Proton NMR Study of Atacamite Cu₂Cl(OH)₃

The magnetic properties of single-crystalline atacamite, Cu2Cl(OH)3

“…From our experimental data we cannot unambiguously decide whether the magnetic structure proposed in Ref. [5] is consistent with our magnetic propagation vector q. In order to resolve this question, further single-crystal neutron diffraction experiments will be carried out in the near future, this way to firmly establish the ordered magnetic structure of atacamite.…”

“…Notably, an antiferromagnetic transition at T N = 9.0 K has been observed, further, previous susceptibility measurements indicate a Curie-Weiss temperature |Θ CW | T N . So far, attempts have been undertaken to determine the symmetry of the magnetic ground state of this material by means of µSR and NMR measurements [4,5]. The conclusions drawn from these studies, however, are contradictory: From the µSR measurements on mineral atacamite, a disordered state was suggested [4], whereas from the proton NMR spectra of synthetic samples an all-in all-out alignment of the Cu magnetic moments on crystallographic inequivalent sites was deduced (µ I = 1.12 µ B and µ II = 0.25 µ B ), similar to the magnetic structure of an antiferromagnetic pyrochlore lattice [5].…”

Structural complexity and crystallization: the Ostwald sequence of phases in the Cu2(OH)3Cl system (botallackite–atacamite–clinoatacamite)

“…3a, b) are based on threedimensional Cu arrays of the distorted pyrochlore type (Fig. 3c, d) that define their magnetic behavior as pyrochlore antiferromagnets [20]. The pyrochlore-type Cu array corresponds to the array of the O atoms in cristobalite, SiO 2 , and can be derived from the diamond (dia) threedimensional net through its expansion.…”

“…Its crystal structure was first solved by Brasseur and Toussaint [17] and later refined by Wells [18] and Parise and Hyde [19]. The latter authors pointed out the similarity of the Cu cationic array in atacamite to those in spinel and pyrochlore, and this similarity was later supported by the magnetic measurements [20]. Botallackite, a-Cu 2 (OH) 3 Cl, was first described by Church [21] and structurally characterized by Voronova and Vainstein [22] and Hawthorne [23].…”

“…All three known Cu 2 (OH) 3 Cl polymorphs have been the subject of extensive studies due to their interesting magnetic properties [20,29,30], their occurrence in patinas forming on the surface of bronze artifacts [31][32][33] and their general mineralogical and geochemical importance [34][35][36][37]. Their relative stabilities and thermodynamic properties were investigated by several authors (see [38] and references therein).…”