New S=1/2 Kagome antiferromagnets A2Cu3SnF12: A=Cs and Rb

We synthesized single crystals of the new hexagonal compounds A2Cu3SnF12 with A=Cs and Rb, and investigated their magnetic properties. These compounds are composed of Kagome layers of corner-sharing CuF6-octahedra. Cs2Cu3SnF12 has the proper Kagome layer at room temperature, and undergoes structural phase transition at Tt ≅ 185 K. The temperature dependence of the magnetic susceptibility in Cs2Cu3SnF12 agrees well with the result of the numerical calculation for S = 1/2 two-dimensional Heisenberg Kagome antiferromagnet down to Tt with the nearest exchange interaction J/kB ≅ 240 K. Although the magnetic susceptibility deviates from the calculated result below T < Tt, the rounded maxima were observed at approximately T ≅ (1/6)J/kB as predicted by the theory. Cs2Cu3SnF12 undergoes three-dimensional magnetic ordering at TN = 20 K. Rb2Cu3SnF12 has the Kagome layer, whose unit cell is enlarged by 2a x 2a as compared with the proper Kagome layer even at room temperature. From the viewpoint of crystal structure, the exchange interactions between nearest neighbor Cu2+-ions are classified into four kinds. From the magnetic susceptibility and high-field magnetization measurements, it was found that the ground state is a disordered singlet with the spin gap, as predicted by recent theory. Exact diagonalization method with 12-site Kagome cluster was performed to analyze the magnetic susceptibility. By comparing the calculated results with the experimental data, the individual exchange interactions were evaluated.