Geometrically frustrated spin systems continue to serve as a fertile ground for realizing nontrivial quantum states, making the exploration of new materials in this category a central pursuit. In this work, we report the synthesis and investigation of a new family of antiferromagnets, $\mathrm{LiBa}{\mathrm{RE}}_2{\left(\mathrm{B}{\mathrm{O}}_3\right)}_3$ (RE = Pr, Nd, Sm–Tb). These compounds crystallize in the trigonal $P\overline{3}m1$ space group, where neighboring $\mathrm{RE}{\mathrm{O}}_9$ polyhedra share faces to form spatially confined ${\mathrm{RE}}_6$ spin hexamers. These hexamers interconnect within the $ab$ plane to construct a two-dimensional frustrated triangular spin-hexamer lattice, featuring hierarchical spin interactions with strong intracluster and weak intercluster couplings. Magnetic susceptibility measurements down to 1.8 K reveal no signs of long-range magnetic order or spin freezing across the entire series, despite the presence of dominant antiferromagnetic interactions. Further magnetization studies on a representative member $\mathrm{LiBaG}{\mathrm{d}}_2{\left(\mathrm{B}{\mathrm{O}}_3\right)}_3$ down to 0.4 K uncover a long-range antiferromagnetic transition at ∼0.91 K. Quasi-adiabatic demagnetization experiments on this material demonstrate outstanding cooling effect, reaching a pronounced temperature minimum at ∼146 mK near the antiferromagnetic critical field ${\mu }_0{H}_c\sim 0.75$ T. Notably, this temperature valley is markedly broadened over an extended field interval, indicating the presence of a quantum critical state near ${\mu }_0{H}_c$ with persistent spin fluctuations. Altogether, $\mathrm{LiBa}{\mathrm{RE}}_2{\left(\mathrm{B}{\mathrm{O}}_3\right)}_3$ represents a versatile platform of frustrated spin clusters, offering new possibilities for exploring unconventional magnetic ground states and potential cryogenic cooling applications.

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• Magnetic phase transitions
• Magnetic susceptibility
• Magnetism