Site Specific X-ray Anomalous Dispersion of the Geometrically Frustrated Kagome Magnet, Herbertsmithite, ZnCu3(OH)6Cl2

Structural characterization, exploiting X-ray scattering differences at elemental absorption edges, is developed to quantitatively determine crystallographic site-specific metal disorder. We apply this technique to the problem of Zn-Cu chemical disorder in ZnCu3(OH)(6)Cl-2. This geometrically frustrated kagome antiferromagnet is one of the best candidates for a spin-liquid ground state, but chemical disorder has been suggested as a mundane explanation for its magnetic properties. Using anomalous scattering at the Zn and Cu edges, we determine that there is no Zn occupation of the intralayer Cu sites within the kagome layer; however there is Cu present on the Zn intersite, leading to a structural formula of (Zn0.85Cu0.15)-Cu-3(OH)(6)Cl-2. The lack of Zn mixing onto the kagome lattice sites lends support to the idea that the electronic ground state in ZnCu3(OH)(6)Cl-2 and its relatives is nontrivial.