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Abstract

Temperature-and field-dependent H-1-, F-19-, and Br-79,Br-81-NMR measurements together with zero -field Br-79,Br-81-NQR measurements on polycrystalline samples of barlowite, Cu-4(OH)(6)FBr are conducted to study the magnetism and possible structural distortions on a microscopic level. The temperature dependence of the Br-79,Br-81-NMR spin-lattice relaxation rates 1/T-1 indicate a phase transition at T-N similar to 15 K which is of magnetic origin, but with an unusually weak slowing down of fluctuations below T-N. Moreover, 1/T1T scales linear with the bulk susceptibility which indicates persisting spin fluctuations down to 2 K. Quadupolare resonance (NQR) studies reveal a pair of zero-field NQR-lines associated with the two isotopes of Br with the nuclear spins of I = 3/2. Quadrupole coupling constants of v(Q) similar or equal to 28.5 MHz and 24.7 MHz for Br-79- and Br-81-nuclei are determined from Br-NMR and the asymmetry parameter of the electric field gradient was estimated to eta similar or equal to 0.2. The Br-NQR lines are consistent with our findings from Br-NMR and they are relatively broad, even above T-N. This broadening and the relative large eta value suggests a symmetry reduction at the Br-site reflecting the presence of a local distortion in the lattice. Our density-functional calculations show that the displacements of Cu2 atoms located between the kagome planes do not account for this relatively large eta. On the other hand, full structural relaxation, including the deformation of kagome planes, leads to a better agreement with the experiment.