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.