Order-disorder transition in the S=1/2 kagome antiferromagnets claringbullite and barlowite

The transition in the quantum magnets barlowite, Cu-4(OH)(6)FBr, and claringbullite, Cu-4(OH)(6)FCl 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.