On the crystal and magnetic ordering structures of clinoatacamite, γ-Cu2(OD)3Cl, a proposed valence bond solid

Frustrated magnetic systems provide routes to exploring degeneracies and creating new electronic states. While there has been extensive theoretical study concerning the effects in the S = 1/2 quantum limit, little is known experimentally because of the scarcity of model materials, in particular systems with the pyrochlore and kagome lattices of corner-sharing tetrahedra and triangles, respectively. Much interest has developed from the discovery of herbertsmithite, ZnCu3(OH)(6)Cl-2, a material for which magnetic order has not yet been found, and its proposal as a model S = 1/2 kagome antiferromagnet. This work reopened the debate over the nature of the ground state for this archetypal quantum magnet and had led to recent theoretical suggestions that resonance effects, similar to those used to explain aromaticity in benzene, crystallize an exotic localized spin structure called a valence bond solid (VBS) from the quantum spin liquid phase. Inspired by the work on herbertsmithite, studies of the parent material clinoatacamite, gamma-Cu-2(OH)(3)Cl, in which the kagome layers, that are separated by Zn2+ in herbertsmithite, are coupled by Cu2+ ions have led to suggestions that the exchange interactions connecting these spins to those in the kagome layers are only weak, which allows a transition to occur at T similar to 18 K to a VBS, made up of singlets formed from the dimerized spins of the kagome planes. We present neutron diffraction studies of the crystallographic and magnetic structures that cast doubt on this interpretation and instead indicate significant coupling of all the Cu spins. Further, we find that the low temperature transition is associated with a canted ferromagnetic structure which is incompatible with the VBS.