Order–disorder transition in the S = ½ kagome antiferromagnets claringbullite and barlowite

The transition in the quantum magnets barlowite, Cu₄(OH)₆FBr, and claringbullite, Cu₄(OH)₆FCl is of an order–disorder type, where at ambient temperature interlayer Cu²⁺ ions are dynamically disordered over three equivalent positions. The disorder becomes static as the temperature is decreased, resulting in a lowering of symmetry. Ab initio density functional theory calculations explain this structural phase transition and provide insights regarding the differences between these two materials.