 (SiO 4 ) 6 (RE = Ho-Yb), which crystallize into the rhombohedral structure with the space group R 3̅. In these serial compounds, magnetic RE 3+ ions are arranged on a perfect honeycomb lattice within the ab -plane and stacked in the "ABCABC"-type fashion along the c -axis. All synthesized Ba 9 RE 2 (SiO 4 ) 6 (RE = Ho-Yb) polycrystals exhibit the dominant antiferromagnetic interaction and absence of magnetic order down to 2 K. In combination with the magnetization and electron spin resonance results, magnetic behaviors are discussed for the compounds with different RE ions. Moreover, the as-grown Ba 9 Yb 2 (SiO 4 ) 6 single crystals show large magnetic frustration with frustration index f = θ CW / T N > 8 and no long-range magnetic ordering down to 0.15 K, being a possible QSL candidate material. These series of compounds are attractive for exploring the exotic magnetic phases of Kitaev materials with 4f electrons.