Single Crystal and Magnetic Characterization of Iodobarlowite Cu ₄ (OH) ₆ FI: A Kagomé Lattice Synthesized from a 2-D Triangular Precursor

A single-step synthetic method to produce single crystals of the barlowite structural family is disclosed with iodobarlowite as a focus. The kagomé lattice of iodobarlowite, Cu4(OH)6FI, is furnished by removal of 1/4 of the triangular lattice sites of iodobotallackite, Cu2(OH)3I. Single-crystal structures have been solved at 100 and 300 K, both of which show I– vacancies within the lattice with no attendant Cu vacancies. X-ray absorption spectroscopy and diffraction studies reveal only the presence of Cu(II), no F– substitution at the I– site, and no Cu vacancies, suggesting that the charge balance to accommodate the I– deficiency occurs through loss of protons from the bridging hydroxide ligands during hydrothermal synthesis. Unlike microcrystalline powders, single-crystalline Cu4(OH)6FI exhibits a single magnetic transition at 15 K, which contrasts with the large Weiss constant of θ = −163(5) K, highlighting the key role of geometric spin frustration in the kagomé lattice to lower the critical temperature of the magnetically ordered state. Temperature-dependent susceptibility measurements indicate that the low temperature magnetic phase is associated with an uncompensated antiferromagnetic ordered state arising from the competing interactions between the four S = 1/2 Cu(II) spins of the iodobarlowite lattice.