Extension of the zinc paratacamite phase diagram: Probing the effect of spin vacancies in an S=1/2 kagome antiferromagnet

We report on the extension of the phase diagram of the magnetic model system zinc paratacamite, ZnxCu4-x(OH)(6)Cl-2 (0 <= x <= 1), with compositions x > 1, up to similar to 1.2. This amounts to diamagnetic doping of the kagome lattice of antiferromagnetically coupled Cu2+ with S = 1/2 in what was hitherto known as the end member in the series: herbertsmithite of stoichiometry ZnCu3(OH)(6)Cl-2. Combining chemical analysis, x-ray diffraction, magnetic susceptibility measurements, Raman spectroscopy and muon spin relaxation spectroscopy we conclude the following: (i) Herbertsmithite can be synthesized in stoichiometric form, but it is affected by antisite disorder of similar to 7% in Cu2+ content, implying that for x = 1, similar to 7% of the Cu kagome sites is occupied by Zn2+. (ii) The Zn content can be increased up to x similar to 1.2. No qualitatively new behavior is found for the diamagnetically doped kagome lattice, only a continuation of the trends observed for samples with x < 1 as x is increased.