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 Cu-4(OH)(6)FBr, Cu3Zn(OH)(6)FBr shows no magnetic phase transition down to 50 mK, and the system is suggested to be a two-dimensional kagome 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)(6)FBr 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 kagome layers. Thus the Zn substitution of the intersite layers is not complete, and likely it intrudes the kagome layers.