  Synthetic barlowite, Cu4(OH)6BrF, has emerged as a new quantum spin liquid
(QSL) host, containing kagome layers of S=1/2 Cu2+ ions separated by interlayer
Cu2+ ions. Similar to synthetic herbertsmithite, ZnCu3(OH)6Cl2, it has been
reported that Zn2+ substitution for the interlayer Cu2+ induces a QSL ground
state. Here we report a scalable synthesis of single crystals of Cu4(OH)6BrF.
Through x-ray, neutron, and electron diffraction measurements coupled with
magic angle spinning 19F and 1H NMR spectroscopy, we resolve the previously
reported positional disorder of the interlayer Cu2+ ions and find that the
structure is best described in the orthorhombic space group, Cmcm, with lattice
parameters a = 6.665(13) A, b = 11.521(2) A, c = 9.256(18) A and an ordered
arrangement of interlayer Cu2+ ions. Infrared spectroscopy measurements of the
O-H and F-H stretching frequencies demonstrate that the orthorhombic symmetry
persists upon substitution of Zn2+ for Cu2+. Specific heat and magnetic
susceptibility measurements of Zn-substituted barlowite, ZnxCu4-x(OH)6BrF,
reveal striking similarities with the behavior of ZnxCu4-x(OH)6Cl2. These
parallels imply universal behavior of copper kagome lattices even in the
presence of small symmetry breaking distortions. Thus synthetic barlowite
demonstrates universality of the physics of synthetic Cu2+ kagome minerals and
furthers the development of real QSL states.
