New S=1/2 Kagome Antiferromagnets A${}_2$Cu${}_3$SnF${}_{12}$: A=Cs and Rb

We synthesized single crystals of the new hexagonal compounds A${}_2$Cu${}_3$SnF${}_{12}$ with A=Cs and Rb, and investigated their magnetic properties. These compounds are composed of Kagom'{e} layers of corner-sharing CuF${}_6$-octahedra. Cs${}_2$Cu${}_3$SnF${}_{12}$ has the proper Kagom'{e} layer at room temperature, and undergoes structural phase transition at $T_{\mathrm{t}}\simeq 185$ K. The temperature dependence of the magnetic susceptibility in Cs${}_2$Cu${}_3$SnF${}_{12}$ agrees well with the result of the numerical calculation for $S=1\mathrm{/}2$ two-dimensional Heisenberg Kagom'{e} antiferromagnet down to $T_{\mathrm{t}}$ with the nearest exchange interaction $J\mathrm{/}{k}_{\mathrm{B}}\simeq 240$ K. Although the magnetic susceptibility deviates from the calculated result below $T<T_{\mathrm{t}}$, the rounded maxima were observed at approximately $T\simeq (1\mathrm{/}6)J\mathrm{/}{k}_{\mathrm{B}}$ as predicted by the theory. Cs${}_2$Cu${}_3$SnF${}_{12}$ undergoes three-dimensional magnetic ordering at $T_{\mathrm{N}}=20$ K.