Strong magnetic frustration in Y3Cu9(OH)19Cl8: a distorted kagome antiferromagnet

We present the crystal structure and magnetic properties of Y3Cu9(OH)(19)Cl-8, a stoichiometric frustrated quantum spin system with slightly distorted kagome layers. Single crystals of Y3Cu9(OH)(19)Cl-8 were grown under hydrothermal conditions. The structure was determined from single crystal X-ray diffraction and confirmed by neutron powder diffraction. The observed structure reveals two different Cu-positions leading to a slightly distorted kagome layer in contrast to the closely related YCu3(OH)(6)Cl-3. Curie-Weiss behavior at high-temperatures with a Weiss-temperature theta(W) of the order of -100 K, shows a large dominant antiferromagnetic coupling within the kagome planes. Specific-heat and magnetization measurements on single crystals reveal an antiferromagnetic transition at T-N = 2.2 K indicating a pronounced frustration parameter of theta(W)/T-N approximate to 50. Optical transmission experiments on powder samples and single crystals confirm the structural findings. Specific-heat measurements on YCu3(OH)(6)Cl-3 down to 0.4 K confirm the proposed quantum spin-liquid state of that system. Therefore, the two Y-Cu-OH-Cl compounds present a unique setting to investigate closely related structures with a spin-liquid state and a strongly frustrated AFM ordered state, by slightly releasing the frustration in a kagome lattice.