Structural Phase Transitions in the S=1/2 Kagome Antiferromagnets Barlowite and Claringbullite*

Barlowite (Cu4(OH)6FBr) and claringbullite (Cu4(OH)6FCl) are atacamite minerals related to the quantum spin liquid (QSL) candidate herbertsmithite ZnCu3(OH)6Cl2, the popular S = ½ antiferromagnet with a geometrically perfect kagome lattice. The kagome lattices of claringbullite and barlowite are stacked perfectly on top of one another and show promise as QSL candidates. Experiments suggest that at room temperature both materials have a hexagonal crystal structure with P6₃/mmc symmetry and undergo temperature-dependent phase transitions. Ab-initio density functional theory calculations were performed to explain these structural phase transitions. The transitions are likely influenced by the presence of defects, so it is expected that the transition temperature varies from sample to sample and will be different for crystals grown by different methods.