Chemical and structural disorder in the kagome spin S=12 systems ZnCu3(OH)6Cl2 and YCu3(OH)6Br2[Brx(OH)1-x]

By single crystal diffraction we characterize the chemostructural disorder introduced by Zn-Cu site mixing in the kagome spin S = 21 systems herbertsmithite ZnCu3(OH)6Cl2 and YCu3(OH)6Br2[Brx(OH)1-x]. For an untwinned single crystal of herbertsmithite of composition Zn0.95(1)Cu2.99(3)O5.9(1)H5.8 (1)Cl2 we find substitution by Cu of the Zn atoms in the layers separating the kagome layers as well as substantial Zn substitution for Cu in the kagome layers. In YCu3(OH)6Br2[Brx(OH)1-x] site mixing disorder is present for intermediate x. Analogous to the Cl homologous system in crystals with x = 1/3 disorder is absent and a low-temperature structural transition emerges driven by strong magnetophonon coupling as a release of frustration. Apart from this structural anomaly we find the physical properties of these crystals unchanged compared to intermediate x and closely resembling the Cl homologue where long-range magnetic order was observed.