Title: Strong magnetic frustration in Y$_{3}$Cu$_{9}$(OH)$_{19}$Cl$_{8}$: a distorted kagome antiferromagnet,
Abstract: We present the crystal structure and magnetic properties of
Y$_{3}$Cu$_{9}$(OH)$_{19}$Cl$_{8}$, a stoichiometric frustrated quantum spin
system with slightly distorted kagome layers. Single crystals of
Y$_{3}$Cu$_{9}$(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 distored kagome layer in contrast
to the closely related YCu$_{3}$(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 $\theta_{W}/T_{N}\approx50$. Optical transmission
experiments on powder samples and single crystals confirm the structural
findings. Specific-heat measurements on YCu$_{3}$(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.