When the bonds of a quantum magnet are modulated with a periodic pattern, exotic quantum ground states may emerge. Newly synthesized crystalline barlowite (Cu 4 (OH) 6 FBr) and Zn-substituted barlowite demonstrate the delicate interplay between singlet states and spin order on the spin-1 2 kagome lattice. Our new variant of barlowite maintains hexagonal symmetry at low temperatures with an arrangement of distorted and undistorted kagome triangles, for which numerical simulations predict a pinwheel valence bond crystal (VBC) state instead of a quantum spin liquid (QSL). The presence of interlayer spins eventually leads to novel pinwheel q=0 magnetic order. Partially Zn-substituted barlowite (Cu 3.44 Zn 0.56 (OH) 6 FBr) has an ideal kagome lattice and shows QSL behavior, demonstrating the robustness of the QSL against local impurities. This system is a unique playground displaying QSL, VBC, and spin order, furthering our understanding of these highly competitive quantum states.Identifying the ground state for interacting quantum spins on the kagome lattice is an important unresolved question in condensed matter physics owing to the great difficulty in selecting amongst competing states that are very close in energy. Antiferromagnetic (AF) spins on this lattice are highly frustrated, and for spin-1 2 the ground state does not achieve magnetic order and is believed to be a quantum spin liquid (QSL). (1-9) The QSL is an unusual magnetic ground state, characterized by long-range quantum entanglement of the spins with the absence of long-range magnetic order. (10-13) The recent identification of herbertsmithite (Cu 3 Zn(OH) 6 Cl 2 ) (14-17) as a leading candidate QSL material has ignited intense interest in further understanding similar kagome materials.Often, real kagome materials have interactions that relieve the frustration and drive the moments to magnetically order. (18,19) In contrast, for the ideal S = 1 2 kagome Heisen-

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The mineral barlowite, Cu 4 (OH) 6 FBr, has been the focus of recent attention due to the possibility of substituting the interlayer Cu 2+ site with non-magnetic ions to develop new quantum spin liquid materials. We re-examine previous methods of synthesizing barlowite and describe a novel hydrothermal synthesis method that produces large single crystals of barlowite and Zn-substituted barlowite (Cu 3 Zn x Cu 1−x (OH) 6 FBr). The two synthesis techniques yield barlowite with indistinguishable crystal structures and spectroscopic properties at room temperature; however, the magnetic ordering temperatures differ by 4 K and the thermodynamic properties are clearly different. The dependence of properties upon synthetic conditions implies that the defect chemistry of barlowite and related materials is complex and significant. Zn-substituted barlowite exhibits a lack of magnetic order down to T = 2 K, characteristic of a quantum spin liquid, and we provide a synthetic route towards producing large crystals suitable for neutron scattering.