Hexagonal antiferromagnets Cs2Cu3MF12 (M=Zr, Hf, and Sn) have uniform kagome lattices of Cu^2+ with S=1/2, whereas Rb2Cu3SnF12 has a 2a×2a enlarged cell as compared to the uniform kagome lattice. The crystal data of Cs2Cu3SnF12 synthesized in the present work are reported. We performed magnetic-susceptibility measurements on this family of kagome antiferromagnet using single crystals. In the Cs2Cu3MF12 systems, structural phase transitions were observed at Tt=225, 172, and 185 K for M=Zr, Hf, and Sn, respectively. The magnetic susceptibilities observed for T>Tt are almost perfectly described using theoretical results obtained by exact diagonalization for the 24-site kagome cluster with J/kB=244, 266, and 240 K, respectively. Magnetic ordering accompanied by the weak ferromagnetic moment occurs at TN=23.5, 24.5, and 20.0 K, respectively. The origins of the weak ferromagnetic moment should be ascribed to the lattice distortion that breaks the hexagonal symmetry of the exchange network for T<Tt and the Dzyaloshinsky-Moriya interaction. Rb2Cu3SnF12 is magnetically described as a modified kagome antiferromagnet with four types of neighboring exchange interactions. Neither structural nor magnetic phase transition was observed in Rb2Cu3SnF12. Its magnetic ground state was found to be a spin singlet with a triplet gap. Using exact diagonalization for a 12-site kagome cluster, we analyzed the magnetic susceptibility and evaluated individual exchange interactions. The causes leading to the different ground states in Cs2Cu3SnF12 and Rb2Cu3SnF12 are discussed.
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