  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.
