X-ray absorption investigation of the site occupancies of the copper element in nominal Cu₃Zn(OH)₆FBr

With Zn substitution to the three-dimensional antiferromagnetically ordered barlowite Cu₄(OH)₆FBr, Cu₃Zn(OH)₆FBr shows no magnetic phase transition down to 50 mK, and the system is suggested to be a two-dimensional kagomé quantum spin liquid Chin. Phys. Lett. 34 077502 (2017). A key issue to identify such phase diagram is the exact chemical formula of the substituted compound. With Cu L-edge x-ray absorption spectrum (XAS) combined with the MultiX XAS calculations, we evaluate the Cu concentration in a nominal Cu₃Zn(OH)₆FBr sample. Our results show that although the Cu concentration is 2.80, close to the expected value, there is 34% residual Cu occupation in intersite layers between kagomé layers. Thus the Zn substitution of the intersite layers is not complete, and likely it intrudes the kagomé layers.