The deformed pyrochlore lattice compound clinoatacamite, Cu 2 (OH) 3 Cl, shows intriguing frustrated magnetism and 1/4 substitution of Cu with nonmagnetic Zn leads to the two-dimensional kagome lattice ZnCu 3 Cl 2 (OH) 6 that exhibits spin liquid behaviour. These findings renewed interest in botallackite Cu 2 (OH) 3 Cl, which is a polymorphous structure of clinoatacamite Cu 2 (OH) 3 Cl possessing a two-dimensional triangular lattice. The present work investigates the effect of halogen ions on the magnetic transitions in the botallackite structure. Similar to the antiferromagnetic Cu 2 (OH) 3 Cl (T N =7.2 K), Cu 2 (OH) 3 Br and Cu 2 (OH) 3 I both showed antiferromagnetic transitions at T N =10 and 14 K, respectively. Long-range order also exists in Cu 2 (OH) 3 Br as exemplified by the μSR study. The experimental results suggest that the magnetic coupling on the triangular lattice is dominated by the super-exchange interaction through the halogen ions. Further detailed comparison studies on these botallackite-structure compounds are expected to clarify the spin configuration on this triangular lattice.
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