Chemo-Structural Disorder in the kagomé spin $S$ = 1/2 systems ZnCu$_3$(OH)$_6$Cl$_2$ and YCu$_3$(OH)$_{6}$Br$_{2}$[Br$_x$(OH)$_{1-x}$]

Abstract: By single crystal diffraction we characterize the chemo-structural disorder introduced by Zn-Cu site mixing in the kagom\'e spin $S$-1/2 systems herbertsmithite ZnCu$3$(OH)$_6$Cl$_2$ and YCu$_3$(OH)${6}$Br${2}$[Br$_x$(OH)${1-x}$]. For an untwinned single crystal of herbertsmithite of composition Zn${0.95(1)}$Cu${2.99(3)}$O${5.9(1)}$H${5.8(1)}$Cl$2$ we find substitution by Cu of the Zn atoms in the layers separating the kagom\'e layers as well as substantial Zn substitution for Cu in the kagom\'e layers. In YCu$_3$(OH)${6}$Br${2}$[Br$_x$(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 magneto-phonon 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.