Symmetry breaking and structural phase transition in frustrated quantum kagome antiferromagnet barlowite via pressure tuning strategy

As promising members of the two-dimensional kagome lattice materials, the copper-based compounds of the atacamite family have attracted much attention due to their unique structures and abundant properties. In this work, research efforts were focused on exploring the structural, optical and magnetic properties, as well as the compression behavior, of barlowite, Cu₄(OH)₆FBr, which possesses a geometrically perfect kagome lattice of Cu²⁺ cations. The powder samples of barlowite were synthesized via a hydrothermal strategy. They exhibited a canted antiferromagnetic behavior below 15 K, above which a magnetic transition to paramagnetism was observed. The high pressure experimental technique was employed to investigate the structural evolution of barlowite samples. The observed stepwise structural transformation from the initial hexagonal to the intermediate orthorhombic and the eventual monoclinic phase, is correlated with a continuous variation of the interlayer Cu occupation, the halide sizes, and the distortion of the kagome lattice within the system under pressure. The phase transition mechanism was also closely related with the combined contributions of an increase in external pressure and the cooperative intrinsic Jahn-Teller distortion. These findings advance the understanding of pressure-induced structural evolution and lattice deformation in barlowite and could be applied to other copper-based quantum kagome antiferromagnet materials.